LEFT JOIN and RIGHT JOIN in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a LEFT JOIN do in SQL?

A LEFT JOIN returns all rows from the left table and the matching rows from the right table. If there is no match, the result will contain NULL for columns from the right table.

beginner

What is the difference between LEFT JOIN and RIGHT JOIN?

LEFT JOIN returns all rows from the left table and matching rows from the right table. RIGHT JOIN returns all rows from the right table and matching rows from the left table.

beginner

In a RIGHT JOIN, what happens if there is no matching row in the left table?

If there is no matching row in the left table, the result will still include the row from the right table, but columns from the left table will be NULL.

intermediate

Write a simple SQL query using LEFT JOIN to combine two tables: employees and departments.

SELECT employees.name, departments.department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;

intermediate

Why might you use a LEFT JOIN instead of an INNER JOIN?

You use LEFT JOIN when you want to keep all rows from the left table even if there is no matching row in the right table. INNER JOIN only returns rows with matches in both tables.

What will a LEFT JOIN return if there is no matching row in the right table?

A All rows from the right table

B No rows

C Only rows with matching keys

D All rows from the left table with NULLs for right table columns

LEFT JOIN returns all rows from the left table and fills NULL for right table columns if no match exists.

Which JOIN returns all rows from the right table and matching rows from the left table?

A LEFT JOIN

B RIGHT JOIN

C INNER JOIN

D FULL JOIN

RIGHT JOIN returns all rows from the right table and matching rows from the left table.

If you want to keep all employees even if they don't belong to a department, which JOIN should you use?

A LEFT JOIN

B RIGHT JOIN

C INNER JOIN

D CROSS JOIN

LEFT JOIN keeps all rows from the left table (employees) even if no matching department exists.

What will happen if you use RIGHT JOIN but the right table has no rows?

A Result will be empty

B All rows from left table returned

C All rows from right table returned

D Error occurs

RIGHT JOIN returns all rows from the right table; if right table is empty, result is empty.

Which JOIN type can be thought of as the opposite of LEFT JOIN?

A INNER JOIN

B FULL JOIN

C RIGHT JOIN

D CROSS JOIN

RIGHT JOIN is the opposite of LEFT JOIN, returning all rows from the right table.

Explain in your own words how LEFT JOIN works and when you would use it.

Think about keeping all your friends in a list even if some don't have phone numbers.

Describe the difference between LEFT JOIN and RIGHT JOIN with an example.

Imagine two lists: one is your friends, the other is their phone numbers.