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

Quote SQL aliases correctly to avoid calculation errors

In SQL, aliases allow assigning temporary names to columns or expressions, making queries more concise and readable. However, you may encounter "unknown column" errors when referencing the alias in subsequent calculations.

Consider the following example:

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

When trying to execute this query, you may encounter an error because column my_num*5 is not recognized in the expression my_num. To fix this, you need to put the alias in the subquery:

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

You can explicitly reference a column that has been given an alias by including my_num in a SELECT statement, such as (SELECT my_num). This ensures that the database understands which column is used in the calculation.

This technique is useful when you need to reuse an alias multiple times in a complex query. By aliasing an expression once and referencing it in the subquery, you avoid potential errors and improve the maintainability of your code.

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