Scalar subqueries in PostgreSQL - Mini Project: Build & Apply

Using Scalar Subqueries in PostgreSQL

📖 Scenario: You are managing a small bookstore database. You want to find the price of each book along with the average price of all books in the store.

🎯 Goal: Build a SQL query that uses a scalar subquery to show each book's title, its price, and the average price of all books.

📋 What You'll Learn

Create a table called books with columns id (integer), title (text), and price (numeric).

Insert exactly three books with these values: (1, 'Book A', 10.00), (2, 'Book B', 15.00), (3, 'Book C', 20.00).

Write a scalar subquery to calculate the average price of all books.

Select the title, price, and the average price from the scalar subquery in the final query.

💡 Why This Matters

🌍 Real World

Scalar subqueries are useful when you want to include a single value calculated from the database inside another query, such as averages, counts, or maximum values.

💼 Career

Understanding scalar subqueries helps you write efficient and readable SQL queries for reports and data analysis tasks in many database-related jobs.

1

Create the books table and insert data

Create a table called books with columns id as integer, title as text, and price as numeric. Then insert these three rows exactly: (1, 'Book A', 10.00), (2, 'Book B', 15.00), and (3, 'Book C', 20.00).

PostgreSQL

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

Hint

Use CREATE TABLE books (id INTEGER, title TEXT, price NUMERIC); and then INSERT INTO books (id, title, price) VALUES (...), (...), (...);

2

Write a scalar subquery to calculate average price

Create a scalar subquery that calculates the average price of all books from the books table. Assign this scalar subquery to a variable or prepare it to use in the next step.

PostgreSQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  price NUMERIC
);

INSERT INTO books (id, title, price) VALUES
  (1, 'Book A', 10.00),
  (2, 'Book B', 15.00),
  (3, 'Book C', 20.00);

-- Write a scalar subquery to calculate average price
-- Your code here

Hint

Use (SELECT AVG(price) FROM books) to get the average price as a scalar subquery.

3

Select book details with average price using scalar subquery

Write a SELECT query that retrieves the title and price from the books table, and also includes the average price of all books using the scalar subquery (SELECT AVG(price) FROM books) as a column named average_price.

PostgreSQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  price NUMERIC
);

INSERT INTO books (id, title, price) VALUES
  (1, 'Book A', 10.00),
  (2, 'Book B', 15.00),
  (3, 'Book C', 20.00);

-- Select title, price, and average price using scalar subquery
SELECT title, price, (SELECT AVG(price) FROM books) AS average_price
FROM books;
-- Your code here

Hint

Use SELECT title, price, (SELECT AVG(price) FROM books) AS average_price FROM books;

4

Complete the query with ordering by price

Add an ORDER BY price ASC clause to the query to show the books sorted by their price from lowest to highest.

PostgreSQL

CREATE TABLE books (
  id INTEGER,
  title TEXT,
  price NUMERIC
);

INSERT INTO books (id, title, price) VALUES
  (1, 'Book A', 10.00),
  (2, 'Book B', 15.00),
  (3, 'Book C', 20.00);

SELECT title, price, (SELECT AVG(price) FROM books) AS average_price
FROM books
ORDER BY price ASC;
-- Your code here

Hint

Add ORDER BY price ASC at the end of the query to sort by price ascending.

Practice

1. What does a scalar subquery return in PostgreSQL?

easy

A. A single value used inside another query

B. Multiple rows and columns

C. Only table names

D. A list of table columns

Solution

1. Step 1: Understand scalar subquery definition

   A scalar subquery returns exactly one value, not multiple rows or columns.

2. Step 2: Compare options with definition

   Only A single value used inside another query describes a single value used inside another query, matching the scalar subquery concept.

3. Final Answer:

   A single value used inside another query -> Option A

4. Quick Check:

   Scalar subquery = single value [OK]

Hint: Scalar subqueries always return one value only [OK]

Common Mistakes:

• Thinking scalar subqueries return multiple rows
• Confusing scalar subqueries with table names
• Assuming scalar subqueries return lists

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

easy

A. SELECT (SELECT MAX(price) FROM products) AS max_price;

B. SELECT MAX(price) FROM products WHERE price = (SELECT);

C. SELECT MAX(price) FROM products SELECT price;

D. SELECT MAX(price) FROM (SELECT price FROM products);

Solution

1. Step 1: Identify correct scalar subquery syntax

   The scalar subquery must be enclosed in parentheses and return a single value, used inside the main SELECT.

