SQL CASE WHEN: Conditional Logic in Queries

SELECT order_id, amount,
  CASE
    WHEN amount < 100 THEN 'Small'
    WHEN amount < 1000 THEN 'Medium'
    ELSE 'Large'
  END AS order_size
FROM orders;

CASE evaluates conditions top to bottom. First match wins. An order of $50 hits 'Small', $500 hits 'Medium', $5000 falls through to 'Large'. The result appears as a new column called order_size.

1. CASE opens the conditional block
2. WHEN amount < 100 THEN 'Small' — first condition checked
3. WHEN amount < 1000 THEN 'Medium' — only checked if first is false
4. ELSE 'Large' — catches everything remaining
5. END AS order_size — closes CASE and names the output column

• Without ELSE, non-matching rows return NULL
• Conditions are evaluated in order — put the most restrictive first
• CASE works in SELECT, WHERE, ORDER BY, GROUP BY, and HAVING
• NULL comparisons need IS NULL, not = NULL