First query execution in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this simple SELECT query?

Consider a table employees with columns id and name. The table has 3 rows:
1, 'Alice'
2, 'Bob'
3, 'Charlie'

What will be the result of this query?

SELECT name FROM employees WHERE id = 2;

MySQL

SELECT name FROM employees WHERE id = 2;

A []

B [{"name": "Bob"}]

C [{"name": "Alice"}]

D [{"id": 2}]

💡 Hint

Look for the row where the id is exactly 2.

The query selects the name from employees where id equals 2. Only Bob has id 2, so the output is Bob's name.

📝 Syntax

intermediate

Which query will cause a syntax error?

Identify which of the following MySQL queries will cause a syntax error when executed.

A SELECT * FROM employees WHERE id = 1;

B SELECT COUNT(*) FROM employees;

C SELECT id, name FROM employees;

D SELECT name FROM employees WHERE id == 1;

💡 Hint

MySQL uses a single equals sign for comparison.

MySQL uses a single '=' for comparison, not '=='. Option D will cause a syntax error.

❓ query_result

advanced

What is the output of this aggregate query?

Given the sales table with columns product and amount containing:
('apple', 10), ('banana', 5), ('apple', 15), ('banana', 10)

What is the result of this query?

SELECT product, SUM(amount) AS total FROM sales GROUP BY product ORDER BY total DESC;

MySQL

SELECT product, SUM(amount) AS total FROM sales GROUP BY product ORDER BY total DESC;

A [{"product": "apple", "total": 10}, {"product": "banana", "total": 5}]

B [{"product": "banana", "total": 15}, {"product": "apple", "total": 25}]

C [{"product": "apple", "total": 25}, {"product": "banana", "total": 15}]

D [{"product": "apple", "total": 15}, {"product": "banana", "total": 10}]

💡 Hint

SUM adds amounts per product, ORDER BY sorts descending by total.

The query sums amounts per product and orders results by total descending. Apple total is 25, banana total is 15.

🔧 Debug

advanced

Why does this query return no rows?

Given a table users with a column username containing 'alice', 'bob', 'charlie', why does this query return no rows?

SELECT * FROM users WHERE username = 'Alice';

A Because the username 'Alice' does not exist exactly with uppercase A.

B Because the query syntax is incorrect.

C Because MySQL string comparison is case-sensitive by default.

D Because the users table is empty.

💡 Hint

Check the exact spelling and case of usernames in the table.

The username 'alice' is lowercase in the table, but the query searches for 'Alice' with uppercase A, so no match is found.

❓ optimization

expert

Which query is more efficient for counting rows?

You want to count how many rows are in the orders table. Which query is more efficient in MySQL?

A SELECT COUNT(*) FROM orders;

B SELECT COUNT(1) FROM orders;

C SELECT COUNT(order_id) FROM orders;

D SELECT COUNT('order_id') FROM orders;

💡 Hint

COUNT(*) counts all rows regardless of NULLs.

COUNT(*) is optimized by MySQL to count rows efficiently. COUNT(1) is similar but less clear. COUNT(order_id) counts only non-NULL order_id values. COUNT('order_id') counts all rows because it's a constant string, but less efficient.