LEFT JOIN execution behavior 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 will contain NULLs for columns from the right table.

beginner

In a LEFT JOIN, what happens when there is no matching row in the right table?

The query returns the left table's row with NULL values for the right table's columns.

intermediate

How does SQL execute a LEFT JOIN internally?

SQL scans the left table first, then for each row, it looks for matching rows in the right table. If matches exist, it combines them; if not, it fills right table columns with NULL.

beginner

What is the difference between LEFT JOIN and INNER JOIN in terms of output?

LEFT JOIN returns all rows from the left table regardless of matches, while INNER JOIN returns only rows where there is a match in both tables.

intermediate

Can a LEFT JOIN return duplicate rows? Why or why not?

Yes, if the right table has multiple matching rows for a single left table row, the LEFT JOIN will return multiple rows for that left row, one for each match.

What will a LEFT JOIN return if the right table has no matching rows?

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

B No rows

C Only rows with matching keys

D All rows from the right table

LEFT JOIN always returns all rows from the left table, filling NULLs for right table columns when no match exists.

Which table's rows are always fully included in a LEFT JOIN result?

A Both tables equally

B Right table

C Left table

D Neither table

LEFT JOIN guarantees all rows from the left table appear in the result.

If a left table row matches multiple right table rows, how many rows appear in the LEFT JOIN result for that left row?

A One

B Multiple, one per matching right row

C Depends on the database

D Zero

LEFT JOIN returns one row for each matching right table row combined with the left row.

What SQL keyword is used to perform a LEFT JOIN?

A INNER JOIN

B FULL JOIN

C RIGHT JOIN

D LEFT JOIN

The keyword LEFT JOIN explicitly performs a left join operation.

Which of these is true about NULLs in a LEFT JOIN result?

A NULLs appear in right table columns when no match

B NULLs appear in left table columns when no match

C NULLs never appear in LEFT JOIN results

D NULLs appear in both tables always

When no matching right table row exists, the right table columns show NULL.

Explain how a LEFT JOIN works and what its output looks like.

Think about what happens when there is no matching row on the right.

Describe the difference between LEFT JOIN and INNER JOIN in SQL.

Focus on which rows appear in the result.