2. Step 2: Check each option

   SELECT (SELECT MAX(price) FROM products) AS max_price; correctly uses (SELECT MAX(price) FROM products) as a scalar subquery. Options A, B, and D have syntax errors or misuse subqueries.

3. Final Answer:

   SELECT (SELECT MAX(price) FROM products) AS max_price; -> Option A

4. Quick Check:

   Scalar subquery syntax = SELECT (SELECT MAX(price) FROM products) AS max_price; [OK]

Hint: Scalar subqueries are enclosed in parentheses inside SELECT [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Using incomplete or invalid subquery syntax
• Confusing subqueries with JOINs

3. Given the tables:
products(id, name, price)
and
sales(product_id, quantity)
What will this query return?

SELECT name, (SELECT SUM(quantity) FROM sales WHERE product_id = products.id) AS total_sold FROM products;

medium

A. Syntax error due to subquery

B. Only product names without sales data

C. Sum of all sales quantities for all products

D. Each product's name with total quantity sold

Solution

1. Step 1: Understand the subquery role

   The subquery sums quantity from sales where product_id matches the current product's id, returning total sold per product.

2. Step 2: Analyze the main query output

   The main query selects product name and the scalar subquery result as total_sold for each product row.

3. Final Answer:

   Each product's name with total quantity sold -> Option D

4. Quick Check:

   Scalar subquery sums sales per product = Each product's name with total quantity sold [OK]

Hint: Scalar subquery calculates per-row related value [OK]

Common Mistakes:

• Thinking subquery sums all sales once
• Ignoring WHERE clause linking tables
• Assuming syntax error without checking carefully

4. Identify the error in this query using a scalar subquery:

SELECT name, (SELECT price FROM products) AS product_price FROM products;

medium

A. Missing alias for subquery

B. Incorrect table name in subquery

C. Subquery returns multiple rows causing error

D. No error, query runs fine

Solution

1. Step 1: Check subquery result cardinality

   The subquery SELECT price FROM products returns multiple prices (rows), not a single value.

2. Step 2: Understand scalar subquery requirement

   Scalar subqueries must return exactly one value; multiple rows cause an error.

3. Final Answer:

   Subquery returns multiple rows causing error -> Option C

4. Quick Check:

   Scalar subquery must return one value [OK]

Hint: Scalar subquery must return one value, not many [OK]

Common Mistakes:

• Ignoring multiple rows returned by subquery
• Assuming alias fixes multiple rows error
• Confusing syntax error with runtime error

5. You want to list each product's name and its price difference from the average price of all products. Which query correctly uses a scalar subquery to achieve this?

hard

A. SELECT name, price - AVG(price) FROM products;

B. SELECT name, price - (SELECT AVG(price) FROM products) AS price_diff FROM products;

C. SELECT name, price - (SELECT price FROM products) AS price_diff FROM products;

D. SELECT name, price - (SELECT SUM(price) FROM products) AS price_diff FROM products;

Solution

1. Step 1: Understand the goal

   We want each product's price minus the average price of all products.

2. Step 2: Analyze each option

   SELECT name, price - (SELECT AVG(price) FROM products) AS price_diff FROM products; correctly uses a scalar subquery to get AVG(price) once and subtracts it from each product's price. SELECT name, price - AVG(price) FROM products; lacks GROUP BY and will error. SELECT name, price - (SELECT price FROM products) AS price_diff FROM products; returns multiple prices, causing error. SELECT name, price - (SELECT SUM(price) FROM products) AS price_diff FROM products; subtracts sum, not average, which is incorrect.

3. Final Answer:

   SELECT name, price - (SELECT AVG(price) FROM products) AS price_diff FROM products; -> Option B

4. Quick Check:

   Scalar subquery for AVG(price) = SELECT name, price - (SELECT AVG(price) FROM products) AS price_diff FROM products; [OK]

Hint: Use scalar subquery for single aggregate value [OK]

Common Mistakes:

• Using aggregate without GROUP BY causing error
• Subquery returning multiple rows instead of one
• Confusing SUM with AVG in calculation