Question

I would like to extract a value or its inverse from database. I would like to find a way to do the if statement in SQL, not in PHP.

My table looks like : id, rate, from_value, to_value

I have $from_value and $to_value variables, and would like to extract the corresponding rate value or its inverse.

I would like to

SELECT `rate` if `from_value` = $from_value and `to_value` = $to_value

or

SELECT (1/`rate`) if `from_value` = $to_value AND `to_value` = $from_value 

with only one request.

I have searched a lot of ways to do that with CASE, IF, but I didn’t find a working solution.
I probably missed something. Thanks

Answer 1

Here is a portable method using CASE. I have it returning NULL if neither condition is met.

SELECT 
  CASE
    WHEN `from_value` = $from_value AND `to_value` = $to_value THEN rate
    WHEN `from_value` = $to_value AND `to_value` = $from_value THEN (1.0 / rate)
    ELSE NULL
  END AS rate
FROM tbl