LEFT JOIN in MySQL - 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 NULL for columns from the right table.

beginner

Write the basic syntax of a LEFT JOIN.

SELECT columns FROM left_table LEFT JOIN right_table ON left_table.key = right_table.key;

intermediate

Why might you use a LEFT JOIN instead of an INNER JOIN?

Use LEFT JOIN when you want to keep all records from the left table, even if there are no matching records in the right table. INNER JOIN only returns rows with matches in both tables.

beginner

In a LEFT JOIN, what happens if the right table has no matching row?

The columns from the right table will contain NULL values for that row in the result.

intermediate

Can a LEFT JOIN return more rows than the left table originally has? Why or why not?

Yes, if a single row in the left table matches multiple rows in the right table (one-to-many relationship). A LEFT JOIN always returns at least as many rows as the left table, since every left row is included at least once.

What does a LEFT JOIN include in its result?

A Only rows from the right table

B Only rows that match in both tables

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

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

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

If a row in the left table has no match in the right table, what will the right table columns show?

A NULL

B Zero

C Empty string

D Error

When no match exists, the right table columns show NULL in the result.

Which SQL keyword is used to perform a LEFT JOIN?

A INNER JOIN

B LEFT JOIN

C RIGHT JOIN

D FULL JOIN

The keyword LEFT JOIN is used to perform a left join operation.

Can a LEFT JOIN result have fewer rows than the left table?

A Yes, if the right table has no matches

B No, it always has more rows

C No, it always has the same or more rows

D Yes, if the left table is empty

LEFT JOIN returns all rows from the left table (at least once), so the result has the same number or more rows (if multiple matches in right table).

Which join would you use to keep all rows from the left table regardless of matches?

A LEFT JOIN

B INNER JOIN

C RIGHT JOIN

D CROSS JOIN

LEFT JOIN keeps all rows from the left table, matching rows from the right or NULL if no match.

Explain in your own words what a LEFT JOIN does and when you might use it.

Think about keeping all your main list and adding details if available.

Describe the difference between LEFT JOIN and INNER JOIN with an example scenario.

Imagine a list of customers and their orders.

Practice

1. What does a LEFT JOIN do in SQL?

easy

A. Deletes rows from the left table that do not have matches in the right table.

B. Returns only rows that have matching values in both tables.

C. Returns all rows from the right table and matching rows from the left table.

D. Returns all rows from the left table and matching rows from the right table, NULL if no match.

Solution

1. Step 1: Understand LEFT JOIN behavior

   A LEFT JOIN keeps all rows from the left table regardless of matches in the right table.

2. Step 2: Check what happens with unmatched rows

   If no matching row exists in the right table, the result shows NULL for right table columns.

3. Final Answer:

   Returns all rows from the left table and matching rows from the right table, NULL if no match. -> Option D

4. Quick Check:

   LEFT JOIN = all left rows + matched right rows [OK]

Hint: LEFT JOIN keeps all left rows, fills right with NULL if no match [OK]

Common Mistakes:

• Confusing LEFT JOIN with INNER JOIN
• Thinking it returns all right table rows
• Assuming unmatched rows are dropped

2. Which of the following is the correct syntax for a LEFT JOIN in MySQL?

easy

A. SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

B. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id;

C. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id;

D. SELECT * FROM table1 LEFT JOIN table2 WHERE table1.id = table2.id;

Solution

1. Step 1: Review standard LEFT JOIN syntax

   The correct syntax uses LEFT JOIN followed by ON condition to specify matching columns.

2. Step 2: Check each option

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; uses correct syntax: LEFT JOIN with ON clause. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id; has JOIN LEFT which is invalid. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON, which is incorrect for JOIN condition. SELECT * FROM table1 LEFT JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON, invalid syntax.

3. Final Answer:

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; -> Option A

4. Quick Check:

   LEFT JOIN syntax = LEFT JOIN ... ON ... [OK]

Hint: Use LEFT JOIN ... ON ... for correct syntax [OK]

Common Mistakes:

• Using JOIN LEFT instead of LEFT JOIN
• Using WHERE instead of ON for join condition
• Putting join condition directly in WHERE after table

3. Given these tables:

Employees(id, name)
Departments(id, dept_name, emp_id)

What is the output of this query?

