Question

In my table, I have a nullable bit column (legacy system…) and another developer recently made a change to a stored procedure to only show values where the bit column was not true (1). Because this is a nullable column, we noticed that if the column was NULL, the record was not being picked up. WHY is this?

Both the other developer and I agree that NULL <> 1… Is this a bug in SQL or was this designed this way? Seems like a design flaw.

Current Code:

(VoidedIndicator <> 1) 

Proposed Fix:

(VoidedIndicator <> 1 OR VoidedIndicator IS NULL) 

Clarification (By Jon Erickson)

VoidedIndicator is a nullable bit field so it can have the following values: NULL, 0, or 1

When a SQL statement is created with a where clause such as (VoidedIndicator <> 1) we only get records returned that have VoidedIndicator == 0, but we were expecting both VoidedIndicator == 0 and VoidedIndicator IS NULL. Why is this?

Answer 1

From the Wikipedia entry on NULL:

For example, a WHERE clause or conditional statement might compare a column’s value with a constant. It is often incorrectly assumed that a missing value would be "less than" or "not equal to" a constant if that field contains Null, but, in fact, such expressions return Unknown. An example is below:

-- Rows where num is NULL will not be returned, -- contrary to many users' expectations. SELECT * FROM sometable WHERE num <> 1;                  

Basically, any comparison between NULL and something else, whether it’s with = or <> will not be true.

As another reference, the MSDN T-SQL page on <> states:

Compares two expressions (a comparison operator). When you compare nonnull expressions, the result is TRUE if the left operand is not equal to the right operand; otherwise, the result is FALSE. If either or both operands are NULL, see SET ANSI_NULLS (Transact-SQL).

The SET ANSI_NULLS page then states:

When SET ANSI_NULLS is ON, a SELECT statement that uses WHERE column_name = NULL returns zero rows even if there are null values in column_name. A SELECT statement that uses WHERE column_name <> NULL returns zero rows even if there are nonnull values in column_name.

…

When SET ANSI_NULLS is ON, all comparisons against a null value evaluate to UNKNOWN. When SET ANSI_NULLS is OFF, comparisons of all data against a null value evaluate to TRUE if the data value is NULL.