In MySQL, an alias (or alias name) is a temporary name used to rename a table or column for the duration of a query. Aliases are helpful for simplifying queries, making results easier to read, and improving the clarity of your SQL statements. They are especially useful when working with complex queries or when combining multiple tables.

Syntax for Column Aliases

To create an alias for a column, use the following syntax:

SELECT column_name AS alias_name
FROM table_name;

• column_name: The name of the column you want to alias.
• alias_name: The new name you want to assign to the column for the query result.

Syntax for Table Aliases

To create an alias for a table, use the following syntax:

SELECT column_name
FROM table_name AS alias_name;

• table_name: The name of the table you want to alias.
• alias_name: The new name you want to assign to the table for the query.

Examples

1. Column Alias

   Suppose you have a table named employees with a column first_name, and you want to display it as Employee Name in your results:

   SELECT first_name AS 'Employee Name'
   FROM employees;
   

   This query will return the first_name column but label it as Employee Name in the result set.

2. Table Alias

   Table aliases are often used to simplify queries involving multiple tables, especially when joining tables:

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

   Here, employees is aliased as e. This allows you to refer to the employees table as e in the query.

3. Alias in Joins

   Aliases are useful when performing joins to make the query more readable:

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

   In this query, employees is aliased as e and departments as d. This simplifies the query and makes it easier to read, especially with multiple tables.

4. Alias for Aggregate Functions

   You can also use aliases for aggregate functions to label the results:

   SELECT SUM(salary) AS total_salary
   FROM employees;
   

   This query calculates the total salary and labels the result as total_salary.

5. Alias in Subqueries

   When using subqueries, aliases can help clarify the results:

   SELECT employee_name
   FROM (SELECT first_name AS employee_name
         FROM employees) AS subquery;
   

   In this example, the subquery is aliased as subquery, and the first_name column is aliased as employee_name.

Best Practices

• Clarity: Use meaningful aliases to make your queries more readable and self-explanatory.
• Consistency: Be consistent with alias naming conventions to maintain code readability and manageability.
• Avoid Reserved Words: Avoid using SQL reserved words or special characters as aliases to prevent errors.