Scalar subquery in SELECT in SQL

Introduction

A scalar subquery in SELECT lets you get a single value from another table or query to use in each row of your main query. It helps add extra info without joining tables.

When you want to show a related value from another table for each row, like the latest order date for each customer.

When you need to calculate a small summary value, like the average score, to show alongside each record.

When you want to add a single value result from a query inside your main query without making it complex.

When you want to compare each row's value with a single value from another query.

Syntax

SQL

SELECT column1, (SELECT single_value FROM table2 WHERE condition) AS alias_name FROM table1;

The subquery inside SELECT must return exactly one value (one row, one column).

If the subquery returns no rows, the result is NULL for that row.

Examples

Get each student's name and their highest test score using a scalar subquery.

SQL

SELECT name, (SELECT MAX(score) FROM tests WHERE tests.student_id = students.id) AS max_score FROM students;

Show each product with the total number of sales it has.

SQL

SELECT product_name, (SELECT COUNT(*) FROM sales WHERE sales.product_id = products.id) AS total_sales FROM products;

Add the department name for each employee using a scalar subquery.

SQL

SELECT employee_name, (SELECT department_name FROM departments WHERE departments.id = employees.department_id) AS dept FROM employees;

Sample Program

This query lists each employee's name and their department name by using a scalar subquery in the SELECT clause.

SQL

CREATE TABLE employees (id INT, name VARCHAR(20), department_id INT);
CREATE TABLE departments (id INT, department_name VARCHAR(20));

INSERT INTO employees VALUES (1, 'Alice', 10), (2, 'Bob', 20), (3, 'Charlie', 10);
INSERT INTO departments VALUES (10, 'HR'), (20, 'Sales');

SELECT name, (SELECT department_name FROM departments WHERE departments.id = employees.department_id) AS department FROM employees ORDER BY id;

Important Notes

Scalar subqueries must return only one value; otherwise, SQL will give an error.

If the subquery finds no matching row, the result is NULL for that row in the main query.

Scalar subqueries can be slower than joins for large data, so use them wisely.

Summary

Scalar subqueries in SELECT let you add one value from another query for each row.

They must return exactly one value (one row, one column).

Useful for adding related info without complex joins.

Practice

1. What does a scalar subquery in the SELECT clause return?

easy

A. Only column names

B. Multiple rows and columns

C. A single value (one row, one column)

D. Only table names

Solution

1. Step 1: Understand scalar subquery definition

   A scalar subquery returns exactly one value, meaning one row and one column.

2. Step 2: Compare with other subquery types

   Unlike table subqueries, scalar subqueries cannot return multiple rows or columns.

3. Final Answer:

   A single value (one row, one column) -> Option C

4. Quick Check:

   Scalar subquery = single value [OK]

Hint: Scalar subquery returns one value only, not a table [OK]

Common Mistakes:

• Thinking scalar subquery returns multiple rows
• Confusing scalar subquery with table subquery
• Assuming scalar subquery returns column names only

2. Which of the following is the correct syntax for using a scalar subquery in the SELECT clause?

easy

A. SELECT name, SELECT MAX(score) FROM scores AS max_score FROM students;

B. SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students;

C. SELECT name, MAX(score) FROM scores AS max_score FROM students;

D. SELECT name, (MAX(score) FROM scores) AS max_score FROM students;

Solution

1. Step 1: Identify correct scalar subquery syntax

   The scalar subquery must be enclosed in parentheses and used inside the SELECT clause.

2. Step 2: Check each option

   SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students; correctly uses parentheses around the subquery. Others miss parentheses or have wrong placement.

3. Final Answer:

   SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students; -> Option B

4. Quick Check:

   Scalar subquery syntax = parentheses [OK]

Hint: Always put scalar subquery inside parentheses in SELECT [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Placing SELECT keyword incorrectly
• Using aggregate functions without subquery

3. Given tables employees(id, name, dept_id) and departments(id, dept_name), what is the output of this query?

SELECT name, (SELECT dept_name FROM departments WHERE id = employees.dept_id) AS department FROM employees ORDER BY name;

medium

A. Syntax error due to subquery

B. [{"name": "Alice", "department": null}, {"name": "Bob", "department": null}]

C. [{"name": "Alice", "department": "IT"}, {"name": "Bob", "department": "HR"}]

D. [{"name": "Alice", "department": "HR"}, {"name": "Bob", "department": "IT"}]

Solution

1. Step 1: Understand query logic

   For each employee, the scalar subquery fetches the department name matching their dept_id.

