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CASE Clause
Although SQL is not a procedural programming language, it
does allow conditional control over the data returned from a
query.
The WHERE clause uses comparisons to control row selection.
The CASE statement allows comparisons in column output.
Example:
<![CDATA[
SELECT id, name,
CASE
WHEN semester < 3 THEN 'junior'
ELSE 'senior'
END
FROM stud ;
id | name | case
----+------+--------
1 | fred | junior
3 | tom | junior
4 | john | senior
2 | lisa | junior
(4 rows)
]]>
CASE allows the creation of conditional values, which can
be used for output or for further processing in the same query.