FULL OUTER JOIN in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a FULL OUTER JOIN do in SQL?

It returns all rows from both tables, matching rows where possible. If there is no match, it fills with NULLs for missing columns.

beginner

In a FULL OUTER JOIN, what happens when a row in the left table has no matching row in the right table?

The row from the left table appears in the result with NULLs in the columns from the right table.

intermediate

Write a simple FULL OUTER JOIN query to combine tables employees and departments on department_id.

SELECT * FROM employees FULL OUTER JOIN departments ON employees.department_id = departments.department_id;

intermediate

How is FULL OUTER JOIN different from LEFT JOIN and RIGHT JOIN?

LEFT JOIN returns all rows from the left table and matched rows from the right. RIGHT JOIN returns all rows from the right table and matched rows from the left. FULL OUTER JOIN returns all rows from both tables, matched or not.

beginner

Why might FULL OUTER JOIN be useful in real life?

It helps find all records from two lists, including those that don't match, like finding all customers and all orders even if some customers have no orders or some orders have no customers.

What does FULL OUTER JOIN return?

A All rows from both tables, matched where possible, NULLs where no match

B Only rows that match in both tables

C All rows from the left table only

D All rows from the right table only

FULL OUTER JOIN returns all rows from both tables, matching rows where possible, and NULLs for missing matches.

If a row in the right table has no match in the left table, what does FULL OUTER JOIN do?

A Excludes the row

B Includes the row with NULLs for right table columns

C Only includes if left table has a match

D Includes the row with NULLs for left table columns

FULL OUTER JOIN includes unmatched rows from both tables, filling NULLs for missing side columns.

Which SQL keyword is used to perform a FULL OUTER JOIN?

A FULL OUTER JOIN

B INNER JOIN

C LEFT JOIN

D RIGHT JOIN

The keyword FULL OUTER JOIN explicitly performs a full outer join.

What will be the result of a FULL OUTER JOIN if both tables have no matching rows?

A Empty result

B All rows from both tables with NULLs in unmatched columns

C Only rows from the left table

D Only rows from the right table

FULL OUTER JOIN returns all rows from both tables, even if no matches exist, filling NULLs accordingly.

Which join type would you use to find all records from two tables including unmatched rows?

A LEFT JOIN

B INNER JOIN

C FULL OUTER JOIN

D CROSS JOIN

FULL OUTER JOIN returns all records from both tables including unmatched rows.

Explain in your own words what a FULL OUTER JOIN does and when you might use it.

Think about combining two lists fully, even if some items don't match.

Write a simple SQL query using FULL OUTER JOIN to combine two tables on a common column.

Use SELECT * FROM table1 FULL OUTER JOIN table2 ON table1.col = table2.col;