1 select * from t_um_user 2 --2204 3 select count(*) from t_um_user 4 --2204 5 6 select name from t_um_user --包含null 7 --2204 8 select count(name) from t_um_user --排除null（6个） 9 --2198 10 11 select distinct name from t_um_user --包含null 12 --1973 13 is SELECT COUNT ( DISTINCT name) from t_um_user - negative null 14 - 1972 15 16 SELECT name from t_um_user Group by name . 17 - 1973 18 is SELECT name, COUNT (name) AS iCount from t_um_user Group by name - name comprising null, but the count (name) does not contain a null . 19 - 1973 (name of the row is null, iCount = 0) 20 is SELECT name,COUNT ( * ) AS iCount from t_um_user Group by name - name contains null 21 is - 1973 (name of the row is null, iCount =. 6) 22 is ================= ============================== 23 - table t_user only name a field 24- the SELECT name from t_user 25 - 2204 26 the SELECT COUNT (name) from t_user - exclude null 27 - 2198 28 the SELECT COUNT ( * )from t_user - contain null 29 - 2204 (although * looks equal name, but the calculated result is not the same)