Subqueries in WHERE with IN in PostgreSQL

Introduction

We use subqueries with IN to find rows where a column matches any value from another list of values. It helps us filter data based on related information.

You want to find customers who have placed orders by checking order records.

You want to list products that appear in any sales transactions.

You want to get employees who work in departments located in a specific city.

You want to find students enrolled in courses offered this semester.

Syntax

PostgreSQL

SELECT column1, column2
FROM table1
WHERE columnX IN (SELECT columnY FROM table2 WHERE condition);

The subquery inside IN returns a list of values to compare against.

The main query returns rows where columnX matches any value from the subquery.

Examples

Finds customers who have made at least one order.

PostgreSQL

SELECT name
FROM customers
WHERE id IN (SELECT customer_id FROM orders);

Lists products that belong to the 'Books' category.

PostgreSQL

SELECT product_name
FROM products
WHERE category_id IN (SELECT id FROM categories WHERE name = 'Books');

Finds employees working in departments located in New York.

PostgreSQL

SELECT employee_name
FROM employees
WHERE department_id IN (SELECT id FROM departments WHERE location = 'New York');

Sample Program

This example creates two tables: departments and employees. It inserts some data, then finds employees who work in departments located in New York.

PostgreSQL

CREATE TABLE departments (id SERIAL PRIMARY KEY, name TEXT, location TEXT);
CREATE TABLE employees (id SERIAL PRIMARY KEY, employee_name TEXT, department_id INT);

INSERT INTO departments (name, location) VALUES
('Sales', 'New York'),
('Engineering', 'San Francisco'),
('HR', 'New York');

INSERT INTO employees (employee_name, department_id) VALUES
('Alice', 1),
('Bob', 2),
('Charlie', 3),
('Diana', 1);

SELECT employee_name
FROM employees
WHERE department_id IN (SELECT id FROM departments WHERE location = 'New York');

Important Notes

Subqueries inside IN must return a single column.

Using IN with subqueries is easy to read and understand for beginners.

Performance can be slower with large data; sometimes JOINs are faster.

Summary

Use subqueries with IN to filter rows based on a list from another query.

The subquery returns values that the main query compares against.

This helps connect related data across tables simply.

Practice

1. What does the IN keyword do in a SQL query when used with a subquery in the WHERE clause?

easy

A. It filters rows where a column's value matches any value returned by the subquery.

B. It joins two tables together based on matching columns.

C. It updates rows in the table with values from the subquery.

D. It deletes rows that do not exist in the subquery.

Solution

1. Step 1: Understand the role of IN with subqueries

   The IN keyword checks if a value exists in a list of values returned by the subquery.

2. Step 2: Identify the filtering effect

   Rows are kept only if their column value matches any value from the subquery's result.

3. Final Answer:

   It filters rows where a column's value matches any value returned by the subquery. -> Option A

4. Quick Check:

   IN filters rows by subquery values [OK]

Hint: Think of IN as 'is in this list' from the subquery [OK]

Common Mistakes:

• Confusing IN with JOIN
• Thinking IN updates or deletes rows
• Assuming IN returns a single value only

2. Which of the following is the correct syntax to use a subquery with IN in PostgreSQL?

easy

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

B. SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments);

C. SELECT * FROM employees WHERE department_id IN SELECT id FROM departments;

D. SELECT * FROM employees WHERE department_id = IN (SELECT id FROM departments);

Solution

1. Step 1: Check the correct use of parentheses

   The subquery must be enclosed in parentheses after IN.

2. Step 2: Verify the subquery returns a single column

   The subquery should return one column of values to compare against.

3. Final Answer:

   SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments); -> Option B

4. Quick Check:

   Subquery in parentheses with single column [OK]

Hint: Always put subquery inside parentheses after IN [OK]

Common Mistakes:

• Using = IN instead of just IN
• Omitting parentheses around subquery
• Selecting multiple columns in subquery

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

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

medium

A. An error because subquery returns multiple rows.

B. Names of all employees regardless of department.

C. Names of employees who do not work in Sales.

D. Names of employees who work in the Sales department.

Solution

1. Step 1: Understand the subquery filtering departments

   The subquery selects department IDs where the department name is 'Sales'.

2. Step 2: Filter employees by matching department IDs

   The main query returns employee names whose department_id matches those IDs.

3. Final Answer:

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

4. Quick Check:

   IN filters employees by Sales department IDs [OK]

Hint: Subquery filters departments, main query filters employees [OK]

Common Mistakes:

• Thinking it returns all employees
• Confusing NOT IN with IN
• Assuming subquery causes error with multiple rows

4. Identify the error in this query:

SELECT * FROM employees WHERE department_id IN SELECT id FROM departments WHERE name = 'HR';

medium

A. Missing parentheses around the subquery after IN.

B. Using IN instead of EXISTS.

C. Subquery returns multiple columns.

D. department_id cannot be compared with id.

Solution

1. Step 1: Check syntax for subquery with IN

   The subquery must be enclosed in parentheses after the IN keyword.

2. Step 2: Identify missing parentheses causing syntax error

   The query lacks parentheses around the subquery, causing invalid syntax.

3. Final Answer:

   Missing parentheses around the subquery after IN. -> Option A

4. Quick Check:

   Subquery must be in parentheses after IN [OK]

Hint: Always wrap subqueries in parentheses after IN [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Confusing IN with EXISTS syntax
• Assuming subquery returns multiple columns

5. You want to find all customers who have placed orders for products in the 'Electronics' category. Given tables:
customers(id, name)
orders(id, customer_id, product_id)
products(id, name, category)
Which query correctly uses a subquery with IN to get these customers?

hard

A. SELECT name FROM customers WHERE id IN (SELECT id FROM products WHERE category = 'Electronics');

B. SELECT name FROM customers WHERE id = IN (SELECT customer_id FROM orders WHERE product_id IN (SELECT id FROM products WHERE category = 'Electronics'));

C. SELECT name FROM customers WHERE id IN (SELECT customer_id FROM orders WHERE product_id IN (SELECT id FROM products WHERE category = 'Electronics'));

D. SELECT name FROM customers WHERE id IN (SELECT product_id FROM orders WHERE customer_id IN (SELECT id FROM products WHERE category = 'Electronics'));

Solution

1. Step 1: Identify the correct subquery for products in 'Electronics'

   The innermost subquery selects product IDs where category is 'Electronics'.

2. Step 2: Use these product IDs to find matching orders

   The middle subquery selects customer IDs from orders with those product IDs.

3. Step 3: Select customer names whose IDs match those customer IDs

   The main query selects customer names where their ID is in the list from the orders subquery.

4. Final Answer:

   SELECT name FROM customers WHERE id IN (SELECT customer_id FROM orders WHERE product_id IN (SELECT id FROM products WHERE category = 'Electronics')); -> Option C

5. Quick Check:

   Nested IN subqueries filter customers by product category [OK]

Hint: Use nested IN subqueries to filter step-by-step [OK]

Common Mistakes:

• Using = IN instead of IN
• Mixing up customer_id and product_id columns
• Selecting wrong columns in subqueries