Correlated subqueries in MySQL - Step-by-Step Execution

Concept Flow - Correlated subqueries

Start Outer Query Row

↓

Execute Subquery Using Outer Row Value

↓

Subquery Returns Result

↓

Use Subquery Result in Outer Query Condition

↓

Decide to Include Outer Row

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Move to Next Outer Query Row or End

For each row in the outer query, the subquery runs using that row's data, then the outer query uses the subquery result to decide if the row qualifies.

Execution Sample

MySQL

SELECT e1.name
FROM employees e1
WHERE e1.salary > (
  SELECT AVG(e2.salary)
  FROM employees e2
  WHERE e2.department = e1.department
);

Find employees whose salary is greater than the average salary in their own department.

Execution Table

Step	Outer Row (e1.name, e1.salary, e1.department)	Subquery Condition (e2.department = e1.department)	Subquery Result (AVG salary)	Outer Condition (e1.salary > AVG)	Include Row in Result
1	(Alice, 7000, Sales)	e2.department = 'Sales'	6000	7000 > 6000 = True	Yes
2	(Bob, 5000, Sales)	e2.department = 'Sales'	6000	5000 > 6000 = False	No
3	(Charlie, 8000, HR)	e2.department = 'HR'	7500	8000 > 7500 = True	Yes
4	(Diana, 7500, HR)	e2.department = 'HR'	7500	7500 > 7500 = False	No
5	(Eve, 9000, IT)	e2.department = 'IT'	8500	9000 > 8500 = True	Yes
6	No more rows	-	-	-	End

💡 All outer rows processed; query ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
e1.name	None	Alice	Bob	Charlie	Diana	Eve	None
e1.salary	None	7000	5000	8000	7500	9000	None
e1.department	None	Sales	Sales	HR	HR	IT	None
Subquery AVG salary	None	6000	6000	7500	7500	8500	None
Include Row?	None	Yes	No	Yes	No	Yes	None

Key Moments - 3 Insights

Why does the subquery run multiple times instead of just once?

Because the subquery depends on the current outer row's department (e1.department), it must run for each outer row to get the correct average salary for that department, as shown in the execution_table rows 1 to 5.

What happens if the subquery returns NULL or no rows?

If the subquery returns NULL, the comparison in the outer query (e1.salary > NULL) evaluates to false, so that outer row is excluded. This is not shown in this example but is important to remember.

How does the outer query decide to include a row?

It compares the outer row's salary to the subquery's average salary result. If the outer salary is greater, the row is included (see execution_table column 'Include Row in Result').

Visual Quiz - 3 Questions

Look at the execution_table, what is the subquery average salary when processing Diana's row?

A 6000

B 7500

C 8500

D None

💡 Hint

Check the 'Subquery Result (AVG salary)' column at step 4 in the execution_table.

At which step does the outer condition become false for the first time?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Outer Condition (e1.salary > AVG)' column in the execution_table and find the first 'False'.

If Bob's salary was 6500 instead of 5000, how would the 'Include Row in Result' change at step 2?

A Change to Yes

B No change, still No

C Change to NULL

D Query would error

💡 Hint

Compare Bob's new salary 6500 to the subquery average 6000 at step 2 in the execution_table.

Concept Snapshot

Correlated subqueries run once per outer row using that row's data.
Syntax: Outer query references outer row in subquery condition.
Subquery result affects outer row filtering or selection.
Useful for comparisons within groups or related data.
Runs slower than simple subqueries because of repetition.

Full Transcript

Correlated subqueries execute the subquery for each row of the outer query using that row's values. For example, to find employees earning more than their department's average salary, the subquery calculates the average salary for the current employee's department. The outer query then compares the employee's salary to this average. This process repeats for each employee row. The execution table shows each step: the outer row, the subquery condition using the outer row's department, the average salary result, the comparison, and whether the row is included. Variables like e1.name, e1.salary, and e1.department change as we move through rows. Key points include understanding why the subquery runs multiple times and how the outer query uses the subquery result to filter rows. Visual quiz questions help reinforce these ideas by referencing specific steps and values in the execution table.