HAVING clause in MySQL - Mini Project: Build & Apply

Filter Groups Using HAVING Clause in MySQL

📖 Scenario: You are managing a small store's sales database. You want to find which products have sold more than a certain number of units in total. This helps you understand your best-selling products.

🎯 Goal: Build a MySQL query that groups sales by product and uses the HAVING clause to show only products with total sales greater than a set threshold.

📋 What You'll Learn

Create a table called sales with columns product (text) and quantity (integer).

Insert the exact sales data provided.

Set a variable min_sales to 10 to use as the threshold.

Write a query that groups sales by product and sums quantity.

Use the HAVING clause to filter groups where total quantity is greater than min_sales.

💡 Why This Matters

🌍 Real World

Filtering grouped data is common in sales reports, inventory management, and any situation where you want to analyze summarized data.

💼 Career

Knowing how to use GROUP BY with HAVING is essential for data analysts, database administrators, and backend developers working with SQL databases.

1

Create the sales table and insert data

Create a table called sales with columns product (VARCHAR) and quantity (INT). Then insert these exact rows: ('Apples', 5), ('Bananas', 8), ('Apples', 7), ('Oranges', 3), ('Bananas', 4), ('Oranges', 10).

MySQL

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

Hint

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

2

Set the minimum sales threshold

Create a variable called min_sales and set it to 10. This will be the threshold for filtering products by total sales.

MySQL

CREATE TABLE sales (
  product VARCHAR(50),
  quantity INT
);

INSERT INTO sales (product, quantity) VALUES
('Apples', 5),
('Bananas', 8),
('Apples', 7),
('Oranges', 3),
('Bananas', 4),
('Oranges', 10);

-- Set min_sales to 10
-- Your code here

Hint

Use SET @min_sales = 10; to create a user variable in MySQL.

3

Write the query to group sales by product and sum quantities

Write a SELECT query that groups rows by product and calculates the total quantity sold for each product using SUM(quantity). Use GROUP BY product.

MySQL

CREATE TABLE sales (
  product VARCHAR(50),
  quantity INT
);

INSERT INTO sales (product, quantity) VALUES
('Apples', 5),
('Bananas', 8),
('Apples', 7),
('Oranges', 3),
('Bananas', 4),
('Oranges', 10);

SET @min_sales = 10;

-- Write SELECT query to group by product and sum quantity
-- Your code here

Hint

Use SELECT product, SUM(quantity) AS total_quantity FROM sales GROUP BY product;

4

Add HAVING clause to filter products with total sales greater than min_sales

Add a HAVING clause to the query to show only products where SUM(quantity) is greater than the variable @min_sales.

MySQL

CREATE TABLE sales (
  product VARCHAR(50),
  quantity INT
);

INSERT INTO sales (product, quantity) VALUES
('Apples', 5),
('Bananas', 8),
('Apples', 7),
('Oranges', 3),
('Bananas', 4),
('Oranges', 10);

SET @min_sales = 10;

SELECT product, SUM(quantity) AS total_quantity
FROM sales
GROUP BY product
-- Add HAVING clause to filter total_quantity > @min_sales
-- Your code here

Hint

Use HAVING SUM(quantity) > @min_sales after the GROUP BY clause.

Practice

1. What is the main purpose of the HAVING clause in SQL?

easy

A. To filter groups after aggregation

B. To filter rows before grouping

C. To sort the result set

D. To join two tables

Solution

1. Step 1: Understand filtering in SQL

   The WHERE clause filters rows before grouping, but HAVING filters after groups are formed.

2. Step 2: Identify the role of HAVING

   HAVING is used to filter groups based on aggregate conditions like COUNT or SUM.

3. Final Answer:

   To filter groups after aggregation -> Option A

4. Quick Check:

   HAVING filters groups, not rows [OK]

Hint: HAVING filters groups after aggregation, WHERE filters rows before [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax to use HAVING with GROUP BY in MySQL?

easy

A. SELECT col1 FROM table HAVING COUNT(col2) > 5 GROUP BY col1;

B. SELECT col1, COUNT(col2) FROM table GROUP BY col1 HAVING COUNT(col2) > 5;

C. SELECT col1, COUNT(col2) FROM table HAVING COUNT(col2) > 5;

D. SELECT col1, COUNT(col2) FROM table GROUP BY col1 WHERE COUNT(col2) > 5;

Solution

1. Step 1: Recall correct order of clauses

   In SQL, GROUP BY comes before HAVING. The correct order is SELECT -> FROM -> GROUP BY -> HAVING.

