MEPX
Chapter 6 of 10All chapters
  1. 1What SQL is
  2. 2SELECT and WHERE
  3. 3NULL
  4. 4Sorting and limiting
  5. 5Aggregates and GROUP BY
  6. 6Joins
  7. 7Keys and relationships
  8. 8Subqueries and CTEs
  9. 9Indexes and performance
  10. 10Changing data

Chapter 6 of 10

Joins

Combining rows from two tables.

The four you need

INNER JOIN keeps rows with a match on both sides. LEFT JOIN keeps every left row and fills missing right columns with NULL. RIGHT is the mirror, and FULL keeps both.

  • The ON clause says which columns must line up.
  • A missing ON clause produces a cross join, every row against every row.

Finding what is missing

A LEFT JOIN with WHERE right.id IS NULL returns rows that have no match, which is how you find customers with no orders or records with a broken reference.