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