🗄️Data Management: Data Profiling II (Using CLR functions)

Data Management Series

In the previous post on Data Profiling I introduced basic data profiling techniques, the queries used for exemplification being quite simple. Probably many people will ask themselves on whether is possible to automate data profiling for a whole range of tables/datasets and the attributes they contain. The answer is simple, this relies entirely on databases’ capabilities of running dynamic scripts in an elegant manner – many such solutions relying on the use of cursors in which the dynamic created scripts are run by a stored procedure (e.g. EXEC or sp_executesql in SQL Server). There are also solution in which the dynamic queries are created, run and managed by third-party tools. In what concerns SQL Server, the CLR functions could prove to be an elegant solution for this purpose.

In a previous post, “Number of Records – The CLR Version”, I described the steps I followed in order to create a CLR function in order to get the number of records for a set of tables, the name of the table being provided as parameter. Instead of giving as parameter only the name of the table it could be provided instead the whole query string, thus having the possibility of running multiple types of query that return only a scalar. 

The problem is that the CLR function returns a value only of a certain data type, thus we could transform all the values to string or create a function for each data type. I will try to describe in this post both approaches, a reason for that being also the fact that initially I thought that the second solution is more practical, and probably easier to debug, though as I discovered the first technique is more flexible allowing to encapsulate the logic in a single query.

Let’s consider the following functions that could be added in a new Database Project or in the project created in the previous post on CLR functions. As mentioned above, is created a function for each important data type, thus ExecuteScalarToInt returns an integer value, ExecuteScalarToDate a date value, ExecuteScalarToString a string value, ExecuteScalarToDecimal a decimal value, amd ExecuteScalarToBoolean a boolean value:

Partial Public Class UserDefinedFunctions 
<SqlFunction(DataAccess:=DataAccessKind.Read, SystemDataAccess:=SystemDataAccessKind.Read)> _ Public Shared Function ExecuteScalarToInt(ByVal SqlScript As String) As Integer     
Using conn As New SqlConnection("context connection=true") 
conn.Open()     
Dim cmd As New SqlCommand(SqlScript, conn) 
Return CType(cmd.ExecuteScalar(), Integer)     
End Using End Function 

<SqlFunction(DataAccess:=DataAccessKind.Read, SystemDataAccess:=SystemDataAccessKind.Read)> _ Public Shared Function ExecuteScalarToDate(ByVal SqlScript As String) As Date     
Using conn As New SqlConnection("context connection=true") 
    conn.Open()     

    Dim cmd As New SqlCommand(SqlScript, conn)     

    Return CType(cmd.ExecuteScalar(), Date)     
End Using End Function 

<SqlFunction(DataAccess:=DataAccessKind.Read, SystemDataAccess:=SystemDataAccessKind.Read)> _ Public Shared Function ExecuteScalarToString(ByVal SqlScript As String) As String     
Using conn As New SqlConnection("context connection=true") 
    conn.Open()     

    Dim cmd As New SqlCommand(SqlScript, conn)   
  
    Return CType(cmd.ExecuteScalar(), String)     
End Using End Function 

<SqlFunction(DataAccess:=DataAccessKind.Read, SystemDataAccess:=SystemDataAccessKind.Read)> _ Public Shared Function ExecuteScalarToDecimal(ByVal SqlScript As String) As Decimal     
Using conn As New SqlConnection("context connection=true") 
    conn.Open()     

    Dim cmd As New SqlCommand(SqlScript, conn)     

    Return CType(cmd.ExecuteScalar(), Decimal)     
End Using End Function 

<SqlFunction(DataAccess:=DataAccessKind.Read, SystemDataAccess:=SystemDataAccessKind.Read)> _ Public 
Shared Function ExecuteScalarToBoolean(ByVal SqlScript As String) As Boolean     
Using conn As New SqlConnection("context connection=true") 
    conn.Open()     

    Dim cmd As New SqlCommand(SqlScript, conn)   
  
    Return CType(cmd.ExecuteScalar(), Boolean)     
End Using End Function 
End Class

After compiling the project and re-linking the assembly, must be declared the following functions:

-- returns int values 
CREATE FUNCTION dbo.ExecuteScalarToInt(@sql nvarchar(max)) 
RETURNS bigint  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].ExecuteScalarToInt 
GO 

-- returns decimal values 
CREATE FUNCTION dbo.ExecuteScalarToDecimal(@sql nvarchar(max)) 
RETURNS decimal(18,2)  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].ExecuteScalarToDecimal 
GO 

-- returns nvarchar values 
CREATE FUNCTION dbo.ExecuteScalarToString(@sql nvarchar(max)) 
RETURNS nvarchar(max)  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].ExecuteScalarToString 
GO 

-- returns date values 
CREATE FUNCTION dbo.ExecuteScalarToDate(@sql nvarchar(max)) 
RETURNS date  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].ExecuteScalarToDate 
GO 

-- returns boolean values 
CREATE FUNCTION dbo.ExecuteScalarToBoolean(@sql nvarchar(max)) 
RETURNS bit  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].ExecuteScalarToBoolean 
GO 


-- testing the functions    
SELECT dbo.ExecuteScalarToInt('SELECT count(*) FROM Production.Product') NumberRecords 
SELECT dbo.ExecuteScalarToString('SELECT Max(ProductNumber) FROM Production.Product') GreatestStringValue 
SELECT dbo.ExecuteScalarToString('SELECT Max(ListPrice) FROM Production.Product') GreatestNumericValue 
SELECT dbo.ExecuteScalarToDate('SELECT Max(SellStartDate) FROM Production.Product') GreatestDateValue 
SELECT dbo.ExecuteScalarToString('SELECT CASE WHEN count(1)>1 THEN 1 ELSE 0 END FROM Production.Product') HasRecords

 


  The first example that returns the number of records could be easily adapted in order to return the respective value for a given set of tables:

