create table a(
id int,
username varchar2(20)
);create table b(
id int,
username varchar2(20)
);insert into a values(1,'小明');
insert into a values(2,'小红');
insert into a values(3,'小君');
commitselect * from a
select * from b
delete from a where id>=4insert into b values(2,'小红');
insert into b values(4,'小星');
insert into b values(5,'小刘');
去重 （查人数）
select * from a
union
select * from b
去重另一种写法
select distinct * from (
select * from a
union all
select * from b
)
不去重（速度快）
select * from a
union all
select * from b
求红色区域
先查idselect id from a where a.id in (select b.id from b)
再查是谁
select * from a where id in(
select id from b where b.id in (select a.id from a)
)
另一种方法
select * from b where b.id in (select a.id from a)
还有一种方法
select b.* from a,b where a.id = b.id
求红色区域
select * from a where id not in (select id from b)
not in 在oracle大数据时，速度非常慢，不建议使用
select * from a where id in ( select id from a
minus
select id from b)