2. Step 2: Match employees to departments

   Alice's dept_id matches HR, Bob's matches IT, so the output shows correct department names.

3. Final Answer:

   [{"name": "Alice", "department": "HR"}, {"name": "Bob", "department": "IT"}] -> Option D

4. Quick Check:

   Scalar subquery returns matching department name per employee [OK]

Hint: Scalar subquery returns one value per row, matching join logic [OK]

Common Mistakes:

• Assuming subquery returns multiple rows causing error
• Mixing up department names for employees
• Expecting nulls when matching keys exist

4. Identify the error in this query:

SELECT name, (SELECT dept_name FROM departments WHERE id = employees.dept_id) AS department FROM employees WHERE (SELECT COUNT(*) FROM departments) > 0;

medium

A. Scalar subquery in WHERE is valid but inefficient

B. Scalar subquery in WHERE returns multiple rows

C. Missing alias for subquery in SELECT

D. Subquery in WHERE must return a single value

Solution

1. Step 1: Analyze subquery in WHERE clause

   The subquery in WHERE returns COUNT(*), which is a single value, so it's valid.

2. Step 2: Consider efficiency and logic

   Using a scalar subquery in WHERE like this works but is inefficient; better to check existence differently.

3. Final Answer:

   Scalar subquery in WHERE is valid but inefficient -> Option A

4. Quick Check:

   Scalar subquery in WHERE can be valid but watch efficiency [OK]

Hint: Scalar subquery in WHERE must return one value; check efficiency [OK]

Common Mistakes:

• Thinking scalar subquery in WHERE always causes error
• Confusing alias requirement in SELECT with WHERE
• Assuming subquery returns multiple rows here

5. You want to list all products with their category name, but some products have no category assigned (category_id is NULL). Which query correctly uses a scalar subquery in SELECT to show category names or 'Uncategorized' if none?

Options:
A) SELECT product_name, IFNULL((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products WHERE category_id IS NOT NULL;
B) SELECT product_name, (SELECT category_name FROM categories WHERE id = products.category_id) OR 'Uncategorized' AS category FROM products;
C) SELECT product_name, (SELECT category_name FROM categories WHERE id = products.category_id) AS category FROM products WHERE category_id IS NOT NULL;
D) SELECT product_name, COALESCE((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products;

hard

A. SELECT product_name, COALESCE((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products;

B. SELECT product_name, (SELECT category_name FROM categories WHERE id = products.category_id) OR 'Uncategorized' AS category FROM products;

C. SELECT product_name, (SELECT category_name FROM categories WHERE id = products.category_id) AS category FROM products WHERE category_id IS NOT NULL;

D. SELECT product_name, IFNULL((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products WHERE category_id IS NOT NULL;

Solution

1. Step 1: Handle NULL category_id with scalar subquery

   Use COALESCE to replace NULL result from subquery with 'Uncategorized'.

2. Step 2: Check each option's correctness

   The query with COALESCE((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products; correctly uses COALESCE and includes all products. The query with (SELECT category_name FROM categories WHERE id = products.category_id) OR 'Uncategorized' uses invalid OR syntax. The queries with WHERE category_id IS NOT NULL exclude products with NULL category_id.

3. Final Answer:

   SELECT product_name, COALESCE((SELECT category_name FROM categories WHERE id = products.category_id), 'Uncategorized') AS category FROM products; -> Option A

4. Quick Check:

   Use COALESCE with scalar subquery for NULL handling [OK]

Hint: Use COALESCE to handle NULL from scalar subquery [OK]

Common Mistakes:

• Using OR instead of COALESCE or IFNULL
• Filtering out NULL category_id rows
• Not handling NULL results from subquery