LEFT JOIN with NULL result rows in SQL - 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 shows NULL for columns from the right table.

beginner

Why do some columns show NULL values after a LEFT JOIN?

Columns from the right table show NULL when there is no matching row for the left table's row in the right table.

beginner

Write a simple LEFT JOIN query to get all customers and their orders, including customers with no orders.

SELECT customers.id, customers.name, orders.id AS order_id FROM customers LEFT JOIN orders ON customers.id = orders.customer_id;

beginner

What happens if you use INNER JOIN instead of LEFT JOIN in the same query?

INNER JOIN returns only rows where there is a match in both tables. Customers without orders would be excluded.

intermediate

How can you find rows from the left table that have no matching rows in the right table using LEFT JOIN?

Use a LEFT JOIN and then filter with WHERE right_table.column IS NULL to find unmatched rows.

What does a LEFT JOIN return when there is no matching row in the right table?

A An error

B No rows at all

C Only rows from the right table

D Rows with NULLs in right table columns

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

Which SQL clause filters rows where the right table has no match after a LEFT JOIN?

A WHERE right_table.column IS NULL

B WHERE left_table.column IS NULL

C WHERE right_table.column = 0

D WHERE left_table.column = 0

Filtering with WHERE right_table.column IS NULL finds rows with no matching right table data.

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

A INNER JOIN

B LEFT JOIN

C RIGHT JOIN

D FULL JOIN

LEFT JOIN returns all rows from the left table, matching or not.

What will happen if you replace LEFT JOIN with INNER JOIN in a query?

A Only rows with matches in both tables are returned

B All rows from the left table are returned

C All rows from the right table are returned

D No rows are returned

INNER JOIN returns only rows where both tables have matching data.

In a LEFT JOIN, what does a NULL in the right table columns indicate?

A Matching row with NULL values

B Data error

C No matching row in the right table

D Left table row is missing

NULL in right table columns means no matching row was found for that left table row.

Explain how a LEFT JOIN works and why some columns might show NULL values in the result.

Think about what happens when the right table has no matching data.

Describe how to find rows in the left table that have no matching rows in the right table using a LEFT JOIN.

Filtering NULLs in right table columns helps find unmatched rows.