Subquery in WHERE clause in SQL - Step-by-Step Execution

Concept Flow - Subquery in WHERE clause

Start Query

↓

Evaluate Subquery

↓

Get Subquery Result

↓

Use Result in WHERE Clause

↓

Filter Main Query Rows

↓

Return Filtered Rows

↓

End Query

The main query runs, but first the subquery inside the WHERE clause runs to get values. Then the main query uses those values to filter its rows.

Execution Sample

SQL

SELECT name FROM employees
WHERE department_id IN (
  SELECT id FROM departments WHERE location = 'NY'
);

This query finds employees who work in departments located in NY by using a subquery in the WHERE clause.

Execution Table

Step	Action	Subquery Evaluation	Main Query Filtering	Output Rows
1	Start main query	Not started	No filtering yet	All employees
2	Run subquery	SELECT id FROM departments WHERE location = 'NY'	Waiting for subquery result	No output yet
3	Subquery returns department ids	Result: [2, 5]	Use these ids in WHERE clause	No output yet
4	Filter employees WHERE department_id IN (2,5)	Subquery result used	Filter employees with department_id 2 or 5	Employees in dept 2 or 5
5	Return filtered rows	Subquery done	Filtered employees returned	List of employees in NY departments
6	End query	Complete	Complete	Final result set

💡 Subquery completes and main query filters employees based on subquery result, then returns final rows.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
subquery_result	empty	running	[2,5]	[2,5]	[2,5]
filtered_employees	all employees	all employees	all employees	employees with dept_id 2 or 5	employees with dept_id 2 or 5

Key Moments - 2 Insights

Why does the subquery run before the main query filters rows?

Because the main query needs the subquery result to know which department_ids to filter employees by, as shown in execution_table step 3.

What happens if the subquery returns no rows?

Then the WHERE clause condition becomes false for all rows, so no employees are returned. This is implied after step 3 if subquery_result is empty.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the subquery_result?

A [1, 3]

B [2, 5]

C Empty

D [10, 20]

💡 Hint

Check the 'Subquery Evaluation' column at step 3 in the execution_table.

At which step does the main query start filtering employees?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Main Query Filtering' column to see when filtering begins.

If the subquery returned an empty list, what would be the output rows?

A No employees

B Employees with department_id 0

C All employees

D Employees with department_id NULL

💡 Hint

Refer to key_moments about what happens if subquery returns no rows.

Concept Snapshot

Subquery in WHERE clause:
- Runs subquery first to get values
- Uses those values to filter main query rows
- Syntax: WHERE column IN (subquery)
- Subquery result must be compatible with condition
- If subquery empty, main query returns no rows

Full Transcript

This visual execution shows how a SQL query with a subquery in the WHERE clause runs. First, the subquery runs to find department ids located in NY. Then the main query uses these ids to filter employees who belong to those departments. The execution table traces each step: starting the main query, running the subquery, getting results, filtering employees, and returning the final list. Variables like subquery_result and filtered_employees change as the query progresses. Key moments clarify why the subquery runs first and what happens if it returns no rows. The quiz tests understanding of these steps by asking about subquery results and filtering timing.

Practice

1. What does a subquery in the WHERE clause do in SQL?

easy

A. Creates a new table from existing data

B. Filters rows based on the results of another query

C. Deletes rows from a table

D. Updates values in a table

Solution

1. Step 1: Understand the role of subqueries in WHERE clause

   A subquery inside a WHERE clause is used to filter rows by comparing values to the results of another query.

2. Step 2: Compare with other SQL operations

   Creating tables, deleting, or updating rows are different SQL operations and not the purpose of subqueries in WHERE.

