SQL Server Column Alias

SQL Server column alias

When you use the SELECT statement to query data from a table, SQL Server uses the column names as the column headings for the output. See the following example:

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SELECT     first_name,     last_name FROM     sales.customers ORDER BY     first_name;

As clearly shown in the output, the first_name and last_name column names were used for the column headings respectively.

To get full names of customers, you can concatenate the first name, space, and the last name using the concatenation  + operator as shown in the following query:

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SELECT     first_name + ' ' + last_name FROM     sales.customers ORDER BY     first_name;

SQL Server returned the full name column as ( No column name) which is not meaningful in this case.

To assign a column or an expression a temporary name during the query execution, you use a column alias.

The following illustrates the column alias syntax:

1	column_name | expression  AS column_alias

In this syntax, you use the AS keyword to separate the column name or expression and the alias.

Because the AS keyword is optional, you can assign an alias to a column as follows:

1	column_name | expression column_alias

Back to the example above, you can rewrite the query using a column alias:

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SELECT     first_name + ' ' + last_name AS full_name FROM     sales.customers ORDER BY     first_name;

Note that if the column alias contains spaces, you need to enclose it in quotation marks as shown in the following example:

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SELECT     first_name + ' ' + last_name AS 'Full Name' FROM     sales.customers ORDER BY     first_name;

The following example shows how to assign an alias to a column:

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SELECT     category_name 'Product Category' FROM     production.categories;

In this example, the product category column alias is much more clear than the category_name column name.

When you assign a column an alias, you can use either the column name or the column alias in the ORDER BY clause as shown in the following example:

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SELECT     category_name 'Product Category' FROM     production.categories ORDER BY     category_name;       SELECT     category_name 'Product Category' FROM     production.categories ORDER BY     'Product Category';

Note that the ORDER BY clause is the very last clause to be processed therefore the column aliases are known at the time of sorting.

SQL Server table alias

A table can be given an alias which is known as correlation name or range variable.

Similar to the column alias, a table alias can be assigned either with or without the AS keyword:

1 2	table_name AS table_alias table_name table_alias

See the following example:

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SELECT     sales.customers.customer_id,     first_name,     last_name,     order_id FROM     sales.customers INNER JOIN sales.orders ON sales.orders.customer_id = sales.customers.customer_id;

In this example, both the customers and the orders tables have a column with the same name customer_id, therefore, you need to refer to the column using the following syntax:

1	table_name.column_name

such as:

1 2	sales.custoners.customer_id sales.orders.customer_id

If you did not do so, SQL server would issue an error.

The query above is quite difficult to read. Fortunately, you can improve its readability by using the table alias as follows:

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SELECT     c.customer_id,     first_name,     last_name,     order_id FROM     sales.customers c INNER JOIN sales.orders o ON o.customer_id = c.customer_id;

In this query, c is the alias for the sales.customers table and o is the alias for the sales.orders table.

When you assign an alias to a table, you must use the alias to refer to the table column. Otherwise, SQL Server will issue an error.

In this tutorial, you have learned how to use the SQL Server alias including column alias and table alias.