-- number of records 
SELECT S.name [schema_name] 
, T.name table_name 
, dbo.ExecuteScalarToInt('SELECT count(*) NumberRecords FROM AdventureWorks.' + S.name + '.' + T.name) NumberRecords 
FROM AdventureWorks.sys.tables T 
    JOIN AdventureWorks.sys.schemas S 
      ON T.schema_id = S.schema_id 
WHERE S.name = 'Sales' 
ORDER BY S.name  
, T.name 

 


The attribute-related metrics require a lower level of detail, to the above query adding the sys.colums table, for this purpose I will use the dbo.vTableColumns view created in the previous post on data profiling. And here’s the query based on AdventureWorks database:

-- data profiling attribute level SELECT[Schema_Name]  
, Table_Name  
, Column_Name  
, user_type  
, system_type  
-- number distinct values 
 , CASE  
    WHEN system_type NOT IN ('xml') THEN dbo.ExecuteScalarToInt('SELECT count(DISTINCT [' + Column_Name + ']) NumberRecords FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name)  
     ELSE -1 
END NumberDistinctValues 
-- number/percentage not null values 

, dbo.ExecuteScalarToInt('SELECT count(1) NumberRecords FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name + ' WHERE [' + Column_Name + '] IS NOT NULL') NumberNotNullValues  
, CASE  
    WHEN system_type NOT IN ('xml') THEN dbo.ExecuteScalarToDecimal('SELECT Cast(CASE WHEN count(1) >0 THEN Cast(count([' + Column_Name + ']) as decimal(18,2))/count(1) ELSE 0 END as decimal(5,2)) PercentageNotNullValues FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name) 
    ELSE -1 
END PercentageNotNullValues 
-- min/max length of values 

, CASE  
    WHEN system_type NOT IN ('xml') THEN dbo.ExecuteScalarToInt('SELECT IsNull(Min(Len([' + Column_Name + '])), 0) MinLengthValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name) 
    ELSE -1 
END MinLengthValue 
, CASE  
     WHEN system_type NOT IN ('xml') THEN dbo.ExecuteScalarToInt('SELECT IsNull(Max(Len([' + Column_Name + '])), 0) MaxLengthValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name) 
    ELSE -1 
END MaxLengthValue 
-- Min/Max values , CASE  
    WHEN system_type NOT IN ('varbinary', 'uniqueidentifier', 'xml', 'sysname') THEN  
dbo.ExecuteScalarToString('SELECT IsNull(Cast(Min('  
    + CASE WHEN system_type = 'bit' THEN 'Cast(' ELSE '' END 
    + '[' + Column_Name + ']' 
    + CASE WHEN system_type = 'bit' THEN ' as smallint)' ELSE '' END 
     + ') as nvarchar(max)), ''NULL'') MinValue FROM AdventureWorks.' 
     + [Schema_Name] + '.' + Table_Name) 
   ELSE CAST('' as nvarchar(max))  
END MinValue 
, CASE  
     WHEN system_type NOT IN ('varbinary', 'uniqueidentifier', 'xml', 'sysname') THEN  
dbo.ExecuteScalarToString('SELECT IsNull(Cast(Max('  
    + CASE WHEN system_type = 'bit' THEN 'Cast(' ELSE '' END 
    + '[' + Column_Name + ']' 
    + CASE WHEN system_type = 'bit' THEN ' as smallint)' ELSE '' END 
+ ') as nvarchar(max)), ''NULL'') MaxValue FROM AdventureWorks.'  
    + [Schema_Name] + '.' + Table_Name )  
    ELSE CAST('' as nvarchar(max))  
END MaxValue  
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' 
AND Table_Name = 'Product' ORDER BY [Schema_Name]  
, Table_Name  
, Column_Name  
, column_id 

   Here’s the output, the query run in about 30 seconds for the whole AdventureWorks database’s table:
  


 More likely, the above query requires some explanations. First of all some CASEs have been introduced in order to treat the exceptions, given the fact that not all data types support all the aggregate functions, while the xml data type doesn’t support any aggregate function at all. Hopefully I treated all the scenarios, if not the query could be easily updated. In other cases I applied a simple conversion, as in the case of bit data type casted to a smallint. In case the dynamic queries return other value than integer, the output value has been casted to a nvarchar(max). Most probably for easier understanding of the dynamic queries used is necessary also a review of the corresponding static queries presented in the previous post.

In case we use a specific function for each data type then we either choose to work with a query for each data type, or in case we want to use a UNION (ALL) join, then we’ll have to cast the output to nvarchar. So whatever method we choose, in order to create only one dataset out of our profiling implementation, a cast to nvarchar is required. I will resume myself to provide the queries for each data type, more for exemplifying the use of the above CLR functions:

-- getting Min/Max integer values 
SELECT [Schema_Name] 
, Table_Name  
, Column_Name 
, user_type 
, dbo.ExecuteScalarToInt('SELECT IsNull(Min(' + Column_Name + '), 0) MinValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MinValue 
, dbo.ExecuteScalarToInt('SELECT IsNull(Max(' + Column_Name + '), 0) MaxValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MaxValue 
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' 
AND Table_Name = 'Product' 
AND user_type IN ('int', 'bigint') 

-- getting Min/Max String values 
SELECT [Schema_Name] 
, Table_Name  
, Column_Name 
, user_type 
, dbo.ExecuteScalarToString('SELECT IsNull(Min(' + Column_Name + '), '''') MinValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MinValue 
, dbo.ExecuteScalarToString('SELECT IsNull(Max(' + Column_Name + '), '''') MaxValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MaxValue 
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' AND Table_Name = 'Product' 
AND user_type IN ('nvarchar', 'varchar', 'char', 'nchar') 

