Multiple table JOINs in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a JOIN in SQL?

A JOIN is a way to combine rows from two or more tables based on a related column between them.

beginner

What does INNER JOIN do when joining multiple tables?

INNER JOIN returns only the rows where there is a match in all joined tables based on the join condition.

intermediate

How do you join three tables named A, B, and C using INNER JOIN?

You write: SELECT * FROM A INNER JOIN B ON A.key = B.key INNER JOIN C ON B.key = C.key;

intermediate

What is the difference between INNER JOIN and LEFT JOIN when joining multiple tables?

INNER JOIN returns only matching rows in all tables; LEFT JOIN returns all rows from the left table and matching rows from the right table, filling with NULLs if no match.

beginner

Why is it important to specify join conditions when joining multiple tables?

Without join conditions, the query creates a Cartesian product, combining every row of one table with every row of the other, which is usually not desired.

Which JOIN type returns only rows with matching keys in all joined tables?

A FULL OUTER JOIN

B LEFT JOIN

C RIGHT JOIN

D INNER JOIN

INNER JOIN returns only rows where keys match in all tables.

What happens if you join tables without specifying ON conditions?

A You get a Cartesian product (all combinations)

B You get only matching rows

C The query returns an error

D The database automatically guesses the join condition

Without ON conditions, SQL combines every row of one table with every row of the other.

How do you join three tables A, B, and C correctly?

A SELECT * FROM A INNER JOIN B ON A.id = B.id INNER JOIN C ON B.id = C.id;

B SELECT * FROM A, B, C;

C SELECT * FROM A LEFT JOIN B, C;

D SELECT * FROM A JOIN B JOIN C;

You must specify join conditions for each join to combine tables properly.

Which JOIN type includes all rows from the left table even if there is no match in the right table?

A INNER JOIN

B RIGHT JOIN

C LEFT JOIN

D FULL OUTER JOIN

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

When joining multiple tables, what is the best practice for readability?

A Use commas to separate tables

B Use explicit JOIN syntax with ON conditions

C Join tables without conditions

D Use SELECT * without specifying tables

Explicit JOIN with ON conditions makes queries clear and avoids errors.

Explain how to join three tables using INNER JOIN and why join conditions are important.

Think about how tables connect through keys.

Describe the difference between INNER JOIN and LEFT JOIN when joining multiple tables.

Consider what happens when there is no match.