LEFT JOIN vs RIGHT JOIN decision in SQL - Quick Revision & Key Differences

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 is NULL on the right side.

beginner

What does a RIGHT JOIN do in SQL?

A RIGHT JOIN returns all rows from the right table and the matching rows from the left table. If there is no match, the result is NULL on the left side.

intermediate

When should you use LEFT JOIN instead of RIGHT JOIN?

Use LEFT JOIN when you want to keep all rows from the first (left) table and add matching data from the second (right) table. It is often easier to read and understand.

intermediate

Can a RIGHT JOIN be rewritten as a LEFT JOIN?

Yes. A RIGHT JOIN can be rewritten as a LEFT JOIN by swapping the order of the tables. This helps keep queries consistent and easier to read.

beginner

Why might developers prefer LEFT JOIN over RIGHT JOIN?

Developers often prefer LEFT JOIN because it reads naturally from left to right, matching the order of tables in the query. This improves clarity and reduces confusion.

What does a LEFT JOIN return?

A All rows from both tables

B All rows from the left table and matching rows from the right table

C Only matching rows from both tables

D All rows from the right table and matching rows from the left table

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

How can you rewrite a RIGHT JOIN as a LEFT JOIN?

A Swap the order of the tables and use LEFT JOIN

B Change RIGHT JOIN to INNER JOIN

C Use FULL JOIN instead

D You cannot rewrite RIGHT JOIN as LEFT JOIN

RIGHT JOIN can be rewritten as LEFT JOIN by swapping the tables' order.

Which join is usually easier to read and understand?

A LEFT JOIN

B RIGHT JOIN

C FULL JOIN

D CROSS JOIN

LEFT JOIN is often easier to read because it follows the natural left-to-right order.

If you want all rows from the right table regardless of matches, which join do you use?

A CROSS JOIN

B LEFT JOIN

C INNER JOIN

D RIGHT JOIN

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

What happens to unmatched rows in the right table when using LEFT JOIN?

A They are included with NULLs for left table columns

B They are included with NULLs for right table columns

C They are excluded

D They cause an error

LEFT JOIN excludes unmatched rows from the right table; only unmatched left table rows appear with NULLs on the right.

Explain the difference between LEFT JOIN and RIGHT JOIN in simple terms.

Think about which table's rows you want to keep fully.

Describe when and why you might choose LEFT JOIN over RIGHT JOIN in a query.

Consider readability and table order.