COUNT function in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

COUNT with NULL values

Given the table Employees with columns id, name, and manager_id where some manager_id values are NULL, what is the result of this query?

SELECT COUNT(manager_id) FROM Employees;

MySQL

CREATE TABLE Employees (id INT, name VARCHAR(50), manager_id INT);
INSERT INTO Employees VALUES (1, 'Alice', NULL), (2, 'Bob', 1), (3, 'Charlie', NULL), (4, 'Diana', 2);

A 2

B 4

C 3

D 1

💡 Hint

COUNT(column) counts only non-NULL values in that column.

COUNT(manager_id) counts only rows where manager_id is not NULL. There are 2 such rows: Bob (manager_id=1) and Diana (manager_id=2).

❓ query_result

intermediate

COUNT(*) vs COUNT(column)

Consider the table Orders with columns order_id, customer_id, and shipped_date. Some shipped_date values are NULL. What is the output of this query?

SELECT COUNT(*), COUNT(shipped_date) FROM Orders;

MySQL

CREATE TABLE Orders (order_id INT, customer_id INT, shipped_date DATE);
INSERT INTO Orders VALUES (1, 101, '2024-01-01'), (2, 102, NULL), (3, 103, '2024-01-03'), (4, 104, NULL);

A 4, 2

B 2, 4

C 4, 4

D 2, 2

💡 Hint

COUNT(*) counts all rows, COUNT(column) counts only non-NULL values in that column.

COUNT(*) counts all 4 rows. COUNT(shipped_date) counts only the 2 rows where shipped_date is not NULL.

📝 Syntax

advanced

Invalid use of COUNT in WHERE clause

Which option shows the correct way to count rows with a condition on the count itself?
Given a table Sales with product_id and quantity, you want to find products sold more than 10 times.
Which query is valid?

A SELECT product_id FROM Sales WHERE SUM(quantity) > 10;

B SELECT product_id FROM Sales GROUP BY product_id WHERE COUNT(quantity) > 10;

C SELECT product_id FROM Sales GROUP BY product_id HAVING COUNT(quantity) > 10;

D SELECT product_id FROM Sales WHERE COUNT(quantity) > 10;

💡 Hint

Conditions on aggregate functions must use HAVING, not WHERE.

COUNT is an aggregate function and cannot be used in WHERE. HAVING filters groups after aggregation.

❓ optimization

advanced

Optimizing COUNT with large tables

You have a large table Visits with millions of rows. You want to count how many visits happened today. Which query is the most efficient?

A SELECT COUNT(visit_date) FROM Visits WHERE visit_date = CURDATE();

B SELECT COUNT(DISTINCT visit_date) FROM Visits WHERE visit_date = CURDATE();

C SELECT COUNT(*) FROM Visits;

D SELECT COUNT(*) FROM Visits WHERE visit_date = CURDATE();

💡 Hint

COUNT(*) is optimized to count rows quickly. Filtering by date reduces scanned rows.

Option D counts all rows matching today's date efficiently. COUNT(visit_date) is similar but COUNT(*) is preferred for counting rows. Option D counts all rows ignoring date. Option D counts distinct dates, not visits.

🧠 Conceptual

expert

COUNT with JOIN and NULLs

Given two tables:

• Authors(id, name)
• Books(id, author_id, title)

You want to count how many books each author has, including authors with zero books.
Which query produces the correct result?

A SELECT Authors.name, COUNT(Books.id) FROM Authors JOIN Books ON Authors.id = Books.author_id GROUP BY Authors.name;

B SELECT Authors.name, COUNT(Books.id) FROM Authors LEFT JOIN Books ON Authors.id = Books.author_id GROUP BY Authors.name;

C SELECT Authors.name, COUNT(*) FROM Authors LEFT JOIN Books ON Authors.id = Books.author_id GROUP BY Authors.name;

D SELECT Authors.name, COUNT(Books.author_id) FROM Authors RIGHT JOIN Books ON Authors.id = Books.author_id GROUP BY Authors.name;

💡 Hint

Use LEFT JOIN to include authors without books. COUNT(column) ignores NULLs.

LEFT JOIN keeps all authors. COUNT(Books.id) counts books per author. INNER JOIN (option B) excludes authors with zero books. COUNT(*) counts all joined rows, including NULLs from Books in LEFT JOIN, which can miscount.

Practice

1. What does the COUNT(*) function do in MySQL?

easy

A. Counts all rows in the table, including those with NULL values

B. Counts only rows where all columns are NOT NULL

C. Counts only rows with NULL values in any column

D. Counts the number of columns in the table

Solution

1. Step 1: Understand COUNT(*) behavior

   The COUNT(*) function counts every row in the table regardless of NULL values.

