What is the proper approach to defining a dynamic IN clause, based off of parameter flags?

Question

I apologize if this is a duplicate, but I've scoured this site and I can't find a straightforward answer to what has to be a common problem.

I have a stored procedure that takes multiple parameters but three of them are flags. For each flag, flagged as 'Y', I need to insert a value into an IN clause. In other words, consider the following code.

CREATE PROC testProc
  @FlagA char(1),
  @FlagB char(1),
  @FlagC char(1)
AS

--... Do stuff...

IF (@FlagA = 'Y')
  --Add 'AVal' to IN list
IF (@FlagB = 'Y')
  --Add 'BVal' to IN list
IF (@FlagC = 'Y')
  --Add 'CVal' to IN list

--... Do more stuff...

SELECT * FROM myTable
WHERE testField -- Insert correct IN clause.

This is an extremely simplistic version of why my code is trying to accomplish. I can think of multiple ways of solving this dilemma but none of them feel correct.

The best solution that I can come up with is to define a temp table that has one field. For each flag that is set, I can add a corresponding value into the temp table and then use a ...IN (SELECT * FROM #temp) statement.

Is this the right approach or is there a more correct solution to this problem? I'm fine if my solution is the best answer, however, whenever I see simplistic uses of temp tables I'm often suspicious that there is a better approach.

Any clarity would be appreciated. Thank you.

Answer 1

Please, please, please do not do Dynamic SQL or multiple OR conditions (each with a specific "flag" parameter). There is a very simple and set-based approach that SQL Server will appreciate:

1. Create a temp table (or table variable)
2. Insert the values for flags coming in with a value of "Y" into it
3. Use that temp table (or table variable) in an EXISTS clause

Example:

DECLARE
  @FlagA CHAR(1),
  @FlagB CHAR(1),
  @FlagC CHAR(1);

SELECT
  @FlagA = 'Y',
  @FlagB = 'n',
  @FlagC = 'y';

DECLARE @Options TABLE (Value VARCHAR(50));

IF (@FlagA = 'Y')
BEGIN
  INSERT INTO @Options (Value) VALUES ('AVal');
END;

IF (@FlagB = 'Y')
BEGIN
  INSERT INTO @Options (Value) VALUES ('BVal');
END;

IF (@FlagC = 'Y')
BEGIN
  INSERT INTO @Options (Value) VALUES ('CVal');
END;

SELECT *
FROM   MyTable mt
WHERE  EXISTS (SELECT *
               FROM   @Options op
               WHERE  op.Value = mt.[testField]
              );

Of course, it might not hurt to compare performance against your proposed IN list:

SELECT *
FROM   MyTable mt
WHERE  mt.[testField] IN (SELECT op.Value FROM @Options op);

And, since the value to search for can only appear once, that is technically an INNER JOIN, right? It is not usually my preference to use an INNER JOIN purely as a filter, but can't hurt testing :). And this version might work better if [testField] is indexed.

SELECT *
FROM   MyTable mt
INNER JOIN @Options op
        ON op.Value = mt.[testField];

In either case, using this approach (any one of them) is preferred because it lets SQL Server do what it is tuned to do: work with sets. The EXISTS is helpful in that it will stop searching once the first row is found for its query. I suspect that EXISTS is better than an IN list but with only 3 values it should be tested. However, this approach is also more easily adaptable to adding more "flag" values in the future as that won't change the query.

If you want to do the INNER JOIN option to a table being OUTER JOINed:

SELECT *
FROM   PrimaryTable pt
LEFT JOIN (
             MyTable mt
  INNER JOIN @Options op
          ON op.Value = mt.[testField]
          ) ON mt.ID = pt.ID;

Answer 2

You'd have to check performance on this, but you could use conditional WHERE clauses as an alternative. This may not be a great solution, depending on how complicated your actual procedure is, but it gives you something else to consider:

DECLARE @FlagA CHAR(1) = 'Y' ,
        @FlagB CHAR(1) = 'N' ,
        @FlagC CHAR(1) = 'Y'

SELECT  *
FROM    MyTable
WHERE   ( @FlagA = 'Y' AND testField = AVal )
     OR ( @FlagB = 'Y' AND testField = BVal )
     OR ( @FlagC = 'Y' AND testField = CVal )

With this, the WHERE clause on each line will only be evaluated if the first condition is met.

Answer 3

Not sure it is better but another approach

  where @flag1 + testField = 'YAval'
     or @flag2 + testField = 'YBval'
     or @flag2 + testField = 'YCval'

or

where 'Y' + testField = in ( @flag1 + 'Aval', @flag2 + 'Bval', @flag3 + 'Cval')