Joins & Subqueries

INNER JOIN

Combine rows from two tables where a condition matches

LEFT JOIN

Keep all left rows — even when there's no match on the right

RIGHT JOIN

Keep all right rows — the mirror of LEFT JOIN

FULL OUTER JOIN

Keep every row from both sides — the union of LEFT and RIGHT

SELF JOIN

Join a table to itself — for hierarchies and row comparisons

CROSS JOIN

Generate every possible combination of rows from two tables

Subqueries

Nest one query inside another for multi-step logic

EXISTS vs IN

Two ways to check for related rows — and when each one wins