sdfor sdfor - 3 months ago 23
SQL Question

Can I reuse a calculated field in a SELECT query?

Is there a way to reuse a calculated field within a mysql statement. I get the error "unknown column total_sale" for:

SELECT
s.f1 + s.f2 as total_sale,
s.f1 / total_sale as f1_percent
FROM sales s


or do I have to repeat the calculation, which would make for a very long SQL statement if I added all the calculations I need.

SELECT
s.f1 + s.f2 as total_sale,
s.f1 / (s.f1 + s.f2) as f1_percent
FROM sales s


of course I can do all the calculations in my php program.

Answer

Yes, you can reuse variables. This is how you do it:

SELECT 
    @total_sale := s.f1 + s.f2 as total_sale, 
    s.f1 / @total_sale as f1_percent
FROM sales s

Read more about it here: http://dev.mysql.com/doc/refman/5.0/en/user-variables.html

[Note: This behavior is undefined. According to the MySQL docs:]

As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed.