Column aliases with AS in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a column alias in SQL?

A column alias is a temporary name given to a column in the result set of a query to make the output easier to read or understand.

beginner

How do you use the AS keyword in SQL?

You use AS to assign a new name (alias) to a column or expression in the SELECT statement, like: SELECT column_name AS alias_name FROM table_name;

beginner

Why would you use a column alias in a query?

To make column names more readable, to rename complex expressions, or to prepare output for reports or user interfaces.

intermediate

Can you use column aliases without the AS keyword?

Yes, in many SQL dialects you can write SELECT column_name alias_name FROM table_name; but using AS improves clarity.

beginner

Example: Write a query to select the column 'price' from 'products' table and rename it to 'Cost'.

SELECT price AS Cost FROM products;

What does the AS keyword do in SQL?

A Deletes a column from a table

B Creates a new table

C Assigns a temporary name to a column or expression

D Filters rows in a query

AS is used to give a temporary name (alias) to a column or expression in the query result.

Which of the following is a correct way to alias a column named 'salary' as 'Income'?

A SELECT salary AS Income FROM employees;

B SELECT salary Income FROM employees;

C SELECT salary = Income FROM employees;

D SELECT salary INTO Income FROM employees;

The correct syntax uses AS to assign the alias: SELECT salary AS Income FROM employees;

Is the AS keyword mandatory when creating a column alias?

A Only mandatory in SELECT * queries

B Yes, always mandatory

C Only mandatory for table aliases

D No, it is optional in many SQL dialects

AS is optional in many SQL dialects but recommended for clarity.

What will this query return? SELECT price AS Cost FROM products;

A A column named 'Cost' with data from 'price'

B All columns from products table

C An error because AS is not allowed

D A column named 'price' with original data

The query returns the 'price' column data but names the column 'Cost' in the output.

Why might you want to use a column alias?

A To change the data type of a column

B To make output easier to read or understand

C To permanently rename a column in the table

D To delete a column from the table

Column aliases help make query results clearer without changing the actual table.

Explain what a column alias is and how to use the AS keyword in SQL.

Think about how you rename columns temporarily in query results.

Describe why column aliases are useful when writing SQL queries.

Consider how output looks to users or reports.

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