SELECT * FROM customers c WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

EXISTS returns true if subquery returns any rows

Subquery can return any columns (SELECT 1 common)

Correlated subquery references outer query

NOT EXISTS for anti-join pattern

Short-circuits at first match

Core references in this topic include WHERE, =, <, >, <=, >=. Learn what each one does, when to use it, and the execution or engine rules that matter.

WHERE

Filters rows before projection and sorting. It decides which rows continue through the query pipeline.

SELECT ... FROM table WHERE condition;

Most performance issues start with a weak WHERE clause or a missing supporting index.

=

Returns rows where the left and right values are exactly equal.

column = value

Use with exact matches. Do not use = NULL.

<, >, <=, >=

Range comparison operators for less-than, greater-than, and inclusive boundary checks.

salary >= 80000

BETWEEN

Checks whether a value falls inside an inclusive lower/upper range.

order_total BETWEEN 100 AND 500

EXISTS

Tests whether a correlated or non-correlated subquery returns at least one row.

WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id)

ANY / ALL

Compares one value against every or at least one value from a subquery result.

salary > ALL (SELECT salary FROM interns)

INTERVAL

Represents a duration that can be added to or subtracted from dates and timestamps.

order_date + INTERVAL '7 days'

CHECK

Validates a row-level rule whenever data is inserted or updated.

CHECK (salary >= 0)

COUNT

Counts rows or non-NULL values depending on the argument.

COUNT(*)

FILTER

Applies an aggregate only to rows that satisfy an extra predicate.

COUNT(*) FILTER (WHERE status = 'active')

DISTINCT