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DISTINCT predicate
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Function:
Specify a test of whether two row values are distinct.
Author:
Oleg Loa <loa@mail.ru>
Dmitry Yemanov <yemanov@yandex.ru>
Syntax rules:
<value> IS [NOT] DISTINCT FROM <value>
Scope:
DSQL, PSQL
Example(s):
1. SELECT * FROM T1 JOIN T2 ON T1.NAME IS NOT DISTINCT FROM T2.NAME;
2. SELECT * FROM T WHERE T.MARK IS DISTINCT FROM 'test'
Note(s):
1. A DISTINCT predicate evaluates very similar to an equality predicate with
the only difference that two NULL values are considered not distinct. As a result,
this predicate never evaluates to UNKNOWN truth value (the same as IS [NOT] NULL
predicate behaves).
2. The NOT DISTINCT predicate can be optimized via an index, if exists.