-- getting Min/Max Date values 
SELECT [Schema_Name] 
, Table_Name  
, Column_Name 
, user_type 
, dbo.ExecuteScalarToDate('SELECT IsNull(Min(' + Column_Name + '), Cast(''1800-01-01'' as date)) MinDateValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MinDateValue 
, dbo.ExecuteScalarToDate('SELECT IsNull(Max(' + Column_Name + '), Cast(''1800-01-01'' as date)) MaxDateValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MaxDateValue 
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' 
AND Table_Name = 'Product' 
AND user_type IN ('datetime', 'date') 

-- getting Min/Max Boolean values 
SELECT [Schema_Name] 
, Table_Name  
, Column_Name 
, user_type 
, dbo.ExecuteScalarToBoolean('SELECT IsNull(Min(Cast(' + Column_Name + ' as smallint)), 0) MinValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MinValue 
, dbo.ExecuteScalarToBoolean('SELECT IsNull(Max(Cast(' + Column_Name + ' as smallint)), 0) MaxValue FROM AdventureWorks.' + [Schema_Name] + '.' + Table_Name ) MaxValue 
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' 
--AND Table_Name = 'Products' AND user_type IN ('bit') 

Notes:
1.   The problem with actual form of the functions is that when the query returns a null value, the database engine throws the following error message:
Msg 6522, Level 16, State 1, Line 2
A .NET Framework error occurred during execution of user-defined routine or aggregate "ExecuteScalarToDate":
System.InvalidCastException: Conversion from type 'DBNull' to type 'Date' is not valid.
System.InvalidCastException:
   at Microsoft.VisualBasic.CompilerServices.Conversions.ToDate(Object Value)
   at SqlServerProject1.UserDefinedFunctions.ExecuteScalarToDate(String SqlScript)
    
The error could be avoided by handling the NULL values with a default value, for example 0 for numeric values, the empty string for string values, a minimal value date for date values, and 0 for bit values. If for numeric and string value this translation has a minimum of impact, for date and 0 values kind of reduces from functionality.

2. SQL Server 2008 provides also a Data Profiling Task as part of its SSIS solution, quite easy to use, however in comparison with it, the above functions allow to profile not only a table but a set of tables, the statistics about attributes could be brought easily together in a tabular format and the logic could be further encapsulated in a UDF or view for reuse.

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🗄️Data Management: Data Profiling I (An Introduction)

Data Management Series

Some Generalities

Jack Olson defines data profiling as "the use of analytical techniques to discover the true content, structure, and quality of data" [1]. Understanding the structure of data is quintessential for database developers in order to provide accurate technical solutions (reports, database objects for various purposes), though in order to answer business questions is important to understand also the content, the projection of business processes in database environment and, not less important, is the Data Quality reflected in the value the data has for an enterprise, being highly dependent on the understanding of data content and structure.

Typically, DBMS store also metadata about the data chunks and data structures they contain – tables, the columns they contain and their data types, the relations between tables, various types of indexes and other constraints, database objects like views, stored procedures, or user-defined functions. When taken together they provide together with the logical and semantical data models a good overview over the data structure, though the complexity of such systems, sometimes made of hundreds of tables, if often hard to comprehend, especially when one or more layers of abstraction reside in between the database structure and consumer. 

Conversely, the structure is one thing, while the data contained by the structure is another thing, attributes being more or less correctly misused for various purposes, with different degrees of completeness, actuality, accuracy, etc. I wrote about Data Quality and the most important dimensions in several posts, highlighting the importance of having data with an acceptable quality. Thus, in Data Quality initiatives, especially the ones done during data migration/conversion between various systems, data profiling occupies an important place.

A natural question (actually two): what analytical metrics could be used in order to understand the data and how complex do they have to be? Nowadays data mining, the process of extracting patterns from data, gains day by day in importance, more and more complex techniques being used, visualization techniques together with complex algorithms allowing us to derive rules from data. Do we have to go so far? Statistics offers us for example a simple set of techniques and concepts which could be used to understand the data, though Statistics deals mainly with quantities. 

Therefore, if we could translate the volume of data into various metrics, we could reuse in theory the metrics as input for statistical reasoning. And again, do we have to go that far? As we will see further on, data profiling could be done using several simple metrics, like the number of records, the number of missing values (see completeness), the number of distinct values and the distribution formed by them, the minimum and maximum values, or the minimum and maximum length of values. There are also more complex techniques that attempt to identify the correlation existing between the various attributes, falling already in the direction of data mining techniques.

Volume of Data

The number of records, as the name intuitively denotes, resumes in identifying the number of records each table/dataset from a set of table/datasets contains. It’s only a dimension of the volume of data, reflected also in the number of tables/datasets in scope, and the number of attributes each of them contains. So, the volume of data could be considered as a measure of number of records, number of tables and number of attributes. If we want to be more accurate in measurement, given the fact that each attribute could have a different maximum length, the maximum length could be considered as a fourth dimension of the volume of data. 

Now each database vendor decides on whether it reserves the respective maximum length or it considers only the actual used length (e.g. an attribute of type string could have a maximum length of 250 characters, but it holds values with a maximum length of 30 characters). Things are even more complicated because the structural length is different than the physical lengths (the number of bits allocated for storage). The physical storage occupied by a database could be in theory easily identified, though if only a subset is considered then it might be needed to consider the actual size of the volume of data in scope. The subset resumes not only to tables, but also to attributes as maybe not all the attributes from a table are used, records as there could be also inactive/disabled or other type of irrelevant records, and even data content, in case data could be stripped of not useful content.

