Error converting data type varchar to float - ISNULL related

Question

Note: I read similar threads on SO but was of no help in this query.

I have reproduced the following code where is similar to my real problem. I have to use this syntax because it is already been used in the database, unless there is a convincing advice, the logic is wrong here.

In my actual problem, this query works most of the time but fails on a certain time. After investigating I found that the problem is

ISNULL(amount,0)

This is because if any category has both values 0,0 or both values null, null, ISNULL makes them a string and therefore I get

Error converting data type varchar to float

The following is my test code with comments

create table #table1 (
id int not null primary key identity,
category varchar(10),
amount float null
)

insert into #table1 values('A',23)
insert into #table1 values('A',23)
insert into #table1 values('B',NULL)
insert into #table1 values('B',0)
insert into #table1 values('C',NULL)
insert into #table1 values('C',NULL)
insert into #table1 values('D',0)
insert into #table1 values('D',0)

select * from #table1 -- works

select category, sum1  -- works
 from 
(select category, SUM(Round(ISNULL(amount,0),0)) as Sum1 from #table1
group by category)  D

select category, sum2 = -- does not work
case Sum1
    when 'A' then Sum1 * 1   -- my problem is here
    when 'B' then Sum1 * 2   -- this is the logic in my actual code
    when 'C' then Sum1 * 3   -- would like to make this query work
    when 'D' then Sum1 * 4
    else Sum1
end 
 from 
(select category, SUM(Round(ISNULL(amount,0),0) ) as Sum1 from #table1
group by category)  D

What am I doing wrong?

Answer 1

Shouldn't your case statement be "case category" and not "case sum1"?

If you change that, the query works as expected.

Answer 2

your case needs to look at category not sum1

select category, sum2 = -- does not work
case category
    when 'A' then Sum1 * 1   -- my problem is here
    when 'B' then Sum1 * 2   -- this is the logic in my actual code
    when 'C' then Sum1 * 3   -- would like to make this query work
    when 'D' then Sum1 * 4
    else Sum1
end 
 from 
(select category, SUM(Round(ISNULL(amount,0),0) ) as Sum1 from #table1
group by category)  D