In MySQL, an alias is a temporary name assigned to a table or column for the duration of a SQL query. Aliases are used to make column names or table names more readable or to provide a shorter name for convenience in your SQL statements. They can be useful when working with complex queries, subqueries, or when joining multiple tables.

Here’s how to use aliases in MySQL:

1. Column Aliases:
   You can assign aliases to column names in the SELECT statement to give them more meaningful or concise names:

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

In this example, “First Name” and “Last Name” are aliases for the first_name and last_name columns, respectively.

2. Table Aliases:
   Table aliases are used when you need to reference the same table multiple times in a query, typically when performing self-joins or using subqueries. Table aliases are also used for brevity:

   SELECT e1.first_name, e2.last_name
   FROM employees AS e1
   JOIN employees AS e2 ON e1.manager_id = e2.employee_id;

Here, e1 and e2 are aliases for the employees table, allowing you to distinguish between the two instances of the same table.

3. Using Aliases in Calculations:
   Aliases can also be used in calculations or expressions within the SELECT statement:

   SELECT product_name, unit_price * quantity AS total_price
   FROM order_details;

Here, total_price is an alias for the result of the multiplication, making it easier to understand the purpose of the column.

4. Subqueries:
   Aliases can be particularly useful in subqueries to make the code more readable:

   SELECT first_name, last_name
   FROM employees
   WHERE department_id = (
       SELECT department_id
       FROM departments
       WHERE department_name = 'Sales'
   ) AS sales_department;

In this case, sales_department is an alias for the result of the subquery.

Aliases help improve the readability and maintainability of SQL code, especially in complex queries. They are not mandatory but are considered good practice, especially in larger database systems where queries can become intricate.

Aliases come in handy when:

1. You’re dealing with multiple tables in a query.
2. You’re using functions in your query.
3. Column names are long or hard to understand.
4. You need to combine two or more columns together.