SQL union與union all 簡單易懂

-union 缺省在合併結果集後消除重複項,
--union all 指定在合併結果集後保留重複項, 
--比如A表的數據是 tbalea{ 1,4,5,9}       tbaleb{2,3,4,5}


create TABLE tbalea (Name varchar(50)) 
INSERT INTO tbalea VALUES('1')
INSERT INTO tbalea VALUES('4')
INSERT INTO tbalea VALUES('5')
INSERT INTO tbalea VALUES('9')


create TABLE tbaleb (Name varchar(50)) 
INSERT INTO tbaleb VALUES('2')
INSERT INTO tbaleb VALUES('3')
INSERT INTO tbaleb VALUES('4')
INSERT INTO tbaleb VALUES('5')




SELECT * FROM tbalea UNION 
SELECT * FROM tbaleb 
--結果是{1,2,3,4,5,9}




SELECT * FROM tbalea UNION all
SELECT * FROM tbaleb 
--結果是{1,2,3,4,4,5,5,9}