SELECT c.* FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.id IS NULL;

Use LEFT JOIN to preserve all left rows

Check right table PRIMARY KEY for NULL

Do not check foreign key for NULL

WHERE clause filters to unmatched rows only

Alternative: NOT EXISTS subquery

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.

!= / <>

Returns rows where the compared values are not equal.

column <> value

SQL supports both <> and != in many engines, but <> is the portable form.

<, >, <=, >=

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

salary >= 80000

IS NULL / IS NOT NULL

Tests whether a value is missing. SQL NULL semantics require dedicated NULL predicates.

manager_id IS NULL

Never use = NULL or != NULL.

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'

PRIMARY KEY

Uniquely identifies each row and implicitly requires NOT NULL.

customer_id INT PRIMARY KEY

FOREIGN KEY

Enforces referential integrity by requiring a matching row in another table.