Multiple LEFT JOINs in one query 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 matched rows from the right table. If there is no match, the result is NULL on the right side.

intermediate

How do multiple LEFT JOINs work in one SQL query?

Multiple LEFT JOINs add more tables to the query, joining each one to the previous result. Each join keeps all rows from the left side, adding matching rows or NULLs from the right side.

beginner

Write a simple SQL query using two LEFT JOINs.

SELECT a.id, b.name, c.status FROM tableA a LEFT JOIN tableB b ON a.id = b.a_id LEFT JOIN tableC c ON a.id = c.a_id;

beginner

Why might you get NULL values in columns from tables joined with LEFT JOIN?

Because LEFT JOIN keeps all rows from the left table, if there is no matching row in the right table, the columns from the right table will show NULL.

intermediate

What is the difference between INNER JOIN and multiple LEFT JOINs?

INNER JOIN returns only rows with matches in both tables. Multiple LEFT JOINs return all rows from the left table and matching rows or NULLs from each joined table.

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

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

B Only matching rows from both tables

C All rows from the right table

D No rows

A LEFT JOIN returns all rows from the left table and fills NULLs for columns from the right table when there is no match.

In a query with multiple LEFT JOINs, what happens if the second joined table has no matching rows?

A An error occurs

B The entire query returns no rows

C Only rows with matches in the second table appear

D Rows from the left table still appear with NULLs for the second joined table

Multiple LEFT JOINs keep all rows from the left table, so unmatched rows in any joined table show NULLs but do not remove rows.

Which SQL keyword is used to join tables and keep all rows from the left table?

A INNER JOIN

B RIGHT JOIN

C LEFT JOIN

D FULL JOIN

LEFT JOIN keeps all rows from the left table and adds matching rows or NULLs from the right table.

If you want to join three tables and keep all rows from the first table, which join type should you use?

A LEFT JOIN for all

B INNER JOIN for all

C RIGHT JOIN for all

D FULL JOIN for all

Using LEFT JOIN for all joins keeps all rows from the first (leftmost) table.

What does the ON clause specify in a LEFT JOIN?

A The columns to select

B The condition to match rows between tables

C The order of rows

D The filter for the left table only

The ON clause defines how rows from the left and right tables match for the join.

Explain how multiple LEFT JOINs work in a SQL query and why they might be useful.

Think about combining data from several tables while keeping all main records.

Describe the difference between INNER JOIN and multiple LEFT JOINs in terms of returned rows.

Focus on how unmatched rows are handled.