CASE Expression

CASE Expression

The CASE expression is like an if-then-else in SQL. It checks conditions and returns a value for the first one that is true.

Basic example

SELECT OrderID, Quantity,
CASE
  WHEN Quantity > 30 THEN 'big'
  WHEN Quantity > 20 THEN 'medium'
  ELSE 'small'
END AS Size
FROM OrderDetails;

How it works

1. Conditions are checked top to bottom.
2. The first TRUE condition wins.
3. If none match, ELSE is returned.
4. END finishes the CASE; AS Size names the column.

ELSE is optional

If there is no ELSE and nothing matches, the result is NULL.

CASE in WHERE

SELECT CustomerName, City
FROM Customers
ORDER BY
(CASE
  WHEN City IS NULL THEN 1
  ELSE 0
END), City;

Sorts NULL cities last.

CASE in ORDER BY

CASE is also handy for custom sorting, like always keeping a certain product on top.

TL;DR

• CASE is if-then-else for SQL.
• First true WHEN wins.
• ELSE is the fallback (NULL if missing).
• Usable in SELECT, WHERE, and ORDER BY.