--===== If the test table already exists, drop it
     IF OBJECT_ID('TempDB..#mytable','U') IS NOT NULL
        DROP TABLE #mytable

--===== Create the test table with 
 CREATE TABLE #mytable 
        (
        id			INT, 
        eq_id			VARCHAR(10),
        bl_id			VARCHAR(10),
        fl_id			VARCHAR(10),
        rm_id			VARCHAR(10)
        )

--===== Insert the test data into the test table
 INSERT INTO #mytable 
       (id, eq_id, bl_id, fl_id, rm_id)
 SELECT 1, 'AAAA','BBBB','CCCC', NULL UNION ALL
 SELECT 2, 'EEEE', NULL, NULL, NULL UNION ALL
 SELECT 3, NULL,'BBBB', 'CCCC', NULL UNION ALL
 SELECT 4, NULL, 'QQQQ',NULL, NULL


select * from #mytable

DECLARE @eq_id varchar(10), @bl_id varchar(10), @fl_id varchar(10), @rm_id varchar(10)
set @eq_id = NULL
set @bl_id = 'BBBB'
set @fl_id = 'CCCC'
set @rm_id = NULL

select * from #mytable
	WHERE     (eq_id = @eq_id or @eq_id IS NULL) AND (bl_id = @bl_id or @bl_id IS NULL) 
			AND (fl_id = @fl_id or @fl_id IS NULL) AND (rm_id = @rm_id or @rm_id IS NULL)

select * from #mytable
    WHERE     (eq_id = @eq_id or eq_id IS NULL) AND (bl_id = @bl_id or bl_id IS NULL) 
	AND (fl_id = @fl_id or fl_id IS NULL) AND (rm_id = @rm_id or rm_id IS NULL)