Subexpressions

'Orange' similar to 'O(ra|ri|ro)nge' -- true 'Orange' similar to 'O(r[a-e])+nge' -- true 'Orange' similar to 'O(ra){2,4}nge' -- false 'Orange' similar to 'O(r(an|in)g|rong)?e' -- true

`IS [NOT] DISTINCT FROM`

Syntax

<operand1> IS [NOT] DISTINCT FROM <operand2>

Two operands are considered DISTINCT (different) if they have a different value or if one of them is NULL and the other non-null.They are considered NOT DISTINCT (equal) if they have the same value or if both of them are NULL.

IS [NOT] DISTINCT FROM always returns TRUE or FALSE and never UNKNOWN (NULL) (unknown value).Operators ‘=’ and ‘<>’, conversely, will return UNKNOWN (NULL) if one or both operands are NULL.

Table 1. Results of Various Comparison Predicates
Operand values	Result of various predicates
Operand values	`=`	`IS NOT DISTINCT FROM`	`<>`	`IS DISTINCT FROM`
Same value	`TRUE`	`TRUE`	`FALSE`	`FALSE`
Different values	`FALSE`	`FALSE`	`TRUE`	`TRUE`
Both `NULL`	`UNKNOWN`	`TRUE`	`UNKNOWN`	`FALSE`
One `NULL`, one non-`NULL`	`UNKNOWN`	`FALSE`	`UNKNOWN`	`TRUE`

Examples

SELECT ID, NAME, TEACHER
FROM COURSES
WHERE START_DAY IS NOT DISTINCT FROM END_DAY;

-- PSQL fragment
IF (NEW.JOB IS DISTINCT FROM OLD.JOB)
THEN POST_EVENT 'JOB_CHANGED';