彻底理解删除重复记录，只保留一条

Posted 2022-12-08 清_澈

已oracle数据库为例：

--1、建表

-- Create table
create table DEMO
(
  ID          INTEGER,
  NAME        VARCHAR2(50),
  IDCARD      VARCHAR2(18),
  AMT         NUMBER(*,2),
  END_DATE    DATE,
  UPDATE_TIME TIMESTAMP(6)
)
tablespace PLATEDATA
  pctfree 10
  initrans 1
  maxtrans 255
  storage
  (
    initial 64K
    next 1M
    minextents 1
    maxextents unlimited
  );
-- Add comments to the columns 
comment on column DEMO.ID
  is '主键';
comment on column DEMO.NAME
  is '姓名';
comment on column DEMO.IDCARD
  is '身份证号';
comment on column DEMO.AMT
  is '金额';
comment on column DEMO.END_DATE
  is '截止时间';
comment on column DEMO.UPDATE_TIME
  is '更新时间';

--2、插入数据 （重复数据为370305197812097560，需要删除 齐白石）

insert into demo (ID, NAME, IDCARD, AMT, END_DATE, UPDATE_TIME)
values (2, '齐白石', '370305197812097560', 50000.00, to_date('16-11-2022', 'dd-mm-yyyy'), '');

insert into demo (ID, NAME, IDCARD, AMT, END_DATE, UPDATE_TIME)
values (1, '苏东坡', '370300198211118113', 20000.00, to_date('17-11-2022', 'dd-mm-yyyy'), '');

insert into demo (ID, NAME, IDCARD, AMT, END_DATE, UPDATE_TIME)
values (3, '鲁赤水', '370305197812097560', 40000.00, to_date('30-11-2022', 'dd-mm-yyyy'), '');

insert into demo (ID, NAME, IDCARD, AMT, END_DATE, UPDATE_TIME)
values (4, '张大千', '370302195602068945', 26000.00, to_date('17-11-2022', 'dd-mm-yyyy'), '');

--3、查询重复的数据

select * from demo t ;
select IDCard from demo t  group by IDCard ; --排重后，一共有这些条数据 
select name,IDCard,end_date from demo t  group by IDCard ,name ,end_date ; 
select IDCard from demo t group by IDCard having count(IDCard) > 1;--查询重复的身份证号                
--查出重复的条数信息
SELECT *  from demo
      where end_date not in
            (select t.max_id
               from (select max(end_date) as max_id from demo group by IDCard) t);  

--4、删除重复数据，只（根据最新时间，或者最大id）保留一条

--按最大id删除
 delete from demo
      where id not in
            (select t.max_id
               from (select max(id) as max_id from demo group by IDCard) t);  
 --或者按最新时间删除
delete  from demo
      where end_date not in
            (select t.max_id
               from (select max(end_date) as max_id from demo group by IDCard) t);    
--此语句不起效，还未查明原因                 
DELETE  FROM demo t
 WHERE IDCard NOT IN
       (SELECT t.IDCard
          FROM (SELECT IDCard
                  FROM demo t1
                 WHERE t1.end_date =
                       (SELECT max(t2.end_date)
                          FROM demo t2
                         WHERE t2.IDCard = t1.IDCard group by IDCard )) t);