Question

I have a table Tran with transaction records.

I have two tables, Parameters1 and Parameters2, which are used to filter results when selecting from the Tran table a la:

SELECT *  
FROM  
     Tran t JOIN  
     Parameters1 p1 ON T.code1 = p1.code1  
     Parameters2 p2 ON T.code2 = p2.code2  
WHERE p1.code1 is not null AND
      p2.code2 is not null

Sometimes Parameters1 or Parameters2 is empty.

What I want is for the filtering to happen only if the Parameters tables have records. In other words, if Parameters1 is empty, don’t use it to filter, and vice versa. Same goes for Parameters2.

I’m stumped.

Answer 1

You’d normally do this using dynamic SQL, since coding up dynamic search conditions with static SQL is generally hackish. If you are interested in this approach I highly recommend Erland Sommarskog’s page on dynamic SQL for SQL Server 2005. He has a great case study with plenty of example code on how to do exactly what you’re asking for.

1. Setup

Whether you use dynamic or static SQL, you should first determine upfront what filters apply:

DECLARE @p1_filter BIT;
DECLARE @p2_filter BIT;

SET @p1_filter = 0;
SET @p2_filter = 0;

IF EXISTS (SELECT TOP (1) * FROM Parameters1)
BEGIN
   SET @p1_filter = 1;
END;

IF EXISTS (SELECT TOP (1) * FROM Parameters2)
BEGIN
   SET @p2_filter = 1;
END;

2.a. Dynamic SQL

If you’re going the dynamic SQL route you’d then do:

DECLARE @sql NVARCHAR(MAX);

SET @sql = 'SELECT * FROM Tran T ';

IF (@p1_filter = 1)
BEGIN
   SET @sql = @sql + 'INNER JOIN Parameters1 p1 ON T.code1 = p1.code1 ';
END;

IF (@p2_filter = 1)
BEGIN
   SET @sql = @sql + 'INNER JOIN Parameters2 p2 ON T.code2 = p2.code2 ';
END;

EXECUTE sp_executesql
   @statement = @sql
;

2.b.i. Static SQL: Sub-SELECTs in the WHERE clause

Otherwise, you’d continue with this ugly (and probably poorly performing) static SQL:

SELECT *
FROM Tran T
WHERE 
   (
         T.code1 IN (SELECT code1 FROM Parameters1)
      OR (@p1_filter = 0)
   )
   AND
   (
         T.code2 IN (SELECT code2 FROM Parameters2)
      OR (@p2_filter = 0)
   )
;

Erland also presents static SQL approaches like this one on that same page I referred to earlier. You may find a better approach there.

2.b.ii. Static SQL: LEFT OUTER JOIN

You can also go with this approach, which you proposed in the comments on this answer:

SELECT *
FROM 
                   Tran T
   LEFT OUTER JOIN Parameters1 p1
      ON T.code1 = p1.code1
   LEFT OUTER JOIN Parameters2 p2
      ON T.code2 = p2.code2
WHERE
       T.code1 = 
          CASE
             WHEN @p1_filter = 1
                THEN p1.code1
             ELSE T.code1
          END
   AND T.code2 =
          CASE
             WHEN @p2_filter = 1
                THEN p2.code2
             ELSE T.code2
          END
;