Considering a given table/dataset the above-mentioned metrics could be obtained fairly easy by running queries against the table/dataset itself or against the metadata. As I highlighted above, database vendors that store metadata about their data structure, of importance in this context being the tables, the attributes, attributes’ data type and maximum length. There are also vendors  that store the number of records as statistics for various purposes, though the statistics are not always up to date, now it depends also on profiling requirements. On the other side getting the number of records is a simple task, even in an automated manner. Considering for exemplification the AdventureWorks database coming with SQL Server, the number of records from a table can be easily identified with the help of the count aggregate function available in all important RDBMS:  

-- the number of records (returns 54) 
SELECT count(1) NumberRecords 
FROM AdventureWorks.Production.Product 

The number of attributes the table contains could be obtained by doing a simple count against the number of attributes returned by a SELECT * statement or by querying directly the metadata:  

-- the number of attributes (returns 25) 
SELECT COUNT(1) 
FROM sys.columns C 
    JOIN sys.tables T 
      ON C.object_id = T.object_id 
WHERE T.name = 'Product' 

The same tables (catalog views starting with SQL Server 2005) could be used in order to retrieve the name of the attributes, their data type, length related attributes and whether the attribute accepts NULL values. Given the fact that the respective views will be used in multiple queries, it makes sense to encapsulate them in a view:

-- table columns view 
CREATE VIEW dbo.vTableColumns 
AS  
SELECT s.name [schema_name] 
, T.name Table_Name 
, C.name Column_Name 
, C.column_id  
, C.max_length 
, C.precision 
, C.scale 
, C.is_nullable 
, C.is_identity 
, UT.name user_type 
, ST.name system_type 
FROM sys.columns C 
    JOIN sys.types as UT 
       ON C.user_type_id = UT.user_type_id 
    JOIN sys.types ST 
      ON C.system_type_id = ST.user_type_id 
    JOIN sys.tables T 
       ON C.object_id = T.object_id 
         JOIN sys.schemas S 
            ON T.schema_id = S.schema_id 

 
Thus, the needed query becomes: 

-- metadata details 
SELECT * 
FROM dbo.vTableColumns 
WHERE [Schema_Name] = 'Production' 
   AND Table_Name = 'Product' 
ORDER BY column_id 

Output:

schema_name	Table_Name	Column_Name	column_id	max_length	precision	scale	is_nullable	is_identity	user_type	system_type
Production	Product	ProductID	1	4	10	0	0	1	int	int
Production	Product	Name	2	100	0	0	0	0	Name	nvarchar
Production	Product	ProductNumber	3	50	0	0	0	0	nvarchar	nvarchar
Production	Product	MakeFlag	4	1	1	0	0	0	Flag	bit
Production	Product	FinishedGoodsFlag	5	1	1	0	0	0	Flag	bit
Production	Product	Color	6	30	0	0	1	0	nvarchar	nvarchar
Production	Product	SafetyStockLevel	7	2	5	0	0	0	smallint	smallint
Production	Product	ReorderPoint	8	2	5	0	0	0	smallint	smallint
Production	Product	StandardCost	9	8	19	4	0	0	money	money
Production	Product	ListPrice	10	8	19	4	0	0	money	money

Note:
For more information on the meaning of user_type and system_type and the other related metadata stored, or on the length of each data types in SQL Server please check the local or online SQL Server documentation (see: Mapping System Tables to System Views, respectively Data Types).

Analyzing the Data Content

Analyzing the content resumes in studying the completeness for each attribute, what values are used and their distribution, the minimal/maximal/average values and minimal/maximal/average length of values.

Completeness could be studied as a metric of the number of records that have not-null values, or the complementary metric, the number of records that have null values, or as a percentage of the two from the total number of records. For example, in order to study the completeness of “Color” attribute could be used the following query:

-- completeness 
SELECT count(1) NumberRecords 
, count(Color) NumberNotNullValues 
, count(1)-count(Color) NumberNullValues 
, CASE  
    WHEN count(1)>0 THEN Cast(100*Cast(count(Color) As decimal (18,2))/count(1) AS decimal (5,2)) 
     ELSE 0.00  
END PercentageNotNullValues 
, CASE  
    WHEN count(1)>0 THEN Cast(100*Cast((count(1)-count(Color)) As decimal (18,2))/count(1) AS decimal (5,2)) 
     ELSE 0.00  
END PercentageNullValues  
FROM Production.Product 

Output:

NumberRecords	NumberNotNullValues	NumberNullValues	PercentageNotNullValues	PercentageNullValues
504	256	248	50.79	49.21

Studying completeness of attributes is important especially for the cases in which the attribute becomes mandatory, the number of null records equating with the number of changes that must be made to make the respective values available. In contrast, the not-null values could be a base for further review.

The next step is determining the values used in attributes, in other words the number of distinct values appearing in an attribute. Eventually, especially for list of values, it could be considered as a percentage from the number of values available. For example it could be studied how many accounts are used from the number of accounts defined. On the other side there are also attributes for which the list of possible values are not explicitly delimited, as in the case of colors available for Products, though that doesn’t necessarily happen in all business cases. To determine the number of distinct colors used for queries could be used the following query:
   

-- number distinct values 
 SELECT count(DISTINCT Color) NumberDistinctColors 
, count(DISTINCT IsNull(Color, 'n/a')) NumberDistinctColorsIncludingNulls 
FROM Production.Product 

Output:

NumberDistinctColors	NumberDistinctColorsIncludingNulls
9	10

Note:
Please note that the NULL value is ignored by count aggregate function. I’m mentioning that because there are cases in which the NULL value has a meaning of its own, in such cases the NULL could be placed with a meaningful (e.g. ‘’n/a in this case).

Attributes’ usage is quite an important subject when considering metadata, being for example important to study which attributes are referenced in various tables. For example, is of importance studying which Products appear in Purchases, Sales Orders, Inventory, etc., especially the respective Products have to receive special attention in what concerns Data Quality.

The number of distinct values is just a measure in studying attributes’ usage, a more complex analysis being performed based on the frequency distribution of values based on the number of references in a given table/dataset. This can be easily done using a count together with a grouping based on the studied attribute. Here’s the example for color: 

-- distribution of values 

SELECT Color 
, count(1) NumberRecords 
FROM Production.Product 
GROUP BY Color 
ORDER BY NumberRecords DESC

Output:

Color	NumberRecords
NULL	248
Black	93
Silver	43
Red	38
Yellow	36
Blue	26
Multi	8
Silver/Black	7
White	4
Grey	1

As can be seen from the above output (called a univariate frequency table), the result shows not only the number of records with NULL values, but also the number of distinct values. Attributes’ usage could be used also as a criterion for prioritizing Data Cleansing by focusing first on the attributes with the highest frequency. Eventually the frequencies could be expressed also as percentages in ratio with the number of records, the percentages being often considered as more representative values in visualization of data.

