HAVING for filtering groups in PostgreSQL - Mini Project: Build & Apply

Filtering Groups with HAVING in PostgreSQL

📖 Scenario: You are managing a small bookstore database. You want to find authors who have sold more than a certain number of books in total.

🎯 Goal: Build a query that groups sales by author and uses HAVING to filter authors with total sales above a threshold.

📋 What You'll Learn

Create a table called book_sales with columns author (text) and copies_sold (integer).

Insert the exact sales data provided.

Create a variable min_sales to set the minimum total copies sold to filter authors.

Write a query that groups sales by author and sums copies_sold.

Use HAVING to filter groups where total copies sold is greater than min_sales.

💡 Why This Matters

🌍 Real World

Filtering grouped data is common in sales reports, analytics, and business intelligence to find top performers or categories.

💼 Career

Understanding how to use HAVING helps database analysts and developers write efficient queries that summarize and filter grouped data.

1

Create the book_sales table and insert data

Create a table called book_sales with columns author (text) and copies_sold (integer). Then insert these exact rows: ('Alice Munro', 120), ('Alice Munro', 80), ('Gabriel Garcia Marquez', 200), ('Gabriel Garcia Marquez', 150), ('Haruki Murakami', 90).

PostgreSQL

-- Create table book_sales and insert data
-- Your code here

Hint

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

2

Set the minimum sales threshold

Create a variable called min_sales and set it to 200 to represent the minimum total copies sold to filter authors.

PostgreSQL

CREATE TABLE book_sales (
    author TEXT,
    copies_sold INTEGER
);

INSERT INTO book_sales (author, copies_sold) VALUES
('Alice Munro', 120),
('Alice Munro', 80),
('Gabriel Garcia Marquez', 200),
('Gabriel Garcia Marquez', 150),
('Haruki Murakami', 90);

-- Create variable min_sales and set to 200
-- Your code here

Hint

Use \set min_sales 200 to create a psql variable for filtering.

3

Write a query to group sales by author and sum copies

Write a query that selects author and the sum of copies_sold as total_sold. Group the results by author.

PostgreSQL

CREATE TABLE book_sales (
    author TEXT,
    copies_sold INTEGER
);

INSERT INTO book_sales (author, copies_sold) VALUES
('Alice Munro', 120),
('Alice Munro', 80),
('Gabriel Garcia Marquez', 200),
('Gabriel Garcia Marquez', 150),
('Haruki Murakami', 90);

\set min_sales 200

-- Write query to select author and sum(copies_sold) as total_sold grouped by author
-- Your code here

Hint

Use GROUP BY author and SUM(copies_sold) to get total sales per author.

4

Add HAVING clause to filter authors by total sales

Add a HAVING clause to the query to only include authors where SUM(copies_sold) is greater than the variable min_sales.

PostgreSQL

CREATE TABLE book_sales (
    author TEXT,
    copies_sold INTEGER
);

INSERT INTO book_sales (author, copies_sold) VALUES
('Alice Munro', 120),
('Alice Munro', 80),
('Gabriel Garcia Marquez', 200),
('Gabriel Garcia Marquez', 150),
('Haruki Murakami', 90);

\set min_sales 200

SELECT author, SUM(copies_sold) AS total_sold
FROM book_sales
GROUP BY author
-- Add HAVING clause to filter authors with total_sold > min_sales
-- Your code here

Hint

Use HAVING SUM(copies_sold) > :min_sales to filter groups after grouping.

Practice

1. What is the main purpose of the HAVING clause in a PostgreSQL query?

easy

A. To join two tables

B. To filter rows before grouping

C. To sort the result set

D. To filter groups after aggregation

Solution

1. Step 1: Understand the role of HAVING

   The HAVING clause is used to filter groups created by GROUP BY based on aggregate conditions.

2. Step 2: Differentiate HAVING from WHERE

   WHERE filters rows before grouping, HAVING filters groups after aggregation.

