IS [NOT] DISTINCT_FROM

Compares whether two expressions are equal (or not equal) with awareness of nullability, meaning it treats NULLs as known values for comparing equality. Note that this is different from the comparision operators, which will return NULL if the arguments are NULL.

Syntax

<expr1> IS [ NOT ] DISTINCT FROM <expr2>

Arguments

Arguments	Description
<expr1>	Any general expression which will be evaluated as the value.
<expr2>	Any general expression which will be evaluated as the value.

Return Type

For IS DISTINCT FROM operator, if x and y are NULL, it returns 0; and if only one of x and y is NULL, it returns 1; otherwise it returns the result of x<>y, and vice versa for IS NOT DISTINCT FROM.

Examples

CREATE TABLE t_null (a INT NULL, b INT UNSIGNED NULL);

INSERT INTO t_null VALUES(1, NULL), (NULL, 2), (NULL, NULL),  (1, 2), (3, 3);

SELECT a, b FROM t_null WHERE a IS DISTINCT FROM b;
+------+------+
| a    | b    |
+------+------+
|    1 | NULL |
| NULL |    2 |
|    1 |    2 |
+------+------+

SELECT a, b FROM t_null WHERE a IS NOT DISTINCT FROM b;
+------+------+
| a    | b    |
+------+------+
| NULL | NULL |
|    3 |    3 |
+------+------+