LEFT JOIN in MySQL - Step-by-Step Execution

Concept Flow - LEFT JOIN

Start with LEFT table

↓

For each row in LEFT table

↓

Find matching rows in RIGHT table

Yes No↓

Combine

↓

Add to result

↓

Next row

↓

End

LEFT JOIN takes all rows from the left table and matches rows from the right table. If no match, it fills with NULLs.

Execution Sample

MySQL

SELECT A.id, A.name, B.score
FROM Students A
LEFT JOIN Scores B ON A.id = B.student_id;

This query lists all students with their scores if available; if no score, shows NULL.

Execution Table

Step	Current LEFT row (A.id, A.name)	Matching RIGHT rows found?	Action	Output row
1	(1, 'Alice')	Yes (score=85)	Combine A and B row	(1, 'Alice', 85)
2	(2, 'Bob')	No	Combine A row with NULLs	(2, 'Bob', NULL)
3	(3, 'Charlie')	Yes (score=92)	Combine A and B row	(3, 'Charlie', 92)
4	(4, 'Diana')	No	Combine A row with NULLs	(4, 'Diana', NULL)
End	No more rows in LEFT table	-	-	Query complete

💡 All rows from LEFT table processed; unmatched RIGHT rows ignored.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Current LEFT row	None	(1, 'Alice')	(2, 'Bob')	(3, 'Charlie')	(4, 'Diana')	None
Matching RIGHT rows	None	Yes (score=85)	No	Yes (score=92)	No	None
Output rows count	0	1	2	3	4	4

Key Moments - 2 Insights

Why do some output rows have NULL values?

When no matching row is found in the RIGHT table for a LEFT row, LEFT JOIN fills those columns with NULLs, as shown in steps 2 and 4 in the execution_table.

Are rows from the RIGHT table without matches included?

No, LEFT JOIN includes all LEFT table rows but excludes RIGHT table rows without matches, as seen in the exit_note and no extra rows beyond LEFT table count.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output row at step 3?

A (3, 'Charlie', NULL)

B (3, 'Charlie', 92)

C (3, 'Charlie', 85)

D (3, 'Charlie', 0)

💡 Hint

Check the 'Output row' column in row with Step '3' in execution_table.

At which step does the LEFT JOIN fill NULLs for the RIGHT table columns?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at 'Matching RIGHT rows found?' and 'Output row' columns for NULL values in execution_table.

If a new LEFT row has no matching RIGHT row, what happens to the output?

A It appears with NULLs for RIGHT table columns

B It is excluded from the result

C It causes an error

D It duplicates the last matched RIGHT row

💡 Hint

Refer to steps 2 and 4 in execution_table where no matches lead to NULLs.

Concept Snapshot

LEFT JOIN syntax:
SELECT columns
FROM left_table
LEFT JOIN right_table ON condition;

Behavior:
- Includes all rows from left_table
- Matches rows from right_table
- Fills NULLs if no match
- Ignores unmatched right_table rows

Full Transcript

LEFT JOIN takes every row from the left table and tries to find matching rows in the right table using the join condition. For each left row, if a matching right row exists, it combines them into one output row. If no match is found, it still outputs the left row but fills the right table columns with NULL values. Rows in the right table without matches are not included in the result. This ensures all left table data is preserved, with right table data added when available.

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