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7.13.3 NULLIF
NULLIF(value1, value2)
The NULLIF function returns a null value if
value1 and value2
are equal; otherwise it returns value1.
This can be used to perform the inverse operation of the
COALESCE example given above:
SELECT NULLIF(value, '(none)') ...
If value1 is (none), return a null,
otherwise return value1.
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