Column aliases with AS in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of column alias in SELECT

What is the output of this SQL query?

SELECT first_name AS name FROM employees;

Assume the employees table has these rows:

first_name
John
Jane
Bob

SQL

SELECT first_name AS name FROM employees;

A

name
John
Jane
Bob

B

name
first_name
first_name

C

first_name
John
Jane
Bob

D Error: Invalid column alias

💡 Hint

The AS keyword renames the column in the output.

Using AS changes the column header in the result to the alias name.

❓ query_result

intermediate

Multiple column aliases in SELECT

What is the output of this SQL query?

SELECT first_name AS fname, last_name AS lname FROM employees;

Assume employees table rows:

first_name | last_name
Alice      | Smith
Bob        | Brown

SQL

SELECT first_name AS fname, last_name AS lname FROM employees;

A

first_name | last_name
Alice      | Smith
Bob        | Brown

B

fname | lname
Alice | Smith
Bob   | Brown

C

fname | last_name
Alice | Smith
Bob   | Brown

D Error: AS keyword used incorrectly

💡 Hint

Each column alias changes the column header separately.

Both columns are renamed in the output using AS.

📝 Syntax

advanced

Identify the syntax error with AS alias

Which option contains a syntax error when using column alias with AS?

A SELECT salary AS monthly_salary FROM payroll;

B SELECT city AS location FROM addresses;

C SELECT age AS FROM users;

D SELECT price AS cost FROM products;

💡 Hint

AS must be followed by a valid alias name.

Option C misses the alias name after AS, causing syntax error.

❓ query_result

advanced

Effect of alias on ORDER BY clause

What is the output order of this query?

SELECT first_name AS name FROM employees ORDER BY name DESC;

Employees table:

first_name
Anna
Zack
Mike

SQL

SELECT first_name AS name FROM employees ORDER BY name DESC;

A

name
Zack
Mike
Anna

B

name
Anna
Mike
Zack

C

first_name
Anna
Zack
Mike

D Error: Cannot use alias in ORDER BY

💡 Hint

Aliases can be used in ORDER BY to sort results.

The query orders by the alias 'name' in descending order.

🧠 Conceptual

expert

Why use column aliases with AS?

Which is the best reason to use column aliases with AS in SQL queries?

A To speed up query execution by indexing aliases

B To change the data type of a column in the table

C To permanently rename columns in the database schema

D To rename columns in the output for clarity or formatting

💡 Hint

Think about what aliases affect: output or schema?

Aliases only rename columns in the query result, not the table.

Practice

1. What is the purpose of using AS in an SQL query when selecting columns?

easy

A. To filter rows based on a condition

B. To rename the column in the output result

C. To join two tables together

D. To delete a column from the table

Solution

1. Step 1: Understand the role of AS in SQL SELECT

   The AS keyword is used to give a temporary name (alias) to a column or expression in the output.

2. Step 2: Identify what AS does in the query result

   It changes how the column header appears, making the output easier to read or understand.

3. Final Answer:

   To rename the column in the output result -> Option B

4. Quick Check:

   Column alias = rename column [OK]

Hint: AS renames columns in output, not data itself [OK]

Common Mistakes:

• Thinking AS filters rows
• Confusing AS with JOIN
• Believing AS deletes columns

2. Which of the following SQL queries correctly uses AS to alias the column price as cost?

easy

A. SELECT price IS cost FROM products;

B. SELECT price TO cost FROM products;

C. SELECT price AS cost FROM products;

D. SELECT price = cost FROM products;

Solution

1. Step 1: Review correct syntax for column aliasing

   The correct syntax uses AS between the original column name and the alias.

2. Step 2: Check each option for correct usage

   Only SELECT price AS cost FROM products; uses AS properly: price AS cost. Others use invalid keywords or operators.

3. Final Answer:

   SELECT price AS cost FROM products; -> Option C

4. Quick Check:

   Correct alias syntax uses AS [OK]

Hint: Use AS between column and alias name [OK]

Common Mistakes:

• Using TO instead of AS
• Using IS or = instead of AS
• Omitting AS but using invalid syntax

3. Given the table employees with columns first_name and last_name, what will be the output column name of this query?

SELECT first_name AS fname, last_name AS lname FROM employees;

medium

A. fname, lname

B. first_name, last_name

C. first_name AS fname, last_name AS lname

D. fname AS first_name, lname AS last_name

Solution

1. Step 1: Understand alias effect on output column names

   The AS keyword renames the output columns to the alias names given.

2. Step 2: Identify the aliases used in the query

   The columns first_name and last_name are aliased as fname and lname respectively.

3. Final Answer:

   fname, lname -> Option A

4. Quick Check:

   Aliases become output column names [OK]

Hint: Output columns show alias names, not original names [OK]

Common Mistakes:

• Expecting original column names in output
• Confusing alias syntax with output format
• Thinking AS changes table column names

4. Identify the error in this SQL query:

SELECT product_name AS, price FROM products;

medium

A. No error, query is correct

B. AS keyword should be lowercase

C. Comma should be after AS

D. Missing alias name after AS

Solution

1. Step 1: Check the syntax of the alias usage

   The AS keyword must be followed by the alias name immediately.

2. Step 2: Identify the missing alias name

   In the query, product_name AS is incomplete because no alias name is given after AS.

3. Final Answer:

   Missing alias name after AS -> Option D

4. Quick Check:

   AS must be followed by alias name [OK]

Hint: AS must always have a name after it [OK]

Common Mistakes:

• Leaving AS without alias name
• Thinking AS case matters
• Misplacing commas around AS

5. You want to display the total sales amount from a sales table with column amount. Which query correctly sums the amounts and shows the result with the column name Total_Sales?

hard

A. All of the above

B. SELECT SUM(amount) Total_Sales FROM sales;

C. SELECT SUM(amount) AS 'Total_Sales' FROM sales;

D. SELECT SUM(amount) AS Total_Sales FROM sales;

Solution

1. Step 1: Understand aliasing with aggregate functions

   You can alias the result of SUM(amount) using AS or by just placing the alias after the expression.

2. Step 2: Check each option for correctness

   SELECT SUM(amount) AS Total_Sales FROM sales; uses AS with underscore alias, valid.
   SELECT SUM(amount) Total_Sales FROM sales; omits AS but alias after expression is valid.
   SELECT SUM(amount) AS 'Total_Sales' FROM sales; uses AS with quoted alias, also valid in many SQL dialects.

3. Step 3: Confirm all options produce correct output

   All options correctly rename the output column to the desired alias.

4. Final Answer:

   All of the above -> Option A

5. Quick Check:

   Aliases can use AS, omit AS, or use quotes [OK]

Hint: Aliases can be with AS, without AS, or quoted [OK]

Common Mistakes:

• Thinking AS is always required
• Not quoting aliases with spaces
• Confusing alias with column name