SELECT e.name, d.dept_name FROM Employees e LEFT JOIN Departments d ON e.id = d.emp_id ORDER BY e.id;

Assuming:
Employees: (1, 'Alice'), (2, 'Bob'), (3, 'Carol')
Departments: (10, 'HR', 1), (20, 'IT', 3)

medium

A. [('Alice', NULL), ('Bob', NULL), ('Carol', NULL)]

B. [('Alice', 'HR'), ('Bob', 'IT'), ('Carol', NULL)]

C. [('Alice', 'HR'), ('Bob', NULL), ('Carol', 'IT')]

D. [('Alice', 'HR'), ('Carol', 'IT')]

Solution

1. Step 1: Match Employees with Departments using LEFT JOIN

   LEFT JOIN keeps all Employees. For each employee, it tries to find a matching department where e.id = d.emp_id.

2. Step 2: Check matches and NULLs

   'Alice' (id=1) matches department 'HR' (emp_id=1). 'Bob' (id=2) has no matching department, so dept_name is NULL. 'Carol' (id=3) matches 'IT' (emp_id=3).

3. Final Answer:

   [('Alice', 'HR'), ('Bob', NULL), ('Carol', 'IT')] -> Option C

4. Quick Check:

   LEFT JOIN keeps all left rows, unmatched right columns NULL [OK]

Hint: LEFT JOIN shows all left rows; unmatched right columns are NULL [OK]

Common Mistakes:

• Assuming unmatched rows are dropped
• Mixing up emp_id and id columns
• Ignoring ORDER BY effect

4. Identify the error in this LEFT JOIN query:

SELECT a.id, b.value FROM tableA a LEFT JOIN tableB b ON a.id = b.id WHERE b.value > 10;

medium

A. The WHERE clause filters out rows where b.value is NULL, defeating LEFT JOIN purpose.

B. The ON clause is missing a join condition.

C. LEFT JOIN should be replaced with INNER JOIN for this query.

D. The SELECT statement is missing table aliases.

Solution

1. Step 1: Understand effect of WHERE on LEFT JOIN

   LEFT JOIN keeps all rows from tableA, but WHERE b.value > 10 excludes rows where b.value is NULL (no match), effectively turning it into INNER JOIN.

2. Step 2: Identify the error

   The error is using WHERE to filter on right table columns after LEFT JOIN, which removes unmatched rows.

3. Final Answer:

   The WHERE clause filters out rows where b.value is NULL, defeating LEFT JOIN purpose. -> Option A

4. Quick Check:

   WHERE on right table column after LEFT JOIN removes unmatched rows [OK]

Hint: Use ON for filtering right table, not WHERE, to keep unmatched rows [OK]

Common Mistakes:

• Using WHERE instead of ON for right table filters
• Confusing LEFT JOIN with INNER JOIN
• Ignoring NULLs in join columns

5. You have two tables:

Products(product_id, name)
Sales(sale_id, product_id, quantity)

You want to list all products with total quantity sold, including products with zero sales.
Which query correctly uses LEFT JOIN to achieve this?

hard

A. SELECT p.name, SUM(s.quantity) FROM Products p INNER JOIN Sales s ON p.product_id = s.product_id GROUP BY p.product_id;

B. SELECT p.name, SUM(s.quantity) FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id GROUP BY p.product_id;

C. SELECT p.name, SUM(s.quantity) FROM Sales s LEFT JOIN Products p ON p.product_id = s.product_id GROUP BY p.product_id;

D. SELECT p.name, COUNT(s.sale_id) FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id WHERE s.quantity > 0 GROUP BY p.product_id;

Solution

1. Step 1: Use LEFT JOIN from Products to Sales

   To include all products even with no sales, LEFT JOIN Sales to Products on product_id.

2. Step 2: Aggregate sales quantity and group by product

   Use SUM(s.quantity) to total sales per product. Group by p.product_id to get one row per product.

3. Final Answer:

   SELECT p.name, SUM(s.quantity) FROM Products p LEFT JOIN Sales s ON p.product_id = s.product_id GROUP BY p.product_id; -> Option B

4. Quick Check:

   LEFT JOIN + SUM + GROUP BY lists all products with total sales [OK]

Hint: LEFT JOIN products to sales, SUM quantity, GROUP BY product [OK]

Common Mistakes:

• Using INNER JOIN excludes products with zero sales
• Joining Sales to Products reverses LEFT JOIN logic
• Filtering in WHERE removes unmatched rows