Use CASE condition as result value

Question

I would like to get the greatest value from a set of fields but some of them can be NULL so I can't use GREATEST as it is and I have to transform NULL values to arbitrary values (-1 in this case). However in the result I still have to get NULL value if all the fields were NULL.

SELECT 
CASE WHEN GREATEST(IFNULL(f1, -1), IFNULL(f2, -1)) = -1 THEN 
   NULL 
ELSE 
   GREATEST(IFNULL(f1, -1), IFNULL(f2, -1)) 
END

My problem is, I have twice the calculation of the greatest value so I wondered if there is a way to "store" the value so that mySQL doesn't have to evaluate twice.

Or if there is a better/ more proper way to do what i want.

Thanks.

Answer 1

You may use NULLIF function:

NULLIF(GREATEST(IFNULL(f1, -1), IFNULL(f2, -1)), -1)

It would return NULL if they equal, or the value of the first argument otherwise.

References:

• http://dev.mysql.com/doc/refman/5.6/en/control-flow-functions.html#function_nullif