NULL means unknown

In the context of a database, if a column is set to NULL, it effectively means that the value is unknown. If we compare any other value with NULL, the result of that comparison is also unknown. In other words, a value can never be equal to NULL, as NULL is the absence of a value. This means the expression ColumnValue = NULL will never evaluate to true or false; even if ColumnValue is in fact NULL, it will always evaluate to unknown. To detect if a column value is NULL, we must use the special expressions IS NULL or IS NOT NULL rather than = or <>.

Note

This handling of NULL is not unique to the SQL Database Engine, it is based on the ANSI standard handling of NULL values.

Having NULL values in our database is not an anti-pattern in and of itself, but when we assign a meaning to the value NULL in our application, we may face some challenges when it comes to writing performant T-SQL due to the need for special handling of NULL comparisons.

Let’...