SQL — Aliases

What is an Alias?

An alias gives a column or table a temporary different name using the AS keyword. It makes queries easier to read and is required when you need to reference a calculated column by name.

Column Aliases

SELECT
  first_name AS "First Name",
  last_name AS "Last Name",
  salary AS "Annual Salary"
FROM employees;

The output headers now say "First Name", "Last Name", and "Annual Salary" instead of the raw column names.

Aliases with Calculations

This is where aliases really shine — you can't reference a calculation without one:

SELECT
  product_name,
  price,
  quantity,
  price * quantity AS total_value
FROM inventory;

Without AS total_value, you'd have no way to refer to price * quantity in a WHERE or ORDER BY clause.

Table Aliases

When joining tables, aliases save typing and prevent ambiguity:

SELECT
  e.first_name,
  e.last_name,
  d.department_name
FROM employees AS e
JOIN departments AS d ON e.department_id = d.id;

You can omit the AS keyword — employees e works the same as employees AS e. Most developers skip it for tables but keep it for columns.

Using Aliases in ORDER BY

SELECT
  first_name,
  salary * 12 AS annual_salary
FROM employees
ORDER BY annual_salary DESC;

Aliases in Subqueries

SELECT avg_salary.department, avg_salary.avg_sal
FROM (
  SELECT department_id, AVG(salary) AS avg_sal
  FROM employees
  GROUP BY department_id
) AS avg_salary
WHERE avg_salary.avg_sal > 75000;

Rules for Aliases

• Wrap the alias in double quotes if it contains spaces or special characters
• Aliases only exist for the duration of the query
• You cannot use a column alias in a WHERE clause (use it in HAVING or ORDER BY instead)

Common Mistakes

-- Wrong: can't use alias in WHERE
SELECT salary AS annual_salary
FROM employees
WHERE annual_salary > 50000;

-- Right: repeat the expression or use HAVING
SELECT salary AS annual_salary
FROM employees
WHERE salary > 50000;

Practice

1. Write a query that aliases first_name and last_name as "Full Name" by concatenating them
2. Calculate monthly salary (salary / 12) and give it a readable alias
3. Use table aliases in a three-way JOIN