问题
If I have a query that runs a search with optional parameters, my usual approach is to pass in NULL for any unused criteria, and have a WHERE clause that looks like
WHERE (@Param IS NULL OR Field=@Param) AND (@Param2 IS NULL OR Field2=@Param2)
If I have a condition that maybe is a little more complex to evaluate, say a LIKE clause, is SQL Server going to evaluate the IS NULL and then short circuit the more complex condition? Is there something I could look at in the execution plan to find out if it is or not? Is this behavior dependent on the plan chosen by the optimizer, such that the answer might be different in my development and production environments (I've got limited access to see what's going on in production, so I'm hoping behaviors would be consistent).
回答1:
I use CASE Statements.
So yours is like this:
WHERE (@Param IS NULL OR Field=@Param) AND (@Param2 IS NULL OR Field2=@Param2)
And I will write it like this:
WHERE CASE WHEN @Param IS NULL THEN 1 ELSE Field END = CASE WHEN @Param IS NULL THEN 1 ELSE @Param END AND
CASE WHEN @Param2 IS NULL THEN 1 ELSE Field2 END = CASE WHEN @Param2 IS NULL THEN 1 ELSE @Param2 END
Have a look at the query execution plans you get too, in a lot of cases when you use the OR condition the Query Optimizer uses a table scan. You can also replace the '=' with 'like' etc, and in fact you can even wrap that up in a case statement too. So you can replace the '=' with CASE WHEN @Param3 = 'eq' THEN = ELSE LIKE END
Hope this helps.
回答2:
I found a more practical solution that does avoid the use of dynamic SQL.
Suppose you have a bit field and the parameter is 1, 0 or (null = both). Then use the following SQL:
WHERE a.FIELD=CASE WHEN ISNULL(@PARAM,-1)=-1 THEN a.FIELD ELSE @PARAM END
With other words: if you dont provide an explicit value for @param then tell that A.field = a.field which is always true. In case of nullable fields you need to apply an ISNULL on that field to avoid that null=null does not come through.
Example: on INT field that can be null,
AND ISNULL(a.CALL_GENERAL_REQUIREMENTS,-1)=CASE WHEN ISNULL(@CALL_GENERAL_REQUIREMENTS,-2)=-2 THEN ISNULL(a.CALL_GENERAL_REQUIREMENTS,-1) ELSE @CALL_GENERAL_REQUIREMENTS END
As you can see I apply -2 to the null value of the param @CALL_GENERAL_REQUIREMENTS this is because real values to select on can be 0 (not passed), 1 (passed), -1 (not evaluated yet). So a -2 means do not select on this field
Example on nullable string field:
AND ISNULL(a.CALL_RESULT,'')=CASE WHEN ISNULL(@CALL_RESULT,'')='' THEN ISNULL(a.CALL_RESULT,'') ELSE @CALL_RESULT END
All works as a charm and avoids a lot of hassle on creating a dynamic SQL string with the concatenation and does not require specific permissions to be assigned to be able to run an exec statement.
Hope this helps anyone as it helped me.
Have nice day
来源：https://stackoverflow.com/questions/5187649/using-dynamic-criteria-in-sql-query