Short Forms for CASE
There are two short forms for special CASE expressions.
The first short form for CASE is NULLIF:
is equivalent toCASE WHEN x1=x2 THEN NULL ELSE x1 END
I.e. if both operands are equal, the NULLIF expression has the value NULL, otherwise it has the value of the first operand.
The second short form for CASE is COALESCE:
is equivalent toCASE WHEN x1 IS NOT NULL THEN x1 ELSE x2 END
is equivalent toCASE WHEN x1 IS NOT NULL THEN x1 ELSE COALESCE(x2,...,xn) END
I.e. the COALESCE expression returns the value of the first non-NULL operand, found by working from left to right, or NULL if all the operands equal NULL.
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