RIGHT JOIN in MySQL - Step-by-Step Execution

Concept Flow - RIGHT JOIN

Start with LEFT table

↓

Match rows in RIGHT table

↓

Include all rows from RIGHT table

↓

If no match, LEFT columns NULL

↓

Combine rows into result

↓

Output final joined table

RIGHT JOIN returns all rows from the right table and matched rows from the left table. If no match, left side is NULL.

Execution Sample

MySQL

SELECT A.id, A.name, B.city
FROM A RIGHT JOIN B ON A.id = B.a_id;

This query returns all rows from table B and matching rows from table A based on id and a_id.

Execution Table

Step	Action	Left Table Row	Right Table Row	Match Condition	Result Row
1	Check B row with a_id=1	A.id=1, name='Ann'	B.a_id=1, city='NY'	1=1 (True)	id=1, name='Ann', city='NY'
2	Check B row with a_id=2	A.id=2, name='Bob'	B.a_id=2, city='LA'	2=2 (True)	id=2, name='Bob', city='LA'
3	Check B row with a_id=4	No matching A row	B.a_id=4, city='SF'	NULL=4 (False)	id=NULL, name=NULL, city='SF'
4	No more B rows	N/A	N/A	N/A	END

💡 All rows from right table B processed; unmatched left rows set to NULL.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
Current B row	None	a_id=1, city='NY'	a_id=2, city='LA'	a_id=4, city='SF'	None
Current A row	None	id=1, name='Ann'	id=2, name='Bob'	No match (NULL)	None
Result row	None	id=1, name='Ann', city='NY'	id=2, name='Bob', city='LA'	id=NULL, name=NULL, city='SF'	None

Key Moments - 2 Insights

Why do some columns from the left table show NULL in the result?

Because RIGHT JOIN includes all rows from the right table even if there is no matching row in the left table, so left table columns are NULL for unmatched rows (see execution_table row 3).

Does RIGHT JOIN include all rows from the left table?

No, RIGHT JOIN includes all rows from the right table only. Left table rows appear only if they match (see execution_table rows 1 and 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the result row at step 3?

A id=4, name='Dan', city='SF'

B id=NULL, name=NULL, city='SF'

C id=3, name='Cara', city=NULL

D No result row

💡 Hint

Check execution_table row 3 under 'Result Row' column.

At which step does the join find no matching left table row?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'No matching A row' in execution_table under 'Left Table Row'.

If we changed RIGHT JOIN to LEFT JOIN, which table's rows would all appear?

A All rows from table A (left table)

B All rows from table B (right table)

C Only matching rows from both tables

D No rows at all

💡 Hint

RIGHT JOIN includes all right table rows; LEFT JOIN includes all left table rows.

Concept Snapshot

RIGHT JOIN syntax:
SELECT columns
FROM left_table RIGHT JOIN right_table
ON join_condition;

Returns all rows from right_table,
matching rows from left_table,
NULLs if no match on left side.

Full Transcript

RIGHT JOIN in SQL returns all rows from the right table and matching rows from the left table. If a row in the right table has no matching row in the left table, the left table columns are filled with NULL. The execution flow starts by scanning each row in the right table, then tries to find matching rows in the left table based on the join condition. Matched rows are combined into the result. Unmatched right table rows still appear with NULLs for left table columns. This is different from LEFT JOIN, which returns all rows from the left table instead. The example query selects id and name from table A and city from table B, joining on A.id = B.a_id. The execution table shows step-by-step how each row from B is matched or not matched with A, and how the result rows are formed.

Practice

1. What does a RIGHT JOIN do in SQL?

easy

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

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

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

D. Returns all rows from both tables, matching where possible.

Solution

1. Step 1: Understand the direction of RIGHT JOIN

   A RIGHT JOIN returns all rows from the right table regardless of matches in the left table.

2. Step 2: Identify matching rows from the left table

   It includes matching rows from the left table and fills NULL where no match exists.

3. Final Answer:

   Returns all rows from the right table and matching rows from the left table. -> Option A

4. Quick Check:

   RIGHT JOIN = all right table rows + matched left rows [OK]

Hint: RIGHT JOIN keeps all right table rows, left table may have NULLs [OK]

Common Mistakes:

• Confusing RIGHT JOIN with LEFT JOIN
• Thinking RIGHT JOIN returns only matching rows
• Assuming RIGHT JOIN returns all rows from left table

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

easy

A. SELECT * FROM table1 RIGHT OUTER JOIN table2 WHERE table1.id = table2.id;

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

C. SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id;

D. SELECT * FROM table1 RIGHT JOIN table2 USING id;

Solution

1. Step 1: Check standard RIGHT JOIN syntax

   The correct syntax is SELECT ... FROM table1 RIGHT JOIN table2 ON condition;.