Often, it’s useful to study the frequency of values based on explicit (e.g. Product Category/Subcategory) or implicit (ad-hoc defined) categories instead of studying values’ distribution.

A third technique used in analyzing attribute’s usage is based on identifying the range of values, here simple statistical tools like minimum, maximum, averages, or standard variation of values could be of importance in understanding the data. The minimum and maximum could be applied for all data types, but they are meaningful mainly for the numeric and date-time data types, and both could take advantage of the whole specific range of statistical tools. Studying the maximum and minimum value within for a string data attribute makes sense only when there is a strong lexicographical motivation, therefore as example I will consider the List Price from same Product table.

-- aggregate values 
 SELECT MIN(ListPrice) MinListPrice 
, MAX(ListPrice) MaxListPrice 
, AVG(ListPrice) AverageListPrice 
, STDEV(ListPrice) SandardDevListPrice  
FROM Production.Product 

Output:

MinListPrice	MaxListPrice	AverageListPrice	SandardDevListPrice
0.00	3578.27	438.6662	773.602842630487

 
The 0 values could be excluded from analysis because at least in this case they represent the lack of a valid List Price. At least in this case the average and standard deviation doesn’t really make sense, while the minimum and maximum reveal the range of values used. As in the case of frequencies, it’s useful to study the frequency of values based on explicit or implicit categories. Here’s an example based on List Price aggregation at Subcategory level:

-- aggregate values per subcategory 
SELECT PS.Name 
, MIN(P.ListPrice) MinListPrice 
, MAX(P.ListPrice) MaxListPrice 
, AVG(P.ListPrice) AverageListPrice 
, STDEV(P.ListPrice) SandardDevListPrice  
FROM Production.Product P 
    LEFT JOIN Production.ProductSubcategory PS 
      ON P.ProductSubcategoryID = PS.ProductSubcategoryID 
WHERE IsNull(P.ListPrice, 0)!=0 
GROUP BY PS.Name 
ORDER BY PS.Name 

Output:

Name	MinListPrice	MaxListPrice	AverageListPrice	SandardDevListPrice
NULL	133.34	196.92	159.1333	28.9631334630769
Bib-Shorts	89.99	89.99	89.99	1.34869915234861E-06
Bike Racks	120.00	120.00	120.00
Bike Stands	159.00	159.00	159.00
Bottles and Cages	4.99	9.99	7.99	2.64575131106459
Bottom Brackets	53.99	121.49	92.24	34.6383097162665
Brakes	106.50	106.50	106.50	0
Caps	8.99	8.99	8.99
Chains	20.24	20.24	20.24
Cleaners	7.95	7.95	7.95

  
I tend to believe that such analysis at category level is less important in Data Profiling, though there could be also exceptions, for example the case of ABC analysis. 

Analyzing the Data Structure

The understanding of the structure of data is facilitated by the metadata available in the database itself, the logical/semantical data models and other types of documentation (e.g. functional specifications, data dictionaries, etc.), but as specified above even if the data are filled in a given predefined structure, when the database is not adequately normalized or that the data attributes are misused, results structures in structures, sometimes people use their own encodings varying from person to person, while other times such encodings are standardized, and even implemented in tools. When we talk about structure, the simplest metrics that could be created are based on the minimal and maximal length of attributes, while the average length could be used for example as a base for calculating the volume of data. Here’s the example script based on Color attribute:

-- aggregate length values 
SELECT MIN(Len(Color)) MinLength 
, MAX(Len(Color)) MaxLength 
, AVG(Len(Color)) AverageLength            
FROM Production.Product

Output:

MinLength	MaxLength	AverageLength
3	12	5

The discovery of special structures (patterns) used inside of an attribute are not so easy to determine using simple scripts, at least not unless the structure is known in advance. There are tools and I’ve also seen some SQL scripts available that allow identifying the patterns in data, though they have relative precision and I believe human intervention is requested (e.g. semi-automatic methods).

Analyzing Data Relations

Talking about patterns, the discovery of relations between attributes could also be considered as special type of pattern discovery, the above last statement applying for them too, the data mining field providing many tools for this type of discovery. In theory the relation between attributes could be discovered by grouping two or more attributes and analyzing the output and interdependence between them. Whether that can be achieved by a database developer depends also on his skills and knowledge in statistics and data mining field, often being required an experienced Data Analyst for this type of tasks.

Closing Remarks

There are more to say about Data Profiling topic, there are many more (complex) techniques that could be used to profile the data, especially in respect to Data Quality, which mainly focuses on 6 important dimensions of data: accuracy, duplicates (or uniqueness), completeness, consistency, conformity, and referential integrity. If in previous posts I addressed the respective topics from a theoretical point of view, I will try to attempt to develop the subjects from Data Profiling perspective in next posts, showing various techniques that could be used by developers for this purpose.

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Written: Jul-2010, Last Reviewed: Mar-2024

References:
[1] Jack E Olson(2003) Data Quality: The Accuracy Dimension

💎SQL Reloaded: Number of Records II (The CLR Version)

While scanning rapidly the daily posts from MSDN blogs, I stopped on Chris Skorlinski’s “Example code executing TSQL for each Table in a database” post, wondering then on whether he’s using other techniques than I used in the past. I run into this type of need several times, especially in cases in which I wanted to report in an automated manner the number of records for a given set of tables, typically during ETL tasks. The problem could be easily solved with the help of a cursor, though it’s not the most elegant technique, even if it solves the problem fairly easy. 

