AS (Alias) in PostgreSQL

The AS keyword in PostgreSQL is used to assign aliases to columns or tables, making query results more readable.

1. Aliasing a Column Name

SELECT salary AS total_salary FROM employees;

✅ The output column will be displayed as total_salary instead of salary.

2. Aliasing Multiple Columns

SELECT first_name AS fname, last_name AS lname FROM employees;

✅ The output columns will be fname and lname instead of first_name and last_name.

3. Using AS in Calculations

SELECT SUM(salary) AS total_salary FROM employees;

✅ The result column will be named total_salary.

4. Aliasing a Table Name

SELECT e.first_name, e.last_name FROM employees AS e;

✅ The alias e is used instead of writing employees each time.

5. Using AS with JOIN

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

✅ Aliases e and d make the query shorter and clearer.

6. Aliasing Subquery Columns

SELECT department, AVG(salary) AS avg_salaryFROM employeesGROUP BY department;

✅ The column name will be avg_salary instead of AVG(salary).

Notes

• The AS keyword is optional in PostgreSQL for column aliases:

  SELECT first_name fname FROM employees;
  

• For table aliases, AS is not allowed, but you can still use an alias:

  SELECT e.first_name FROM employees e; -- ✅ WorksSELECT e.first_name FROM employees AS e; -- ❌ Error