LEFT JOIN preserving all left rows in SQL - Cheat Sheet & Quick Revision

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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.

beginner

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

The query still returns the row from the left table, but the columns from the right table will have NULL values.

beginner

Write a simple SQL 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;

intermediate

Why is LEFT JOIN useful when you want to preserve all rows from the left table?

Because it ensures no rows from the left table are lost, even if there are no matching rows in the right table. This helps to keep all original data from the left side.

beginner

What is the difference between INNER JOIN and LEFT JOIN?

INNER JOIN returns only rows with matching keys in both tables. LEFT JOIN returns all rows from the left table, with matching rows from the right or NULL if no 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 at all

C Only rows with matches in both tables

D Only rows from the right table

LEFT JOIN keeps all rows from the left table and fills right table columns with NULL if no match.

Which SQL keyword is used to keep all rows from the left table regardless of matches?

A LEFT JOIN

B RIGHT JOIN

C FULL JOIN

D INNER JOIN

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

If you want to find customers without orders, which join helps you?

A SELF JOIN

B INNER JOIN

C CROSS JOIN

D LEFT JOIN

LEFT JOIN includes all customers, even those without orders, showing NULL for missing orders.

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

A There is a matching row

B The right table is empty

C No matching row in the right table

D The left table has NULL values

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

Which join type would exclude rows from the left table if no match is found?

A LEFT JOIN

B INNER JOIN

C RIGHT JOIN

D FULL JOIN

INNER JOIN returns only rows with matches in both tables, excluding unmatched left rows.

Explain how a LEFT JOIN preserves all rows from the left table and what happens when there is no match in the right table.

Think about what happens to unmatched rows on the right side.

Write a simple SQL query using LEFT JOIN to list all employees and their departments, including employees without a department.

Use employee table as left and department table as right.