I remember how I tried to find a way to execute a dynamic statement using a dynamic script inside of a  UDF (user-defined function), though that’s not possible because a stored procedure can’t be executed inside of a function. An alternative would be to use the OPENROWSET function and execute a statement using a linked server, though the OPENROWSET function doesn’t work with dynamically created statements.

Five years ago, soon after SQL Server 2005 RTM appeared on the market, I tried to create a UDF programmatically using an example from Bob Beauchemin & co.’s “A First Look at SQL Server 2005 for Developers” book. The example worked then, but not without some inherent headaches, having to spent some time on troubleshooting the received errors, and if I remember correctly they were caused by a change in the way the connection was created from the SQL Server SQLClient. Anyway, that’s already history, though since then I was wondering on whether is possible to use a programmatically created UDF to return the number of records for a given table sent as parameter. Since then I haven’t had the chance to test that, even if it seemed logically possible, but seeing Chris Skorlinski’s post and as last week I installed Visual Studio 2010 on my home computer, I said to myself that that’s a nice way to see the tool at work. Thought and done, but again not without problems, running in several small but time consuming issues.

So armed with some patience, I did a quick search on Microsoft site on how to use CLR scalar-valued functions under SQL Server 2008, running into this MSDN resource, which actually returns the record count using a static query. So I opened Visual Studio 2010 and created a new Database/SQL Server/Visual Basic SQL CLR Database Project named SQLServerProject1 (I forgot to give the project a meaningful name) and added a database reference to the AdventureWorks database using Windows Authentication.



Then in the new created project I’ve added a UDF to the project (Add/New Item/User Defined Function), naming the new created class as CLRLibrary, I replaced the existing function with the one from the MSDN article, added a new String input parameter called TableName, modified the query and changed UDF’s name to NumberRecords as per below piece of code:
 

Imports System 
Imports System.Data 
Imports System.Data.SqlClient 
Imports System.Data.SqlTypes 
Imports Microsoft.SqlServer.Server 
Imports System.Runtime.InteropServices 'added manually 
 

Partial Public Class UserDefinedFunctions 
SqlFunction(DataAccess:=DataAccessKind.Read)> _ 
Public Shared Function NumberRecords(ByVal TableName As String) As Integer 
Using conn As New SqlConnection("context connection=true") 
conn.Open() 
Dim cmd As New SqlCommand("SELECT COUNT(1) AS [NumberRecords] FROM " & TableName, conn) 
Return CType(cmd.ExecuteScalar(), Integer) 
End Using 
End Function 
End Class 

    In addition, I had to add also a reference to System.Runtime.InteropServices, and now the function was ready for testing so I built the solution; then I copied CLRLibrary’s Full Path in SQL Server Management Studio and attempted to follow the steps described in the MSDN article. Now it’s time for some troubleshooting fun… First I enabled the CLR integration configuration option by running the below script:

--enable CLR integration use master 
go sp_configure 'clr enabled', 1  

go 
reconfigure 
go 

       
    Using the master database I attempted to create a reference to the assembly, using the Full Path and the dll name.
 

CREATE ASSEMBLY SqlServerProject1 

FROM 'D:\\SqlServerProject1.dll' 
WITH PERMISSION_SET = EXTERNAL_ACCESS 

   The next step was to declare the function:

CREATE FUNCTION dbo.NumberRecords(@table_name nvarchar(100)) 
RETURNS int  
AS  
EXTERNAL NAME SqlServerProject1.[SqlServerProject1.UserDefinedFunctions].NumberRecords 
GO  
 

    Actually is the final form of the DDL script, because initially I used only a combination of the Project, class and function name (e.g. SqlServerProject1.UserDefinedFunctions.NumberRecords), receiving the following error message:

Msg 6505, Level 16, State 2, Procedure NumberRecords, Line 1
Could not find Type 'UserDefinedFunctions' in assembly 'SqlServerProject1'.

    After a quick search on Google I found out from Eggheadcafe forum that the Namespace was missing from the declaration, and once I made this change the above statement worked without problems. The next step was naturally to test on whether the function is working:

SELECT dbo.NumberRecords('AdventureWorks.HumanResources.Department')  

And it worked! So it’s time to test it against a set of tables from AdventureWorks database:
 

SELECT s.name [schema_name]
   , T.name table_Name 
, dbo.NumberRecords('AdventureWorks.' + s.name + '.' + t.name) NumberRecords 
FROM AdventureWorks.sys.tables t 
      JOIN AdventureWorks.sys.schemas s 
        ON t.schema_id = S.schema_id  
ORDER BY s.name  
, T.name 

  Actually I missed some important thing from the above steps, somewhere while attempting to test the function or to register the assembly, I received the following error message:

A .NET Framework error occurred during execution of user-defined routine or aggregate "NumberRecords": System.InvalidOperationException: Data access is not allowed in this context.  Either the context is a function or method not marked with DataAccessKind.Read or SystemDataAccessKind.Read, is a callback to obtain data from FillRow method of a Table Valued Function, or is a UDT validation method.

   A post on MSDN SQL Server forum remembered me that I forgot to check on whether I’m using the 3.5 or 4.0 .Net Framework. So in the SQLServerProject1 project, from main menu/Project/SQLServerProject1 Properties/Compile/Advice Compile Options/Target Framework, I changed the value to .Net Framework 3.5. This change requires the project to be closed and reopened.

    In between the above steps I had also to check on whether the assembly was registered using the below script:

--assembly file details 
SELECT A.name [Assembly] 
, AF.name File_Path 
FROM sys.assemblies A 
     JOIN sys.assembly_files AF 
        ON A.assembly_id = AF.assembly_id 
WHERE A.is_user_defined=1 
--AND A.name='SqlServerProject1' 

    Also, if you’d like to remove the assembly from the database, you’ll have first to drop the function, and only then drop the assembly:

-- cleaning up
DROP FUNCTION NumberRecords 
DROP ASSEMBLY SqlServerProject1 

    It can be further discussed on whether this approach is acceptable as performance or not, on whether it can be misused, etc. Actually a nice misuse is the following statement which passes a WHERE constraint with the table name.

