Subquery in WHERE clause in SQL - Mini Project: Build & Apply

Filter Employees Using Subquery in WHERE Clause

📖 Scenario: You work in the HR department of a company. You have a database table called employees that stores employee details including their id, name, and department_id. You also have a departments table that stores department_id and department_name.Your manager wants to see the list of employees who work in the 'Sales' department.

🎯 Goal: Build a SQL query that uses a subquery in the WHERE clause to find all employees who belong to the 'Sales' department.

📋 What You'll Learn

Create a table called departments with columns department_id and department_name.

Create a table called employees with columns id, name, and department_id.

Insert the exact data provided into both tables.

Write a SQL query that selects id and name from employees where department_id matches the department_id of the 'Sales' department using a subquery in the WHERE clause.

💡 Why This Matters

🌍 Real World

Filtering employees by department is a common task in HR databases to generate reports or manage staff.

💼 Career

Understanding subqueries in WHERE clauses is essential for database querying roles, data analysis, and backend development.

1

Create the departments table and insert data

Create a table called departments with columns department_id (integer) and department_name (text). Then insert these exact rows: (1, 'Sales'), (2, 'HR'), (3, 'Engineering').

SQL

-- Create the departments table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the employees table and insert data

Create a table called employees with columns id (integer), name (text), and department_id (integer). Then insert these exact rows: (101, 'Alice', 1), (102, 'Bob', 2), (103, 'Charlie', 1), (104, 'Diana', 3).

SQL

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

INSERT INTO departments (department_id, department_name) VALUES
(1, 'Sales'),
(2, 'HR'),
(3, 'Engineering');

-- Create the employees table and insert data
-- Your code here

Hint

Define the employees table with the correct columns and insert the given rows.

3

Write a subquery to find the department_id for 'Sales'

Write a SQL subquery that selects department_id from departments where department_name is 'Sales'. Assign this subquery to be used in the next step.

SQL

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

INSERT INTO departments (department_id, department_name) VALUES
(1, 'Sales'),
(2, 'HR'),
(3, 'Engineering');

CREATE TABLE employees (
    id INTEGER,
    name TEXT,
    department_id INTEGER
);

INSERT INTO employees (id, name, department_id) VALUES
(101, 'Alice', 1),
(102, 'Bob', 2),
(103, 'Charlie', 1),
(104, 'Diana', 3);

-- Write a subquery to select department_id for 'Sales'
-- Your code here

Hint

The subquery should select department_id where the department name is exactly 'Sales'.

4

Use the subquery in WHERE clause to select employees in 'Sales'

Write a SQL query that selects id and name from employees where department_id equals the subquery that selects department_id from departments where department_name is 'Sales'.

SQL

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

INSERT INTO departments (department_id, department_name) VALUES
(1, 'Sales'),
(2, 'HR'),
(3, 'Engineering');

CREATE TABLE employees (
    id INTEGER,
    name TEXT,
    department_id INTEGER
);

INSERT INTO employees (id, name, department_id) VALUES
(101, 'Alice', 1),
(102, 'Bob', 2),
(103, 'Charlie', 1),
(104, 'Diana', 3);

-- Write a query to select id and name from employees
-- where department_id matches the subquery for 'Sales'
-- Your code here

Hint

Use the subquery inside the WHERE clause to filter employees by department.

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