Oracle查询并集、交集、差集

（1）union

求并集，查询表tbl1、表tbl2的所有key，去重

(SELECT key1 AS key FROM tbl1)
UNION 
(SELECT key2 AS key FROM tbl2);

（2） union all
求并集，查询表tbl1、表tbl2的所有key，不去重

(SELECT key1 AS key FROM tbl1)
UNION ALL 
(SELECT key2 AS key FROM tbl2);

（3）intersect
求交集，查询表tbl1和表tbl2都有的key

(SELECT key1 AS key FROM tbl1)
INTERSECT
(SELECT key2 AS key FROM tbl2);

（4）minus
求差集，查询表tbl1有而表tbl2没有的key

(SELECT key1 AS key FROM tbl1)
MINUS
(SELECT key2 AS key FROM tbl2);