sql server CASE: error converting datatype varchar to float

Question

I am trying to add a column dynamically into my query.

I am running the following query:

DECLARE @detMethod varchar(20) = 'MAX(cov)';

WITH myTable as (
                  SELECT user1, (SELECT CASE @detMethod
                                           WHEN 'MAX(cov)' THEN MAX(cov)
                                           ELSE MAX(pcc*cov)
                                        END 
                                ) as method
                  FROM ....
                  WHERE ....
                )

/* some task on myTable */

It is giving me an error on this line:

SELECT user1, (SELECT CASE @detMethod

Error converting data type varchar to float.

One possible alternate is to use dynamic sql and store the whole query in a string, and then execute using sp_executesql. But what exactly is wrong with this code?

Edit: The code is fine, the error was further down in the query in the FROM clause where I had not enclosed @detMethod within the CASE block. The error line number it was giving me was the first line of the derived table.

Answer 1

I see two possibilities:

1. Either one of the columns cov and pcc has a data type of varchar, and contains values that cannot be converted to float;
2. Or the error is further down in your query. Sometimes, the error line number given is not the line of the actual problem expression but the line of the start of the query. Since you're pointing out a line that is very close to the first line of the query, you could just be off by one (easy to do if you have a GO statement near the top of your file), and the error is NOT with this part of the query.

Your pattern of placing the CASE statement in a SELECT is unnecessary, but it's not what's causing the error. You can simplify like this:

WITH myTable AS (
   SELECT
      user1,
      method = -- Putting the alias first is clearer in my opinion
         CASE @detMethod
            WHEN 'MAX(cov)' THEN MAX(cov)
            ELSE MAX(pcc * cov)
         END
   FROM ...
   WHERE ...
)
...

Note that there is nothing wrong with using the alternate shorthand form of the CASE statement that you've chosen, which looks like this:

CASE {expression}
   WHEN {testvalue1} THEN {resultvalue1}
   WHEN {testvalue2} THEN {resultvalue2}
   ...
END

As long as you are testing for equality, then the full syntax is not required. If you use inequality or need to test for NULL or use a different expression in some cases, then obviously you have to use the expanded form:

CASE
   WHEN {expression} = {testvalue1} THEN {resultvalue1}
   WHEN {expression} = {testvalue2} THEN {resultvalue2}
   ...
END

But again, this shouldn't affect your situation.

Last, if your expression always uses Max, consider further simplifying like so:

MAX(
   CASE @detMethod
      WHEN 'MAX(cov)' THEN cov
      ELSE pcc * cov
   END
)

Or even (to better express intent to the next developer):

MAX(
   cov *
      CASE @detMethod
         WHEN 'MAX(cov)' THEN 1
         ELSE pcc
      END
)