💎SQL Reloaded: Updating Data With Values From The Same Table

I have to deal with cases in which I have to update data from a table with values from the same table across records, a simple example I can sketch is the one of Standard Prices, which are normally valid during a year. For example if I would have to reset the Standard Prices from a year to the ones from the previous year, how this can be done? Actually that's pretty sample, for exemplification purposes I will create a table having only 3 columns, ProductID, Year and StandardPrice: 

-- creating the table 
CREATE TABLE dbo.StandardPrices( 
ProductID int 
, [Year] int 
, StandardPrice decimal(13,2) )      

--creating test data 
DECLARE @index int 
SET @index = 0 
  WHILE @index < 50 
  BEGIN 
      INSERT dbo.StandardPrices(ProductID, [Year], StandardPrice) 
      VALUES (@index % 25+1, 2000 + (@index/25), 100 * RAND()) 
      SET @index = @index + 1 
END     

Let's see the content, the RAND() function allows me to assign random prices to each product. As can be seen there are 25 products with Standard Prices for the Years 2000 and 2001.

 --testing StandardPrices table's content 
SELECT * 
FROM dbo.StandardPrices 
ORDER BY ProductID, [Year]     

Let's try to use a normal update: 

--first update attempt 
UPDATE dbo.StandardPrices 
SET StandardPrice = A.StandardPrice 
FROM StandardPrices A 
WHERE StandardPrices.ProductID = A.ProductID 
AND StandardPrices.[Year] = 2001 AND A.[Year] = 2000     

Ups, I got an error (SQL Server 2005):
Msg 4104, Level 16, State 1, Line 1 The multi-part identifier "StandardPrices.ProductID" could not be bound. Msg 4104, Level 16, State 1, Line 1 The multi-part identifier "StandardPrices.Year" could not be bound. 

SQL Server 2000:
Server: Msg 107, Level 16, State 3, Line 1 The column prefix 'StandardPrices' does not match with a table name oralias name used in the query. Server: Msg 107, Level 16, State 1, Line 1 The column prefix 'StandardPrices' does not match with a table name oralias name used in the query.     

It seems we can't use the same table in updates of this type, but we can use a subquery or view which refers to the same table, so we can write: 

  --the update 
UPDATE dbo.StandardPrices 
SET StandardPrice = A.StandardPrice 
FROM ( 
     SELECT ProductID , [Year] , StandardPrice 
     FROM dbo.StandardPrices 
     WHERE [Year] = 2000 
) A 
WHERE StandardPrices.ProductID = A.ProductID 
     AND StandardPrices.[Year] = 2001     

Now there are no errors, 25 records updated, let's see the output: 

--testing table's content after update 
SELECT * 
FROM dbo.StandardPrices 
ORDER BY ProductID
, [Year]

💎SQL Reloaded: Preparing Test Data

There are cases in which we need to create test data in order to check a certain functionality. In this post I'll show how can be created data with the help of loops. For this let's create a simple table containing 5 fields: 

-- creating the test table 
CREATE TABLE Table1( 
ID int NOT NULL 
, [Year] int NOT NULL 
, Product varchar(30) NOT NULL 
, Price decimal(13,2) NOT NULL 
, Quantity decimal(13,3) NOT NULL) 

-- dropping the test table 
--DROP TABLE Table1 

-- deleting the data 
--TRUNCATE TABLE Table1          

        

Usually I'm commenting DROP and TRUNCATES so I can avoid the modification of data by mistake.

 And now the loop, with the help of RAND and modulo the values can be varied:

-- inserting the data
DECLARE @index int 
SET @index = 1 
WHILE @index < 99
BEGIN 
      INSERT Table1(ID, [Year], Product, Price, Quantity) 
      VALUES (@index, 2000 + (@index % 4), 'Product ' + Cast(@index%10 as char(3)), 10 * RAND(), 100 * RAND()) SET @index = @index + 1 
END 


--testing the output 
SELECT * 
FROM Table1     

-- using the data
SELECT [Year], Product, SUM(Price * Quantity) Amount 
FROM Table1 
GROUP BY [Year], Product
ORDER BY [Year], Product