💠🛠️🗒️SQL Server: Nulls [Notes]

Disclaimer: This is work in progress intended to consolidate information from various sources. It considers only on-premise SQL Server, for other platforms please refer to the documentation.

Last updated: 20-Feb-2024

[SQL Server] Null

• {def} keyword that indicates that the value is unknown [1]
• different from an empty or zero value [1]
• no two null values are equal [1]
• comparisons between two null values, or between a null value and any other value, return unknown because the value of each NULL is unknown [1]
• indicates the the value is
• unknown
• not applicable
• to be added later
• ⇒ can't be used as information that is required to distinguish one row in a table from another row in a table [1]
• can be assigned to a value by
• explicitly stating NULL in an INSERT or UPDATE statement [1[
• leaving a column out of an INSERT statement [1]
• {recommendation} test for null values in queries 
• via IS NULL or IS NOT NULL in the WHERE clause [1]
• WHEN present in data, logical and comparison operators can potentially return a third result of UNKNOWN instead of just TRUE or FALSE [1]
•  ⇐ three-valued logic can be the source for many application errors [1]
• ⇐ parameters and variables not explicitly initialized can cause problems in code
• {recommendation} handle null values in logic
• via IsNull or Coalesce functions
• {constraint} [primary kyes] if any of the columns considered in a primary key contain NULL values, the PRIMARY KEY constraint can’t be created [3]
• {constraint} [UNIQUE constraint] allows the columns that make up the constraint to allow NULLs, but it doesn’t allow all key columns to be NULL for more than one row [3]
• [data warehouse] nullability of columns
• {best practice} define columns as NOT NULL when appropriate 
• {benefit} helps the Query Optimizer 
• {benefit} reduces in some cases the storage space required for the data
• {benefit} allows SQL Server to avoid unnecessary encoding in columnstore indexes and during batch mode execution [2]
• {example} [SQL Server 2000+] bigint column
• when the value is defined as NOT NULL , the value fits into a single CPU register
• ⇒ operations on the value can be performed more quickly
• a nullable bigint column requires another, 65th bit to indicate NULL values
• SQL Server avoids cross-register data storage by storing some of the row values (usually the highest or lowest values) in main memory using special markers to indicate it in the data that resides in the CPU cache [2]
• ⇒ adds extra load during execution
• {recommendation} avoid nullable columns in data warehouse environments [2]
• ⇐ the recommendation can apply also to OLTP databases
• there are database designs that enforces not null values for all attributes
• e.g. Dynamics AX 2009/365 F&O
• {benefit} eliminates the need to test for null values in legacy code
• {recommendation} use CHECK and UNIQUE constraints or indexes when overhead introduced by constraints or unique indexes is acceptable [2]
• {recommendation} consider using filtered indexes instead of normal indexes for columns with many null values
• minimizes the waste of storage space
• ⇐ understand the characteristics of the columns used in the queries [3]

See also:  Null-ifying the World, Dynamic Queries, Handling missing dates

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References:

[1] Microsoft Learn (2024) SQL Server 2022: NULL and UNKNOWN (T-SQL)
[2] Dmitri Korotkevitch (2016) Pro SQL Server Internals 2nd Ed.

[3] Microsoft SQL Server 2012 Internals, by Kalen Delaney, Bob Beauchemin, Conor Cunningham, Jonathan Kehayias, Benjamin Nevarez & Paul S. Randal, Microsoft Press, ISBN: 978-0-7356-5856-1 , 2013