How to conditionally filter on a column in a WHERE clause?

Question

OK, the umpteenth conditional column question:

I'm writing a stored proc that takes an input parameter that's mapped to one of several flag columns. What's the best way to filter on the requested column? I'm currently on SQL2000, but about to move to SQL2008, so I'll take a contemporary solution if one's available.

The table queried in the sproc looks like

ID ...  fooFlag  barFlag  bazFlag  quuxFlag
--      -------  -------  -------  --------
01         1        0       0          1
02         0        1       0          0
03         0        0       1          1
04         1        0       0          0

and I want to do something like

select ID, name, description, ...
from myTable
where (colname like @flag + 'Flag') = 1

so if I call the sproc like exec uspMyProc @flag = 'foo' I'd get back rows 1 and 4.

I know I can't do the part in parens directly in SQL. In order to do dynamic SQL, I'll have to stuff the entire query into a string, concatenate the @flag param in the WHERE clause and then exec the string. Aside from the dirty feeling I get when doing dynamic SQL, my query is fairly large (I'm selecting a couple dozen fields, joining 5 tables, calling a couple of functions), so it's a big giant string all because of a single line in a 3-line WHERE filter.

Alternately, I could have 4 copies of the query and select among them in a CASE statement. This leaves the SQL code directly executable (and subject to syntax hilighting, etc.) but at the cost of repeating big chunks of code, since I can't use the CASE on just the WHERE clause.

Are there any other options? Any tricky joins or logical operations that can be applied? Or should I just get over it and exec the dynamic SQL?

Answer 1

There are a few ways to do this:

You can do this with a case statement.

select ID, name, description, ...
from myTable
where CASE
    WHEN @flag = 'foo' then fooFlag
    WHEN @flag = 'bar' then barFlag
END = 1

You can use IF.

IF (@flag = 'foo') BEGIN
    select ID, name, description, ...
    from myTable
    where fooFlag = 1
END ELSE IF (@flag = 'bar') BEGIN
    select ID, name, description, ...
    from myTable
    where barFlag = 1
END

....

You can have a complicated where clause with a lot of parentheses.

select ID, name, description, ...
from myTable
where (@flag = 'foo' and fooFlag = 1)
OR (@flag = 'bar' and barFlag = 1) OR ...

You can do this with dynamic sql:

DECLARE @SQL nvarchar(4000)

SELECT @SQL = N'select ID, name, description, ...
from myTable
where (colname like ''' + @flag + 'Flag'') = 1'

EXECUTE sp_ExecuteSQL @SQL, N''

There are more, but I think one of these will get you going.

Answer 2

"Alternately, I could have 4 copies of the query and select among them in a CASE statement."

You don't need to copy your entire query 4 times, just add all the possibilities into the where clauses in your single copy of the query:

select ID, name, description, ...
from myTable
where (@flag = 'foo' and fooFlag = 1) OR (@flag = 'bar' and barFlag = 1) OR ...

Answer 3

You could have a parameter for each possible flag column, then check if the parameter is null or the value in the column is equal to the parameter. Then you pass in a 1 for the flags that you want to check and leave the others null.

select id, name, description, ...
from myTable
where (@fooFlag is null or fooFlag = @fooFlag) AND
      (@barFlag is null or barFlag = @barFlag) AND
      ...

Honestly, though, this seems like an ideal candidate for building a dynamic LINQ query and skipping the SPROC once you get to SQL2008.

Answer 4

What I would do is CASE some variables at the beginning. Example:

DECLARE
    @fooFlag int,
    @barFlag int,
    @bazFlag int,
    @quuxFlag int

SET @fooFlag = CASE WHEN @flag = 'foo' THEN 1 ELSE NULL END
SET @barFlag = CASE WHEN @flag = 'bar' THEN 1 ELSE NULL END
SET @bazFlag = CASE WHEN @flag = 'baz' THEN 1 ELSE NULL END
SET @quuxFlag = CASE WHEN @flag = 'quux' THEN 1 ELSE NULL END

SELECT ID, name, description, ...
FROM myTable
WHERE (fooFlag >= ISNULL(@fooFlag, 0) AND fooFlag <= ISNULL(@fooFlag, 1))
AND (barFlag >= ISNULL(@barFlag, 0) AND barFlag <= ISNULL(@barFlag, 1))
AND (bazFlag >= ISNULL(@bazFlag, 0) AND bazFlag <= ISNULL(@bazFlag, 1))
AND (quuxFlag >= ISNULL(@quuxFlag, 0) AND quuxFlag <= ISNULL(@quuxFlag, 1))

The good thing about this query is that, because the possible values for "flags" are bounded, you can calculate all your conditionals as prerequisites instead of wrapping columns in them. This guarantees a high-performance index seek on whichever columns are indexed, and doesn't require writing any dynamic SQL. And it's better than writing 4 separate queries for obvious reasons.