How Can I Reuse SQL Aliases in Calculations to Avoid 'Unknown Column' Errors?

Effective use of SQL aliases to simplify calculations

When writing SQL queries, clever use of aliases can simplify complex calculations. However, it is crucial to use aliases correctly, otherwise it is error-prone.

Question:

You may encounter an "unknown column" error when trying to use the alias in subsequent calculations within the same query. For example, the following query will fail:

SELECT 10 AS my_num, my_num*5 AS another_number
FROM table

Solution:

To solve this problem, you can use nested SELECT statements to include reused aliases:

SELECT 10 AS my_num,
       (SELECT my_num) * 5 AS another_number
FROM table

This approach allows you to reference the alias like a subquery, making the calculation efficient. You can use this technique with as many aliases as needed to simplify complex expressions.

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