3. Final Answer:

   Filters rows based on the results of another query -> Option B

4. Quick Check:

   Subquery in WHERE = filter rows [OK]

Hint: Subquery in WHERE filters rows using another query's results [OK]

Common Mistakes:

• Thinking subquery creates or modifies tables
• Confusing subquery with JOIN
• Assuming subquery always returns one value

2. Which of the following is the correct syntax to use a subquery in the WHERE clause to find employees in departments with ID 10 or 20?

easy

A. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20);

B. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20));

C. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20);

D. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20));

Solution

1. Step 1: Identify correct subquery syntax with IN

   The subquery must be enclosed in parentheses and used with IN to match multiple values.

2. Step 2: Check each option's syntax

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); correctly uses IN with a subquery in parentheses. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20)); uses = which expects one value, causing error. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20); misses parentheses around subquery. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20); has incorrect WHERE clause syntax.

3. Final Answer:

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); -> Option D

4. Quick Check:

   Subquery with IN needs parentheses [OK]

Hint: Use IN with parentheses for subqueries returning multiple values [OK]

Common Mistakes:

• Using = instead of IN for multiple values
• Omitting parentheses around subquery
• Incorrect WHERE clause conditions inside subquery

3. Given the tables:
employees(emp_id, name, department_id)
departments(department_id, name)
What will this query return?

SELECT name FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE name = 'Sales');

medium

A. Names of employees who work in the Sales department

B. Names of all employees

C. Names of departments named Sales

D. An error because subquery returns multiple rows

Solution

1. Step 1: Understand the subquery

   The subquery selects department_id from departments where name is 'Sales'. This returns IDs of Sales departments.

2. Step 2: Apply subquery results in main query

   The main query selects employee names where their department_id matches any of those returned by the subquery.

3. Final Answer:

   Names of employees who work in the Sales department -> Option A

4. Quick Check:

   Subquery filters employees by Sales department [OK]

Hint: Subquery filters department IDs, main query filters employees [OK]

Common Mistakes:

• Thinking subquery returns employee names
• Confusing department names with employee names
• Assuming subquery causes error with multiple rows

4. Identify the error in this query:

SELECT * FROM orders WHERE customer_id = (SELECT customer_id FROM customers WHERE city = 'New York');

medium

A. Incorrect table name 'orders'

B. Missing FROM clause in subquery

C. Subquery returns multiple rows causing an error with '=' operator

D. No error, query is correct

Solution

1. Step 1: Analyze subquery result

   The subquery selects customer_id from customers where city is 'New York'. This can return multiple customer IDs.

2. Step 2: Check operator compatibility

   The main query uses '=' which expects a single value, but subquery returns multiple rows, causing an error.

3. Final Answer:

   Subquery returns multiple rows causing an error with '=' operator -> Option C

4. Quick Check:

   Use IN for multiple subquery results [OK]

Hint: Use IN if subquery returns multiple values, not = [OK]

Common Mistakes:

• Using = with subquery returning multiple rows
• Assuming subquery always returns one value
• Ignoring error messages about subquery results

5. You want to find all products that have never been ordered. Given tables:
products(product_id, name)
orders(order_id, product_id)
Which query correctly uses a subquery in the WHERE clause to find these products?

hard

A. SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders);

B. SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders);

C. SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders);

D. SELECT name FROM products WHERE product_id NOT = (SELECT product_id FROM orders);

Solution

1. Step 1: Understand the goal

   We want products that have never been ordered, so their product_id should NOT appear in orders.

2. Step 2: Use NOT IN with subquery

   The subquery selects all product_ids from orders. Using NOT IN filters products not in that list.

3. Step 3: Check other options

   SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders); finds products that have been ordered (opposite). SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders); uses = which expects one value, causing error. SELECT name FROM products WHERE product_id NOT = (SELECT product_id FROM orders); uses NOT = which is invalid syntax for multiple values.

4. Final Answer:

   SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders); -> Option A

5. Quick Check:

   NOT IN filters products never ordered [OK]

Hint: Use NOT IN with subquery to find missing matches [OK]

Common Mistakes:

• Using = instead of IN for multiple values
• Confusing NOT IN with NOT =
• Selecting wrong table columns in subquery