CASE turns values into labels

CASE is SQL's way of saying if this, then that. Each WHEN holds a test and its THEN holds the label to use when that test passes. SQL reads the WHEN tests from the top down and stops at the first one that is true. ELSE gives the label for rows that passed no test, and END closes the CASE.

A tiny example

In plain English: Show me each book's title, labeled short when it is under 100 pages and long otherwise.

SELECT title, CASE WHEN pages < 100 THEN 'short' ELSE 'long' END AS length
FROM books;

What comes back

title	length
The Turnip	short
Dune	long
Wool	long

The Turnip is under 100 pages, so it's 'short'; Dune and Wool failed that test, so ELSE labeled them 'long'.

Related SQL lessons

A query can hold another query inside itA query can hold another query inside it
EXISTS asks 'is there at least one?'EXISTS asks 'is there at least one?'
LAG looks at the row beforeLAG looks at the row before
SUM OVER (ORDER BY ...)SUM OVER (ORDER BY ...) is a running total

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