SQL is null and = null
It's important to note, that NULL doesn't equal NULL.
NULL
is not a value, and therefore cannot be compared to another value.
where x is null
checks whether x is a null value.
where x = null
is checking whether x equals NULL, which will never be true
In SQL, a comparison between a null
value and any other value (including another null
) using a comparison operator (eg =
, !=
, <
, etc) will result in a null
, which is considered as false
for the purposes of a where clause (strictly speaking, it's "not true", rather than "false", but the effect is the same).
The reasoning is that a null
means "unknown", so the result of any comparison to a null
is also "unknown". So you'll get no hit on rows by coding where my_column = null
.
SQL provides the special syntax for testing if a column is null
, via is null
and is not null
, which is a special condition to test for a null
(or not a null
).
Here's some SQL showing a variety of conditions and and their effect as per above.
create table t (x int, y int);
insert into t values (null, null), (null, 1), (1, 1);
select 'x = null' as test , x, y from t where x = null
union all
select 'x != null', x, y from t where x != null
union all
select 'not (x = null)', x, y from t where not (x = null)
union all
select 'x = y', x, y from t where x = y
union all
select 'not (x = y)', x, y from t where not (x = y);
returns only 1 row (as expected):
TEST X Y
x = y 1 1
See this running on SQLFiddle