Select records from a table based on column value from same table

Question

I have already written the following query.

DECLARE @LEVEL NVARCHAR(10),
        @ID INT = 12

SELECT @LEVEL = RTRIM(LEVEL_NEW)
FROM HIER
WHERE ID = @ID

IF (@LEVEL = 'LEVEL 1')
    SELECT *
    FROM HIER
    WHERE FC_LEVEL1 = @ID
ELSE IF (@LEVEL = 'LEVEL 2')
    SELECT *
    FROM HIER
    WHERE FC_LEVEL2 = @ID
ELSE IF (@LEVEL = 'LEVEL 3')
    SELECT *
    FROM HIER
    WHERE FC_LEVEL3 = @ID

The above query works well. But I would like to have a single query with join and case expression. Any help would be highly appreciated.

Answer 1

You can use boolean logic:

select *
from hier
where (@level = 'LEVEL 1' and fc_level1 = @id)
   or (@level = 'LEVEL 2' and fc_level2 = @id)
   or (@level = 'LEVEL 3' and fc_level3 = @id)

Answer 2

To combine this all into one query:

SELECT h.*
FROM HIER h CROSS JOIN
     (SELECT RTRIM(LEVEL_NEW) as LEVEL
      FROM HIER
      WHERE ID = @ID
     ) l
WHERE (L.LEVEL = 'LEVEL 1' AND h.FC_LEVEL1 = @ID) OR
      (L.LEVEL = 'LEVEL 2' AND h.FC_LEVEL2 = @ID) OR
      (L.LEVEL = 'LEVEL 3' AND h.FC_LEVEL3 = @ID) ;