2. Step 2: Validate each option

   SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id; uses correct syntax with ON clause. SELECT * FROM table1 JOIN RIGHT table2 ON table1.id = table2.id; has wrong keyword order. SELECT * FROM table1 RIGHT OUTER JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON. SELECT * FROM table1 RIGHT JOIN table2 USING id; uses invalid syntax (parentheses required: USING (id)).

3. Final Answer:

   SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id; -> Option C

4. Quick Check:

   RIGHT JOIN syntax = FROM table1 RIGHT JOIN table2 ON condition [OK]

Hint: RIGHT JOIN needs ON clause, USING only if columns match exactly [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Incorrect keyword order like JOIN RIGHT
• Confusing RIGHT JOIN with RIGHT OUTER JOIN (both valid but syntax matters)

3. Given these tables:

Employees
id | name
1 | Alice
2 | Bob

Departments
id | dept_name
2 | Sales
3 | Marketing

What is the result of:
SELECT Employees.name, Departments.dept_name FROM Employees RIGHT JOIN Departments ON Employees.id = Departments.id;

medium

A. [('Bob', 'Sales')]

B. [('Alice', NULL), ('Bob', 'Sales')]

C. [('Alice', NULL), ('Bob', 'Sales'), (NULL, 'Marketing')]

D. [('Bob', 'Sales'), (NULL, 'Marketing')]

Solution

1. Step 1: Identify RIGHT JOIN behavior with given tables

   RIGHT JOIN keeps all rows from Departments (right table). Departments have ids 2 and 3.

2. Step 2: Match Employees.id to Departments.id

   Employees with id=2 is Bob, matches Sales. No employee with id=3, so NULL for name and Marketing for dept_name.

3. Final Answer:

   [('Bob', 'Sales'), (NULL, 'Marketing')] -> Option D

4. Quick Check:

   RIGHT JOIN keeps all right rows, unmatched left = NULL [OK]

Hint: RIGHT JOIN shows all right rows, NULL for unmatched left [OK]

Common Mistakes:

• Expecting all employees instead of all departments
• Including unmatched left rows only
• Confusing LEFT JOIN with RIGHT JOIN results

4. Consider this query:

SELECT a.id, b.value FROM tableA a RIGHT JOIN tableB b ON a.id = b.id;

It returns no rows even though tableB has data. What is the most likely cause?

medium

A. The join condition is incorrect or mismatched column names.

B. RIGHT JOIN does not return any rows if left table is empty.

C. RIGHT JOIN requires WHERE clause to filter rows.

D. The query syntax is invalid and causes an error.

Solution

1. Step 1: Understand RIGHT JOIN behavior

   RIGHT JOIN returns all rows from right table even if left table has no matching rows.

2. Step 2: Analyze why no rows returned

   If no rows appear, likely the join condition a.id = b.id does not match any rows due to wrong column names or data mismatch.

3. Final Answer:

   The join condition is incorrect or mismatched column names. -> Option A

4. Quick Check:

   No rows means join condition mismatch [OK]

Hint: No rows? Check join condition and column names first [OK]

Common Mistakes:

• Assuming RIGHT JOIN needs WHERE clause
• Thinking RIGHT JOIN returns no rows if left table empty
• Believing syntax error causes no rows instead of error

5. You have two tables:

Orders
order_id | customer_id
1 | 101
2 | 102

Customers
customer_id | name
101 | John
103 | Alice

You want to list all customers and their orders if any. Which query using RIGHT JOIN gives the correct result?

hard

A. SELECT Customers.name, Orders.order_id FROM Customers RIGHT JOIN Orders ON Customers.customer_id = Orders.customer_id;

B. SELECT Customers.name, Orders.order_id FROM Orders RIGHT JOIN Customers ON Orders.customer_id = Customers.customer_id;

C. SELECT Customers.name, Orders.order_id FROM Orders LEFT JOIN Customers ON Orders.customer_id = Customers.customer_id;

D. SELECT Customers.name, Orders.order_id FROM Customers LEFT JOIN Orders ON Customers.customer_id = Orders.customer_id;

Solution

1. Step 1: Identify which table is right to keep all customers

   We want all customers, so Customers must be the right table in RIGHT JOIN.

2. Step 2: Check join condition and table order

   SELECT Customers.name, Orders.order_id FROM Orders RIGHT JOIN Customers ON Orders.customer_id = Customers.customer_id; uses Orders RIGHT JOIN Customers ON matching customer_id, so all Customers appear with matching Orders or NULL.

3. Final Answer:

   SELECT Customers.name, Orders.order_id FROM Orders RIGHT JOIN Customers ON Orders.customer_id = Customers.customer_id; -> Option B

4. Quick Check:

   RIGHT JOIN keeps all right table rows (Customers) [OK]

Hint: Put table with all rows as right table in RIGHT JOIN [OK]

Common Mistakes:

• Swapping table order and losing all customers
• Using LEFT JOIN instead of RIGHT JOIN incorrectly
• Confusing join condition direction