**How to Correctly Alias Calculated Fields in MySQL and Avoid \'Unknown Column\' Errors?**

Aliasing Fields in MySQL: Avoiding Unknown Column Errors

As part of your MySQL query, you may have encountered the "unknown column" error while attempting to alias a field using the following syntax:

SELECT SUM(field1 + field2) AS col1, col1 + field3 AS col3 from core

This error occurs because the AS keyword aliases the calculated value, not the field itself. To resolve this issue, MySQL provides a different approach that allows you to create an alias for the field while performing calculations simultaneously.

Using the following syntax, you can alias a calculated field:

select @alias:= SUM(field1 + field2), @alias+1 from core

In this example, @alias is the alias assigned to the calculated value of the sum of field1 and field2.

It's important to note, however, that assigning a value to a user variable and reading it within the same statement can be problematic, according to the MySQL 5.6 documentation. While it may produce the intended result in some cases, the order of evaluation for expressions involving user variables is undefined.

Therefore, proceed with caution when using this technique.

The above is the detailed content of **How to Correctly Alias Calculated Fields in MySQL and Avoid \'Unknown Column\' Errors?**. For more information, please follow other related articles on the PHP Chinese website!