Why subqueries are needed in PostgreSQL

Introduction

Subqueries help you get data step-by-step inside a bigger query. They let you ask a question inside another question.

When you want to find data based on a calculation or filter done first.

When you need to compare each row to a group result like max or average.

When you want to break a complex question into smaller parts.

When you want to use the result of one query as input for another.

When you want to filter rows based on values from another table.

Syntax

PostgreSQL

SELECT column1 FROM table WHERE column2 = (SELECT column2 FROM table2 WHERE condition);

A subquery is a query inside parentheses used in WHERE, FROM, or SELECT clauses.

Subqueries run first, then their result is used by the main query.

Examples

Find employees who work in the Sales department by first finding the department id.

PostgreSQL

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

Find products priced higher than the average price.

PostgreSQL

SELECT product_name FROM products WHERE price > (SELECT AVG(price) FROM products);

Find orders made on the most recent date.

PostgreSQL

SELECT customer_id, order_date FROM orders WHERE order_date = (SELECT MAX(order_date) FROM orders);

Sample Program

This query finds all employees who work in the Marketing department by first finding the department's id.

PostgreSQL

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

Important Notes

Subqueries can return one value (scalar), a list of values, or a table.

Using subqueries can make queries easier to read and understand.

Too many subqueries can slow down your query; sometimes joins are faster.

Summary

Subqueries let you ask a question inside another question.

They help break complex queries into smaller steps.

Use them to filter or compare data based on other query results.

Practice

1. Why do we use subqueries in PostgreSQL?
SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments WHERE location = 'NY');

easy

A. To change data types of columns

B. To speed up the database server automatically

C. To ask a question inside another query and filter results based on it

D. To create new tables from existing ones

Solution

1. Step 1: Understand the role of the subquery

   The subquery (SELECT id FROM departments WHERE location = 'NY') finds department IDs located in NY.

2. Step 2: See how the main query uses the subquery

   The main query selects employees whose department_id matches any ID from the subquery result.

3. Final Answer:

   To ask a question inside another query and filter results based on it -> Option C

4. Quick Check:

   Subqueries filter data using inner query results [OK]

Hint: Subqueries answer inner questions to filter outer query [OK]

Common Mistakes:

• Thinking subqueries speed up queries automatically
• Confusing subqueries with table creation
• Using subqueries to change data types

2. Which of the following is the correct syntax for a subquery in PostgreSQL?

easy

A. SELECT * FROM table WHERE column == (SELECT MAX(column) FROM table);

B. SELECT * FROM table WHERE column = (SELECT MAX(column) FROM table);

C. SELECT * FROM table WHERE column = SELECT MAX(column) FROM table;

D. SELECT * FROM table WHERE column IN SELECT MAX(column) FROM table;

Solution

1. Step 1: Identify correct subquery syntax

   Subqueries must be enclosed in parentheses and used with comparison operators like = or IN.

2. Step 2: Check each option

   SELECT * FROM table WHERE column = (SELECT MAX(column) FROM table); correctly uses parentheses and = with a subquery. The other options have syntax errors like using == instead of =, omitting parentheses entirely, or missing them after IN.

3. Final Answer:

   SELECT * FROM table WHERE column = (SELECT MAX(column) FROM table); -> Option B

4. Quick Check:

   Subqueries need parentheses and proper operators [OK]

Hint: Subqueries always go inside parentheses [OK]

Common Mistakes:

• Using double equals (==) instead of single equals (=)
• Omitting parentheses around subqueries
• Using IN without parentheses

3. What will be the output of this query?

SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

medium

A. Syntax error due to subquery

B. Names of employees earning less than the average salary

C. All employee names

D. Names of employees earning more than the average salary

Solution

1. Step 1: Understand the subquery

   The subquery (SELECT AVG(salary) FROM employees) calculates the average salary of all employees.

2. Step 2: Apply the condition in the main query

   The main query selects employee names where their salary is greater than this average.

3. Final Answer:

   Names of employees earning more than the average salary -> Option D

4. Quick Check:

   Subquery returns average; main query filters salaries above it [OK]

Hint: Subqueries can return single values to compare in main query [OK]

Common Mistakes:

• Thinking it returns employees below average
• Assuming syntax error due to subquery
• Confusing AVG with SUM

4. Identify the error in this query:

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

medium

A. Missing parentheses around the subquery

B. Wrong comparison operator used

C. Subquery returns multiple rows causing error

D. Incorrect table name used

Solution

1. Step 1: Check subquery syntax

   The subquery must be enclosed in parentheses to be valid in the WHERE clause.

2. Step 2: Identify the missing parentheses

   The query misses parentheses around SELECT id FROM departments WHERE name = 'Sales', causing syntax error.

3. Final Answer:

   Missing parentheses around the subquery -> Option A

4. Quick Check:

   Subqueries need parentheses to be valid [OK]

Hint: Always put subqueries inside parentheses [OK]

Common Mistakes:

• Forgetting parentheses around subqueries
• Assuming wrong operator causes error
• Thinking table name is incorrect

5. You want to find employees who earn more than the average salary in their own department. Which query correctly uses a subquery to achieve this?

hard

A. SELECT name, salary, department_id FROM employees WHERE salary > (SELECT AVG(salary) FROM employees WHERE department_id = employees.department_id);

B. SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

C. SELECT name FROM employees WHERE salary > ALL (SELECT salary FROM employees);

D. SELECT name FROM employees WHERE salary > ANY (SELECT AVG(salary) FROM employees GROUP BY department_id);

Solution

1. Step 1: Understand the requirement

   We need to compare each employee's salary to the average salary of their own department.

2. Step 2: Analyze each option

   SELECT name, salary, department_id FROM employees WHERE salary > (SELECT AVG(salary) FROM employees WHERE department_id = employees.department_id); uses a correlated subquery that calculates average salary per employee's department. SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees); compares to overall average, ignoring departments. SELECT name FROM employees WHERE salary > ALL (SELECT salary FROM employees); compares salary to all salaries, not averages. SELECT name FROM employees WHERE salary > ANY (SELECT AVG(salary) FROM employees GROUP BY department_id); compares to a list of averages but uses ANY incorrectly.

3. Final Answer:

   SELECT name, salary, department_id FROM employees WHERE salary > (SELECT AVG(salary) FROM employees WHERE department_id = employees.department_id); -> Option A

4. Quick Check:

   Correlated subquery compares salary to department average [OK]

Hint: Use correlated subqueries to compare within groups [OK]

Common Mistakes:

• Comparing to overall average instead of department average
• Using ALL or ANY incorrectly with averages
• Not correlating subquery with outer query