INNER JOIN — combine matching rows from two tables

A JOIN (also written INNER JOIN) combines rows from two tables where a condition is met. Rows without a match in either table are excluded.

Syntax

SELECT t1.col, t2.col
FROM table1 t1
JOIN table2 t2 ON t2.foreign_key = t1.primary_key;

Example: employees with their department name

SELECT
  e.first_name,
  e.last_name,
  d.name AS department
FROM employees e
JOIN departments d ON d.id = e.department_id;

e and d are table aliases — shorter names assigned for the query.

Why alias tables?

• Shorter to type.
• Required when joining the same table twice.
• Disambiguates identical column names (e.id vs d.id).

Joining multiple tables

You can chain JOINs. Each adds one more table:

SELECT e.first_name, d.name AS dept, j.title AS job
FROM employees  e
JOIN departments d ON d.id  = e.department_id
JOIN jobs        j ON j.id  = e.job_id;

Result size

If table A has 100 rows and table B has 50 rows, the JOIN returns at most 100 rows (one per employee). It can return more if the ON condition matches multiple rows in B (a one-to-many match multiplies rows).

Tips

• JOIN and INNER JOIN are identical — both exclude non-matching rows.
• Always specify ON — a JOIN without ON is a CROSS JOIN (cartesian product).
• If a column name is unique across joined tables, you can omit the table prefix.