WHERE

Keeps only the rows for which a condition is TRUE.

Syntax

SELECT ... FROM table
WHERE condition [AND | OR condition ...];

WHERE is evaluated once per row, before grouping. A row is kept only when the condition is TRUE; rows where it is FALSE or NULL (unknown) are dropped.

Combine conditions with AND, OR and NOT. AND binds tighter than OR, so a OR b AND c means a OR (b AND c). Use parentheses whenever you mix them.
Compare text with single-quoted literals: city = 'Riyadh'. Text comparison is case-sensitive.
Dates can be compared with ISO literals: order_date >= '2025-03-01'.
WHERE cannot use aggregates (SUM, COUNT) — use HAVING — and cannot use window functions or select-list aliases, because it runs before those are computed.
NULL: col = NULL is never true. Use col IS NULL. And col <> 'x' silently drops rows where col is NULL.

Examples

Text and number conditions starter

SELECT product_name, category, price
FROM products
WHERE category = 'Kitchen' AND price < 200
ORDER BY price;

Both conditions must be TRUE for a row to stay.

Mixing AND and OR safely starter

SELECT order_id, status, quantity
FROM orders
WHERE (status = 'shipped' OR status = 'processing')
  AND quantity >= 2
ORDER BY order_id;

Without the parentheses, quantity >= 2 would apply only to processing orders.

Date range on a timestamp shop

SELECT order_id, ordered_at, status
FROM orders
WHERE ordered_at >= '2025-03-01' AND ordered_at < '2025-04-01'
ORDER BY ordered_at
LIMIT 10;

A half-open range (>= start, < next start) captures the whole of March, including 31 March 23:59.

Pitfalls

Writing col = NULL instead of col IS NULL (it matches nothing).
Forgetting parentheses when mixing AND and OR.
Filtering a timestamp with BETWEEN '2025-03-01' AND '2025-03-31': it stops at 31 March 00:00:00 and misses the rest of that day.
Using a select-list alias in WHERE (column "alias" does not exist): repeat the expression or wrap the query in a subquery/CTE.
Putting an aggregate in WHERE (aggregate functions are not allowed in WHERE): filter groups with HAVING.