DECLARE @QotD
TABLE   (
        Column_1    TINYINT NOT NULL,
        Column_2    TINYINT NULL
        );
        
INSERT  @QotD VALUES (1, 1);
INSERT  @QotD VALUES (2, NULL);
INSERT  @QotD VALUES (3, 2);
INSERT  @QotD VALUES (4, 2);
INSERT  @QotD VALUES (5, NULL);
INSERT  @QotD VALUES (6, 3);
INSERT  @QotD VALUES (7, 4);
INSERT  @QotD VALUES (8, 5);
INSERT  @QotD VALUES (9, 6);
INSERT  @QotD VALUES (10, 7);

--I.
SELECT  COUNT_BIG(*) FROM @QotD;

--II.
SELECT  COUNT_BIG(Column_2) FROM @QotD;

--III.
SELECT  COUNT_BIG(ALL Column_2) FROM @QotD;

--IV.
SELECT  COUNT_BIG(DISTINCT Column_2) FROM @QotD;