将值与 t-SQL 中的 NULL 进行比较

Question

I was curious if it's legal in t-SQL to compare a NULL to a value?

For instance, if I have:

WITH ctx AS(SELECT 123 AS n0, NULL AS n1)
SELECT n0 FROM ctx
WHERE ctx.n1 < 130

the WHERE clause in that case is always evaluated as FALSE. Is it something I can rely on?

Answer 1

You can't compare NULL with any other value, it will result in 'UNKNOWN' .

From msdn source

A value of NULL indicates that the value is unknown. A value of NULL is different from an empty or zero value. No two null values are equal. Comparisons between two null values, or between a NULL and any other value, return unknown because the value of each NULL is unknown.

Answer 2

All boolean operations in T-Sql with null value returns 'UNKNOWN', which is recognized as false in clauses. You can use ISNULL function when you want set some default value. for example in your case:

WITH ctx AS(SELECT 123 AS n0, NULL AS n1)
SELECT n0 FROM ctx
WHERE isnull(ctx.n1,0) < 130

Answer 3

It depends on the value of ANSI_NULLS.

http://msdn.microsoft.com/en-us/library/ms191270%28v=sql.90%29.aspx

When SET ANSI_NULLS is ON, a comparison in which one or more of the expressions is NULL does not yield either TRUE or FALSE; it yields UNKNOWN.

Transact-SQL supports an extension that allows for the comparison operators to return TRUE or FALSE when comparing against null values. This option is activated by setting ANSI_NULLS OFF. When ANSI_NULLS is OFF, comparisons such as ColumnA = NULL return TRUE when ColumnA contains a null value and FALSE when ColumnA contains some value besides NULL.

Answer 4

The WHERE clause in the following = is also FALSE. You need to be very careful with NULLs

WITH ctx AS
(
SELECT 123 AS n0, NULL AS n1
) 
SELECT *
FROM ctx 
WHERE ctx.n1 = NULL

Answer 5

I've always used the EXISTS keyword along with EXCEPT like so

SELECT 1
WHERE EXISTS ((SELECT 1) EXCEPT (SELECT NULL))