PostgreSQL, Second Edition

The CASE Expression

The CASE expression is a more generic form of NULLIF() and COALESCE(). A CASE expression lets you map any given value into some other value. You can write a CASE expression in two different forms. The first form (called the simple form) looks like this:

CASE expression1
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ...
    [ ELSE resultn ]
END

When PostgreSQL evaluates a simple CASE expression, it computes the value of expression₁ then compares the result to value₁. If expression₁ equals value₁, the CASE expression evaluates to result₁. If not, PostgreSQL compares expression₁ to value₁; if they match, the CASE expression evaluates to result₂. PostgreSQL continues searching through the WHEN clauses until it finds a match. ...