3. Final Answer:

   To filter groups after aggregation -> Option D

4. Quick Check:

   HAVING filters groups = D [OK]

Hint: Remember HAVING filters groups, WHERE filters rows [OK]

Common Mistakes:

• Confusing HAVING with WHERE clause
• Thinking HAVING sorts data
• Using HAVING without GROUP BY

2. Which of the following is the correct syntax to filter groups having a total sales greater than 1000 using HAVING?

easy

A. SELECT store, SUM(sales) FROM sales_data HAVING SUM(sales) > 1000 GROUP BY store;

B. SELECT store, SUM(sales) FROM sales_data GROUP BY store HAVING SUM(sales) > 1000;

C. SELECT store, SUM(sales) FROM sales_data WHERE SUM(sales) > 1000 GROUP BY store;

D. SELECT store, SUM(sales) FROM sales_data GROUP BY store WHERE SUM(sales) > 1000;

Solution

1. Step 1: Identify correct order of clauses

   GROUP BY must come before HAVING in SQL syntax.

2. Step 2: Check aggregate usage in HAVING

   HAVING uses aggregate functions like SUM(sales) to filter groups.

3. Final Answer:

   SELECT store, SUM(sales) FROM sales_data GROUP BY store HAVING SUM(sales) > 1000; -> Option B

4. Quick Check:

   GROUP BY before HAVING = B [OK]

Hint: HAVING always follows GROUP BY clause [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Using WHERE with aggregate functions
• Incorrect clause order

3. Given the table orders with columns customer_id and amount, what will this query return?

SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_id HAVING COUNT(*) > 2;

medium

A. All customers with more than 2 orders

B. All customers with exactly 2 orders

C. All customers with less than 2 orders

D. Syntax error due to HAVING clause

Solution

1. Step 1: Understand the GROUP BY and COUNT(*)

   The query groups rows by customer_id and counts orders per customer.

2. Step 2: Apply HAVING filter

   HAVING COUNT(*) > 2 keeps only customers with more than 2 orders.

3. Final Answer:

   All customers with more than 2 orders -> Option A

4. Quick Check:

   HAVING COUNT(*) > 2 filters groups with >2 rows [OK]

Hint: HAVING filters groups by aggregate results [OK]

Common Mistakes:

• Confusing > 2 with = 2
• Thinking HAVING causes syntax error
• Ignoring GROUP BY effect

4. Identify the error in this query:

SELECT department, AVG(salary) FROM employees HAVING AVG(salary) > 50000 GROUP BY department;

medium

A. HAVING clause used before GROUP BY

B. AVG() cannot be used in HAVING

C. Missing WHERE clause before HAVING

D. No error, query is correct

Solution

1. Step 1: Check clause order

   SQL requires GROUP BY before HAVING clause.

2. Step 2: Identify error in query

   HAVING appears before GROUP BY, which is invalid syntax.

3. Final Answer:

   HAVING clause used before GROUP BY -> Option A

4. Quick Check:

   GROUP BY must precede HAVING [OK]

Hint: GROUP BY comes before HAVING always [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Assuming AVG() not allowed in HAVING
• Confusing WHERE and HAVING

5. You have a sales table with columns region, salesperson, and amount. Write a query to find regions where the total sales exceed 10,000 but only include salespeople who individually sold more than 3,000. Which query correctly applies HAVING to achieve this?

hard

A. SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND SUM(amount) > 10000;

B. SELECT region, SUM(amount) FROM sales GROUP BY region HAVING SUM(amount) > 10000 AND salesperson IN (SELECT salesperson FROM sales GROUP BY salesperson HAVING SUM(amount) > 3000);

C. SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000);

D. SELECT region, salesperson, SUM(amount) FROM sales WHERE SUM(amount) > 3000 GROUP BY region, salesperson HAVING SUM(amount) > 10000;

Solution

1. Step 1: Understand filtering conditions

   We want regions with total sales > 10,000 and salespeople with sales > 3,000.

2. Step 2: Analyze correct logic

   Group by region and salesperson, filter salespeople with >3000 sales using HAVING SUM(amount) > 3000, and filter regions with total sales > 10000 using a subquery in HAVING: region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000).

3. Step 3: Check HAVING usage

   This correctly applies HAVING on both salesperson and region aggregates using subquery.

4. Final Answer:

   SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000); -> Option C

5. Quick Check:

   Use HAVING with subqueries for multi-level group filters [OK]

Hint: Use HAVING with subqueries for multi-level group filters [OK]

Common Mistakes:

• Using WHERE with aggregate functions
• Incorrect clause order
• Combining unrelated HAVING conditions without subqueries