2. Step 2: Compare with other COUNT variants

   Unlike COUNT(column), which counts only non-NULL values in that column, COUNT(*) counts all rows.

3. Final Answer:

   Counts all rows in the table, including those with NULL values -> Option A

4. Quick Check:

   COUNT(*) counts all rows [OK]

Hint: COUNT(*) counts every row, NULL or not [OK]

Common Mistakes:

• Thinking COUNT(*) ignores rows with NULL
• Confusing COUNT(*) with COUNT(column)
• Assuming COUNT(*) counts columns

2. Which of the following is the correct syntax to count non-NULL values in the column age from the table users?

easy

A. SELECT COUNT(*) FROM users;

B. SELECT COUNT(age) FROM users WHERE age != NULL;

C. SELECT COUNT(age) FROM users WHERE age = NULL;

D. SELECT COUNT(age) FROM users;

Solution

1. Step 1: Identify correct COUNT syntax for non-NULL values

   COUNT(column) counts only non-NULL values in that column, so COUNT(age) counts non-NULL ages.

2. Step 2: Check WHERE clause correctness

   SELECT COUNT(*) FROM users; uses COUNT(*) which counts all rows. Using WHERE age IS NOT NULL is valid but unnecessary with COUNT(age). Options B and C use incorrect NULL comparisons.

3. Final Answer:

   SELECT COUNT(age) FROM users; -> Option D

4. Quick Check:

   COUNT(column) counts non-NULL values [OK]

Hint: Use COUNT(column) to count non-NULL values directly [OK]

Common Mistakes:

• Using WHERE age = NULL instead of IS NOT NULL
• Using COUNT(*) to count non-NULL values
• Using != NULL which is invalid in SQL

3. Given the table orders with 5 rows where the discount column has 3 NULL values and 2 non-NULL values, what will this query return?

SELECT COUNT(discount) FROM orders;

medium

A. 5

B. 3

C. 2

D. 0

Solution

1. Step 1: Understand COUNT(column) counts non-NULL values

   The query counts only rows where discount is NOT NULL.

2. Step 2: Apply to given data

   There are 2 non-NULL discount values, so the count is 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   COUNT(discount) = 2 [OK]

Hint: COUNT(column) ignores NULLs, counts only real values [OK]

Common Mistakes:

• Counting total rows instead of non-NULL values
• Confusing NULL count with non-NULL count
• Assuming COUNT(column) counts NULLs

4. Identify the error in this query:

SELECT COUNT(name) FROM employees WHERE name = NULL;

medium

A. Using COUNT(name) instead of COUNT(*)

B. Incorrect NULL comparison with '=' instead of IS NULL

C. Missing GROUP BY clause

D. COUNT cannot be used with WHERE clause

Solution

1. Step 1: Check WHERE clause NULL comparison

   In SQL, NULL cannot be compared with '='; use IS NULL instead.

2. Step 2: Validate COUNT usage

   COUNT(name) is valid and WHERE clause is allowed; no GROUP BY needed here.

3. Final Answer:

   Incorrect NULL comparison with '=' instead of IS NULL -> Option B

4. Quick Check:

   Use IS NULL for NULL checks [OK]

Hint: Use IS NULL, not = NULL, to check for NULL values [OK]

Common Mistakes:

• Using '=' to compare NULL values
• Thinking COUNT can't be combined with WHERE
• Adding unnecessary GROUP BY

5. You have a table sales with columns region and amount. You want to find how many sales records exist for each region, including regions with zero sales. Which query will correctly show all regions with their sales count, including zero counts?

hard

A. SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region;

B. SELECT region, COUNT(*) FROM sales GROUP BY region;

C. SELECT region, COUNT(amount) FROM sales GROUP BY region;

D. SELECT region, COUNT(amount) FROM sales WHERE amount IS NOT NULL GROUP BY region;

Solution

1. Step 1: Understand requirement to include regions with zero sales

   To include regions with zero sales, we must use a LEFT JOIN from all regions to sales.

2. Step 2: Analyze query options

   SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region; uses LEFT JOIN from regions table to sales, counting sales per region including zero counts. Options A, B, D only count existing sales records, missing zero counts.

3. Final Answer:

   SELECT r.region, COUNT(s.amount) FROM regions r LEFT JOIN sales s ON r.region = s.region GROUP BY r.region; -> Option A

4. Quick Check:

   LEFT JOIN + COUNT(column) includes zero counts [OK]

Hint: Use LEFT JOIN with COUNT(column) to include zero counts [OK]

Common Mistakes:

• Using only sales table, missing regions with zero sales
• Using COUNT(*) which counts joined rows, not zero
• Not using LEFT JOIN to include all regions