Why Does NULL = NULL Evaluate to False in SQL Server?

SQL Server's Unique NULL Comparison: Why NULL = NULL is Often False

SQL Server handles NULL values differently than many other programming languages. A common source of confusion is the fact that NULL = NULL typically evaluates to FALSE. This isn't a bug; it's a deliberate design choice rooted in the definition of NULL itself.

In SQL Server, NULL represents an unknown or missing value. Therefore, comparing two NULLs is like comparing two unknowns – it's impossible to definitively say whether they are equal because their values are undefined. The result of NULL = NULL is often FALSE (or sometimes NULL, depending on the context and system settings).

This behavior stems from the ANSI SQL-92 standard. However, it's crucial to understand the impact of the ansi_nulls setting within SQL Server.

The Role of ansi_nulls

The ansi_nulls setting determines how SQL Server interprets NULL comparisons.

• ansi_nulls ON (Default): Under this setting (the default), NULL = NULL evaluates to FALSE, aligning with the ANSI SQL-92 standard.

• ansi_nulls OFF: With this setting, NULL = NULL evaluates to TRUE. This behavior mirrors the way NULL is treated in many other programming languages, where it's often considered a special value representing an undefined state.

Illustrative Example:

The following code snippet demonstrates the difference:

SET ANSI_NULLS OFF;

IF NULL = NULL
    PRINT 'TRUE';
ELSE
    PRINT 'FALSE';

SET ANSI_NULLS ON;

IF NULL = NULL
    PRINT 'TRUE';
ELSE
    PRINT 'FALSE';

With ansi_nulls OFF, both IF statements will print "TRUE". With ansi_nulls ON, the first IF statement will print "FALSE", while the second will also print "FALSE"

Understanding the ansi_nulls setting and the inherent ambiguity of NULL comparisons is essential for writing robust and error-free SQL Server code. Always be mindful of how NULLs are handled in your queries to avoid unexpected results.

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