JOIN (SQL)

Also called: INNER JOIN, LEFT JOIN

Combining rows from two tables that belong together, for example each order with its customer.

Definition

An INNER JOIN keeps only rows that have a match in both tables. A LEFT JOIN keeps every row from the left table and fills the missing side with NULL, which is how testers find records that are missing their partner, such as orders without items.

In practice

orders LEFT JOIN order_items, keeping rows where the item side is NULL, lists every order that has no items.

Learn it in context

Lessons that use this term.

SQL for TestersTutorials

Related terms

SQLThe language used to read and change data in relational databases, like SELECT * FROM orders.
Foreign keyA column that points to the primary key of a row in another table, like orders.user_id → users.id.
NULLA database marker for "no value" or "unknown". It's not zero and not an empty string.