SELECT dbo.NumberRecords('AdventureWorks.HumanResources.Department WHERE Name LIKE ''P%''') 

    It could be created a second parameter for passing the WHERE constraint or, why not, pass the whole query as parameter, as long only a scalar is returned. I hope I haven’t forgot any step, I’m kind of mixing the facts because I attempted also to create the same function using a new created login, running into other type of issues that maybe deserve a second post.

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🛒Market Review: What’s New in Microsoft World II

Microsoft Office - Cloud Computing is the Word

    Two weeks ago, on 15^th of June 2010, Microsoft Office was shipped together with Visio and Project 2010, closing the cycle of releases started with SQL Server 2008 R2, Visual Studio 2010, Sharepoint 2010 (all 3 shipped in April 2010) and Windows Azure (available also in April). The words that describe/unite at best these software tools is cloud computing and collaboration, why that? First we have to consider Azure, the new product from Windows’ portfolio, a framework for cloud computing and SaaS (Software as a Service) architectures, and composed of 3 components, namely Windows Azure which allows running applications and accessing data in the cloud, SQL Azure Database  provides data services in the cloud, while Windows Azure platform AppFabric allows the communication between the applications residing in the cloud. Also MS Office 2010 is part of Microsoft’s strategy toward cloud computing, the weight falling on SharePoint 2010, a business collaboration platform that together with the other MS Office tools allow to manage information, automate and manage business processes, facilitate decision making process, etc. A cornerstone of the framework is the co-authoring tool that “allows multiple people to work on a single copy of a document at the same time or at different times, seamlessly, whether they are online or offline”. As it seems are provided also “community features that allows users to share data as they do on Twitter and Facebook”, a step toward social computing. Microsoft plans to offer an online version of Office 2010, called Office Web Apps (OWA), supposed to be also a competitor for Google Docs.

    There are also people who question the steps done by Microsoft toward cloud computing, but in the end is important to establish the software infrastructure in which cloud computing-based applications could be developed, futures that don’t exist currently could appear in future versions or could be provided by third-party vendors.

    Microsoft comes also with some unpleasant surprises, as it seems Microsoft’s SharePoint Server runs only on 64-bit hardware and requires also a 64 bit SQL Server edition, and this could be quite an important constraint for many customers. The most unpleasant surprise is that Microsoft renounces to the well-known upgrade schema, the reason for that, as mentioned in Ars Technica quoting a Microsoft spokesman, from the need to simplify the product lineup and pricing, based on “partner and customer feedback” (I’m sorry but I can’t really buy that!). The same source expects that upgrades will be available with promotions, after Office’s launch. The only promotion I heard of is Microsoft Office 2010 Technology Guarantee program but if refers only to the customers who “purchased, installed, and activated a qualifying Microsoft Office 2007 product between March 5, 2010, and September 30, 2010”, they being eligible to download Office 2010 at no additional cost. How about the ones who bought a Microsoft Office 2007 copy in 2010 but before 5^th of March (like I did)?!

Microsoft TechEd North America Sessions are Online

    The Microsoft TechEd North America sessions held in New Orleans were made available online (video and slides), an opportunity for technical professionals to get an overview on the new advancements in Microsoft technologies, being approached topics related to the various platforms of Windows, MS Office, Dynamics, Web, Cloud Computing & Online Services, etc. I really like the way Microsoft makes its technologies available to the public, especially the fact that it provides also Express versions of their software, allowing newbies and developers to get acquainted and use essential basic functionality. The MSDN, TechNet, webcasts, Channel9  and community and personal blogs bring the technical and non-technical closer to the company and its technologies.

💎SQL Reloaded: Trivial Equalities in Queries

    If I remember correctly from the Math literature, equalities of the type 0=0, 1=1 or more general n=n could be referred also as trivial equalities. I tried to find, without success though, an exact definition of what is intended by trivial equality, the closest I could get is the Wikipedia’s content on the use of trivial adjective in Mathematics for objects that have a simple structure. As per my perception, not sure if I’m correct, a trivial equality is an equality in which one of the members is a constant, the same definition holding also for trivial inequalities.

    Now, what have trivial equalities like 1=1 have to do with queries?! As 1=1 always equates to true, same as 1=0 equates to false, could be used in certain scenarios as a simple technique to return all the records, respectively no records from a database. For example given the fact that 0=1 equates to false I used such a constraint under ADO in queries like the below one in order to retrieve metadata from a database.

SELECT *  
FROM Production.Product 
WHERE 1=0 

    In the same way 1=1 could be used to retrieve all the records from a table:

SELECT *  
FROM Production.Product 
WHERE 1=1 

    Now if in above query we add different constraints the 1=1 allows to comment/uncomment the constraints as we wish with a minimal of changes.

SELECT *  
FROM Production.Product 
WHERE 1=1 
AND Color = 'Black' 
--AND SafetyStockLevel>800 
AND StandardCost>300 

    This makes it quite useful when creating dynamic queries (see the first constraint from the stored procedure given as example in Just In Case – Part IV: Dynamic Queries post).  I actually have seen quite often this type of writing a query, especially between Oracle developers. Long time ago I asked somebody what’s the consideration behind its use, excepting the fact that it allows to add or remove (comment) constraints. The answer was quite fuzzy, being mentioned a possible improvement in query’s performance. Some time ago I remember somebody was mentioning that in older SQL Server database engines such a constraint could lead to an unexpected query plan, and thus poorer performance. Normally the database engine should recognize such statements as meaningless, and ignore them, however from theory to implementation is a long way and anything is possible. An example in this direction could be found in The real cost of performance, the post and the comments that followed revealing the various facets of using equality sign in programming languages like C# or VB. SQL shouldn’t be so complex, as it doesn’t typically used in the work with objects.

