Column aliases in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this query with column aliases?

Consider a table employees with columns id, first_name, and last_name. What will be the output column names of this query?

SELECT first_name AS fname, last_name AS lname FROM employees;

MySQL

SELECT first_name AS fname, last_name AS lname FROM employees;

A [{"first_name": "John", "lname": "Doe"}, {"first_name": "Jane", "lname": "Smith"}]

B [{"first_name": "John", "last_name": "Doe"}, {"first_name": "Jane", "last_name": "Smith"}]

C [{"fname": "John", "last_name": "Doe"}, {"fname": "Jane", "last_name": "Smith"}]

D [{"fname": "John", "lname": "Doe"}, {"fname": "Jane", "lname": "Smith"}]

💡 Hint

Column aliases rename the output columns in the result set.

Using AS keyword renames the output columns to the alias names specified. So first_name becomes fname and last_name becomes lname in the output.

📝 Syntax

intermediate

Which query uses correct syntax for column aliasing?

Choose the query that correctly aliases the column salary as monthly_salary in MySQL.

A SELECT salary AS monthly_salary FROM employees;

B SELECT salary monthly_salary FROM employees;

C SELECT salary : monthly_salary FROM employees;

D SELECT salary -> monthly_salary FROM employees;

💡 Hint

MySQL uses AS keyword or just a space for aliasing, but some symbols are invalid.

The correct syntax uses AS keyword or a space. Option A is valid in MySQL but less clear. Option A is the standard and recommended syntax. Options C and D use invalid symbols.

❓ query_result

advanced

What is the output of this query with expression alias?

Given a table orders with columns price and quantity, what is the output of this query?

SELECT price * quantity AS total_cost FROM orders;

MySQL

SELECT price * quantity AS total_cost FROM orders;

A [{"total_cost": 200}, {"total_cost": 450}]

B [{"price * quantity": 200}, {"price * quantity": 450}]

C [{"total_cost": "price * quantity"}, {"total_cost": "price * quantity"}]

D [{"price": 200}, {"price": 450}]

💡 Hint

The alias renames the expression column in the output.

The expression price * quantity is calculated for each row and the result is shown under the column named total_cost.

🔧 Debug

advanced

Why does this query cause an error?

This query tries to alias a column but causes an error:

SELECT first_name AS 'fname' FROM employees;

Why?

A AS keyword cannot be used with column aliases.

B Alias names cannot be shorter than original column names.

C Using single quotes around alias name causes a syntax error in MySQL.

D Column names cannot be aliased in SELECT statements.

💡 Hint

MySQL treats single quotes as string literals, not identifiers.

In MySQL, alias names should not be enclosed in single quotes because single quotes denote strings. Use backticks or no quotes for aliases.

🧠 Conceptual

expert

Which statement about column aliases is TRUE?

Choose the correct statement about column aliases in SQL.

A Column aliases must match the original column names exactly.

B Column aliases can be used to rename columns only in the output, not in WHERE clause.

C Column aliases can be used in WHERE clause directly after aliasing.

D Column aliases permanently rename columns in the database table.

💡 Hint

Think about when aliases exist during query execution.

Column aliases exist only in the output of the SELECT statement and cannot be used in WHERE clause because WHERE is evaluated before SELECT.