2. Step 2: Check each option's syntax

   SELECT col1, COUNT(col2) FROM table GROUP BY col1 HAVING COUNT(col2) > 5; correctly places GROUP BY before HAVING and includes aggregate function in HAVING condition.

3. Final Answer:

   SELECT col1, COUNT(col2) FROM table GROUP BY col1 HAVING COUNT(col2) > 5; -> Option B

4. Quick Check:

   GROUP BY before HAVING [OK]

Hint: GROUP BY must come before HAVING in SQL syntax [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Using WHERE with aggregate functions
• Omitting GROUP BY when using HAVING

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

SELECT region, SUM(amount) AS total_sales FROM sales GROUP BY region HAVING total_sales > 1000;

medium

A. All regions with sales less than or equal to 1000

B. Syntax error because alias can't be used in HAVING

C. All regions with total sales greater than 1000

D. Empty result because HAVING filters rows

Solution

1. Step 1: Understand the query logic

   The query groups rows by region and sums amount as total_sales.

2. Step 2: Apply HAVING condition

   It filters groups where total_sales is greater than 1000. Using alias in HAVING is allowed in MySQL.

3. Final Answer:

   All regions with total sales greater than 1000 -> Option C

4. Quick Check:

   HAVING filters groups by aggregate result [OK]

Hint: HAVING filters groups by aggregate values like SUM [OK]

Common Mistakes:

• Thinking HAVING filters rows, not groups
• Assuming alias can't be used in HAVING in MySQL
• Confusing HAVING with WHERE

4. Identify the error in this query:

SELECT department, COUNT(*) FROM employees HAVING COUNT(*) > 10;

medium

A. Missing GROUP BY clause before HAVING

B. COUNT(*) cannot be used in HAVING

C. HAVING should be replaced with WHERE

D. No error, query is correct

Solution

1. Step 1: Check for GROUP BY usage

   The query uses HAVING with COUNT(*) but does not group by department. HAVING requires GROUP BY to define groups.

2. Step 2: Understand HAVING requirements

   HAVING filters groups after aggregation. Without GROUP BY, groups are not defined, causing an error.

3. Final Answer:

   Missing GROUP BY clause before HAVING -> Option A

4. Quick Check:

   HAVING needs GROUP BY to filter groups [OK]

Hint: Always use GROUP BY before HAVING to define groups [OK]

Common Mistakes:

• Using HAVING without GROUP BY
• Replacing HAVING with WHERE incorrectly
• Assuming COUNT(*) can't be in HAVING

5. You have a products table with columns category, price, and stock. Write a query to find categories where the average price is above 50 and total stock is at least 100.
Which query correctly uses HAVING to achieve this?

hard

A. SELECT category FROM products GROUP BY category WHERE AVG(price) > 50 AND SUM(stock) >= 100;

B. SELECT category FROM products HAVING AVG(price) > 50 AND SUM(stock) >= 100 GROUP BY category;

C. SELECT category FROM products WHERE AVG(price) > 50 AND SUM(stock) >= 100 GROUP BY category;

D. SELECT category FROM products GROUP BY category HAVING AVG(price) > 50 AND SUM(stock) >= 100;

Solution

1. Step 1: Recall correct clause order

   The correct order is SELECT -> FROM -> GROUP BY -> HAVING. HAVING filters groups based on aggregate conditions.

2. Step 2: Analyze each option

   SELECT category FROM products GROUP BY category HAVING AVG(price) > 50 AND SUM(stock) >= 100; correctly groups by category and uses HAVING with aggregate functions AVG(price) and SUM(stock).

3. Final Answer:

   SELECT category FROM products GROUP BY category HAVING AVG(price) > 50 AND SUM(stock) >= 100; -> Option D

4. Quick Check:

   HAVING filters grouped data with aggregate conditions [OK]

Hint: Use HAVING after GROUP BY for aggregate filters [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Using WHERE with aggregate functions
• Omitting GROUP BY when using HAVING