    I used such trivial equalities also in cartesian JOINs as an artifice in order to use ANSI syntax in queries, here’s an example using the SplitList table-valued function introduced long time ago:

SELECT * 
FROM dbo.SplitList('1,2,3,4', ',') A  
       JOIN dbo.SplitList('A,B,C,D', ',') B  
          ON 1=1 

    Given the above definition for trivial equalities, in theory all constraints in which one of the members is a hard-coded could be regarded as trivial. Actually all numeric data type constraints could be written as trivial equalities by moving all attributes in one of the members, the other being a constant, for example:

SELECT P.Name Product 
, PM.Name ProductModel 
FROM Production.Product P 
        JOIN Production.ProductModel PM 
            ON P.ProductModelID - PM.ProductModelID = 0 

    Even if such a statement is logically correct, it should be avoided when possible, given the fact that it decreases considerably the performance of the query (just compare the query plan between the above query and the typical join), indexes, if exist, not being used efficiently.

🎡SSIS: Using Oracle as Data Source

Making SSIS work with Oracle doesn’t seem to be a complicated task especially after several years of experience in doing that, though, as usual, something new appears in the landscape – new software versions, new requirements, an environment with its own particularities, etc. In general, when an application needs a connection to an Oracle server, is needed to install on the client computer several components that come with Oracle Client, add/configure the TNS (Transparent Network Substrate) name, eventually set-up some global variables in case the components were not installed in the default location, and depending on the chosen provider might be needed to configure also a DSN (Data Source Name) pointing to the Oracle server. SSIS makes actually no exception from these steps, once the steps performed you should be in theory ready to develop, test, deploy, schedule and run packages using a connection to an Oracle server.

Unfortunately, from my experience, every 2-3 installations, there is a problem with the Oracle Client and its configuration, most of the times the solution being quite simple – removing some declarations from the set-up files, correcting the global variables or the TNS name. As it seems the Oracle Client is quite sensible to the changes in the default installation path, therefore is indicated to install the Client using the default path unless you’d like to gain more experience in troubleshooting such installation issues. In addition, as any respectable company, some of the products come with their own defects, the patching of such issues being not so easy, therefore it’s advisable to check beforehand the known issues coming with the Client version you’d need to install, preferably in case you need to take advantage of the latest Oracle features, it makes sense to install the latest stable Client.  

Because the Oracle Client downloadable package has a few hundred MB, there is a thinner Oracle alternative to Oracle Client, namely Instant Client package, the components could be downloaded from Oracle site (here) and installed individually, as a minimum being necessary to install Instant Client Package – Basic or Basic Lite versions, the ODBC libraries and the SQL *Plus libraries in case you want to test the connection. Of course, after case could be installed also the JDBC, SDK or any other packages Oracle made available.

On 64 bit platforms might be needed to install in parallel the 32 and 64 bit Oracle/Instant Clients (see SSIS, Oracle and X64 post from Business Vision DEV Team), while in order to troubleshoot the various issues could be a good idea to check the differences between 32 bit and 64 bit registry (here). 

There are several drivers that allows you to connect to an Oracle database using SSIS, the most popular ones:
- Microsoft OLE BD Provider for Oracle
- Oracle Provider for OLE DB
- .Net Framework Data Provider for Oracle
- .Net Framework Data Provider for Odbc
- Oracle Data Provider for .NET

The Microsoft drivers come with MDAC (Microsoft Data Access Components), its latest version being MDAC 2.8. Starting with Windows Vista and Windows Server 2008, Microsoft changed MDAC into WDAC (Windows Data Access Components), including it as part of the operating system, removing thus the need to redistribute the components. See Data Access Technologies Road Map, FAQ and Troubleshooting MDAC/WDAC for MDAC/WDAC architecture, components and releases, respectively troubleshooting. Given the various issues that exist with a particular MDAC/WDAC library, see in KB301202 how you could check the current version by using the registers, while for  Windows version up to Windows Server 2003 could be used the Component Checker MDAC Utility.

If .Net Framework Data Provider for Oracle, respectively for Odbc come with the .Net framework, in exchange, Oracle Data Provider for .NET, see also FAQ is Oracle’s implementation for ADO.NET data access,  supposed to take advantage of advanced Oracle database functionality, it comes with ODAC (Oracle Data Access Components). Please note that you might need in install the respective components in addition to the Oracle Client.

There are also third party drivers for Oracle or even particularly for SSIS, for example Microsoft Connectors Version 1.1 for Oracle and Teradata for use with SSIS from Attunity; see also a few tips from SQL Server Performance blog.

Before taking any decision on which driver to use, it might be a good idea to look also at drivers’ limitations and advantages. In the past I often used the Microsoft Oracle ODBC Driver until I run into in an important limitation, namely its inexistent support for unicodes, this residing, according to KB244661, in the fact that “from Microsoft Data Access Components (MDAC) version 2.5 and later versions, both the Microsoft ODBC Driver and OLE DB Provider support ONLY Oracle 7 and Oracle 8i”. Also Oracle Provider for OLE DB seems to have its own limitations.

SQL Server provides linked servers, the powerful functionality of executing commands against OLE DB data sources on remote servers, including Oracle. As linked servers offer the ability to create cross-vendor distributed queries, in certain scenarios they could prove to be a powerful alternative of querying Oracle databases, in such cases not being needed to create in SSIS package an additional connection to Oracle. KB280106 and an article on Oracle Provider for OLE DB describe how to set up and troubleshoot a linked server to an Oracle database in SQL Server.

You might want to check also the SQL Server Integration Services with Oracle Database 10 White Paper coming from Microsoft. “Connectivity and SQL Server 2005 Integration Services”  MSDN article written by Bob Beauchemin, Scott Barrett’s blog or this SSIS 64 bit – Using Oracle Provider post.