Full outer Join

Definition

A Full Outer Join returns all rows from both tables involved in the join. Where a match exists, the joined columns are combined; where no match exists on either side, the missing side's columns appear as NULL.

SELECT * FROM a FULL OUTER JOIN b ON a.id = b.a_id;

When to Use It

Useful for reconciliation-style checks — for example, finding all customers and all orders together, including customers with no orders and orders with no matching customer, which often points to a data integrity problem.

Why Testers Use It

A full outer join is a fast way to spot orphaned records on either side of a relationship during data validation.