select from 具体执行

Posted 2021-01-16 duanwandao

预先准备

create table p(
       name varchar2(10)
);

insert into p values(‘黄伟福‘);
insert into p values(‘赵洪‘);
insert into p values(‘杨华‘);

每条语句执行三次插入

select rowid,p.* from p;

select distinct * from p;

 

 查询最小rowID，只返回了一条数据

select min(rowid) from p p1 where p1.name = p1.name

 

 查询每个重复名字的最小rowID，注意子查询返回的是3条数据，并不是一条，思考一下具体执行顺序

select rowid,p1.* from p p1 where rowid = (select min(rowid) from p p2 where p1.name = p2.name);

 

删除重复数据，只保存数据中的最小rowID ，然后查询

delete from p p1 where rowid > (select min(rowid) from p p2 where p1.name = p2.name);
select rowid,p.* from p;