WHERE inside CASE statement - sql-server

Question

I have seen lots of posts about putting CASE statements inside WHERE clauses. However, in my case, I want to put a WHERE statement inside a CASE statement, which I don't think can be done.
My situation is that I have a view that is going on many, many databases. In half of them, I need to have a WHERE clause. In the other half, I don't need the WHERE clause. Leaving it in isn't harmful, in that I don't get bad data, but it slows the query down considerably given that it has to read and sort a large table when it is unnecessary.
Status_Table
ID Status
1 Active
2 Inactive
3 Unknown --this row of data will not exist in some DBs
Item_table
Item_ID Status value1 value2
1001 3 ..... .....
1002 1 ..... .....
What I want to do is something like this.
-- big nasty ugly query with various CTEs, selects, and joins
CASE WHEN (SELECT MAX(ID) FROM Status_table) = 3
THEN WHERE Item_Table.Status != 3
ELSE WHERE 1=1
END
Ultimately, I can probably swing this by constructing the query using dynamic SQL, but I was hoping for a more elegant solution.

Answer 1

You're trying to build a conditional WHERE clause, yes? I'd think something like this would work pretty well for what you want to do:
WHERE (EXISTS (SELECT 1 FROM Status_table WHERE ID = 3)
AND Item_Table.Status != 3)
OR
NOT EXISTS (SELECT 1 FROM Status_table WHERE ID = 3)

Answer 2

SQL Fiddle
MS SQL Server 2014 Schema Setup:
CREATE TABLE Status_Table ( ID int, [Status] varchar(30) ) ;
INSERT INTO Status_Table ( ID, [Status] )
VALUES
( 1,'Active' ), (2,'Inactive'), (3,'Unknown')
;
CREATE TABLE Item_table ( Item_ID int, [Status] int, value1 varchar(10), value2 varchar(10) ) ;
/* NOTE: Item_table.[Status] should be a descriptive name, like [StatusID], or something not the same name as a different datatype column in the relation table. */
INSERT INTO Item_Table ( Item_ID, [Status], value1, value2 )
VALUES (1001,3,'Bill','Exclude')
,(1002,1,'Ted','Include')
,(1003,3,'Rufus','Exclude')
,(1004,2,'Jay','Include')
,(1005,5,'Bob','BadRecord')
;
Query 1:
SELECT s1.Item_ID, s1.Status, s1.Value1, s1.Value2
FROM (
SELECT i.Item_ID, i.Status, i.Value1, i.Value2
, RANK() OVER (ORDER BY s.ID DESC) AS rn
FROM Item_Table i
RIGHT OUTER JOIN Status_Table s ON i.[Status] = s.ID
) s1
WHERE s1.rn <> 1
Results:
| Item_ID | Status | Value1 | Value2 |
|---------|--------|--------|---------|
| 1004 | 2 | Jay | Include |
| 1002 | 1 | Ted | Include |

Answer 3

If its performance your after start using DECLARE and SET.
It only needs to execute ONCE!!!
DECLARE #UnkownExists AS INT
SET #UnkownExists = (select count(*) from Status_table where ID = 3)
-- big nasty ugly query with various CTEs, selects, and joins
WHERE (#UnkownExists=1 AND Item_Table.Status != 3) OR #NoUnknown=0

Answer 4

You can try this:
CASE WHEN (SELECT MAX(ID) FROM Status_table) = 3
THEN 3
ELSE (SELECT MAX(ID)+1 FROM Status_table) --Invalid value that will not exist
END <> Item_Table.Status