MySQL Alias

 MySQL Alias

Summary: in this tutorial, you will learn how to use MySQL alias to improve the readability of the queries.

MySQL supports two kinds of aliases which are known as column alias and table alias.

MySQL alias for columns

Sometimes, column names are so technical that make the query’s output very difficult to understand. To give a column a descriptive name, you can use a column alias.

The following statement illustrates how to use the column alias:

SELECT 
   [column_1 | expression] AS descriptive_name
FROM table_name;

To assign an alias to a column, you use the AS keyword followed by the alias. If the alias contains spaces, you must quote it as the following:

SELECT 
   [column_1 | expression] AS `descriptive name`
FROM 
   table_name;

Because the AS the keyword is optional, you can omit it in the statement. Note that you can also give an expression of an alias.

Let’s look at the employees table in the sample database.

The following query selects the first names and last names of employees. It uses the CONCAT_WS() function to concatenate first name and last name into full name.

SELECT 
    CONCAT_WS(', ', lastName, firstname)
FROM
    employees;

The column heading is quite difficult to read. To solve this, you can assign the column heading of the output a column alias as shown in the following query:

SELECT
   CONCAT_WS(', ', lastName, firstname) AS `Full name`
FROM
   employees;

In MySQL, you can use the column alias in the ORDER BY, GROUP BY and HAVINGclauses to refer to the column.

The following query uses the column alias in the ORDER BY clause to sort the employee’s full names alphabetically:

SELECT
	CONCAT_WS(', ', lastName, firstname) `Full name`
FROM
	employees
ORDER BY
	`Full name`;

The following statement selects the orders whose total amount is greater than 60000. It uses column aliases in GROUP BY  HAVING clauses.

SELECT
	orderNumber `Order no.`,
	SUM(priceEach * quantityOrdered) total
FROM
	orderDetails
GROUP BY
	`Order no.`
HAVING
	total > 60000;

Notice that you cannot use a column alias in the WHERE clause. The reason is that when MySQL evaluates the WHERE clause, the values of columns specified in the SELECT clause are not been evaluated yet.

MySQL alias for tables

You can use an alias to give a table a different name. You assign a table an alias by using the AS keyword as the following syntax:

table_name AS table_alias

The alias for a table is called a table alias. Like the column alias, the AS keyword is optional so you can omit it.

This query shows how to assign the employees table alias as e:

SELECT * FROM employees e;

Once a table is assigned an alias, you can refer to the table columns using the following syntax:

table_alias.column_name

For example:

SELECT 
    e.firstName, 
    e.lastName
FROM
    employees e
ORDER BY e.firstName;

The table aliases are often used in the statement that contains INNER JOIN, LEFT JOIN, RIGHT JOIN clauses and in subqueries.

Let’s look at the customers and orders tables:

Both tables have the same column name:customerNumber.Without using the table alias to qualify the customerNumber column, you will get an error message like:

Error Code: 1052. Column 'customerNumber' in on clause is ambiguous

To avoid this error, you use a table alias to qualify the customerNumber column:

SELECT
	customerName,
	COUNT(o.orderNumber) total
FROM
	customers c
INNER JOIN orders o ON c.customerNumber = o.customerNumber
GROUP BY
	customerName
ORDER BY
	total DESC;

The query above selects the customer name and the number of orders from the customers and orders tables. It uses c as a table alias for the customers table and o as a table alias for the orders table. The columns in the customers and orders tables are referred to via the table aliases.

If you do not use the alias in the query above, you have to use the table name to refer to its columns, which makes the query lengthy and less readable as the following:

SELECT
	customers.customerName,
	COUNT(orders.orderNumber) total
FROM
	customers
INNER JOIN orders ON customers.customerNumber = orders.customerNumber
GROUP BY
	customerName
ORDER BY
	total DESC

In this tutorial, you have learned how to use MySQL aliases including column and table aliases.