SQL236 删除emp_no重复的记录，只保留最小的id对应的记录

题目描述

删除emp_no重复的记录，只保留最小的id对应的记录。

CREATE TABLE IF NOT EXISTS titles_test (
id int(11) not null primary key,
emp_no int(11) NOT NULL,
title varchar(50) NOT NULL,
from_date date NOT NULL,
to_date date DEFAULT NULL);

insert into titles_test values ('1', '10001', 'Senior Engineer', '1986-06-26', '9999-01-01'),
('2', '10002', 'Staff', '1996-08-03', '9999-01-01'),
('3', '10003', 'Senior Engineer', '1995-12-03', '9999-01-01'),
('4', '10004', 'Senior Engineer', '1995-12-03', '9999-01-01'),
('5', '10001', 'Senior Engineer', '1986-06-26', '9999-01-01'),
('6', '10002', 'Staff', '1996-08-03', '9999-01-01'),

('7', '10003', 'Senior Engineer', '1995-12-03', '9999-01-01');

删除后titles_test表为(注：最后会select * from titles_test表来对比结果)

id	emp_no	title	from_date	to_date
1	10001	Senior Engineer	1986-06-26	9999-01-01
2	10002	Staff	1996-08-03	9999-01-01
3	10003	Senior Engineer	1995-12-03	9999-01-01
4	10004	Senior Engineer	1995-12-03	9999-01-01

SQL Schema

drop table if exists titles_test;
CREATE TABLE titles_test (
   id int(11) not null primary key,
   emp_no  int(11) NOT NULL,
   title  varchar(50) NOT NULL,
   from_date  date NOT NULL,
   to_date  date DEFAULT NULL);

insert into titles_test values
('1', '10001', 'Senior Engineer', '1986-06-26', '9999-01-01'),
('2', '10002', 'Staff', '1996-08-03', '9999-01-01'),
('3', '10003', 'Senior Engineer', '1995-12-03', '9999-01-01'),
('4', '10004', 'Senior Engineer', '1995-12-03', '9999-01-01'),
('5', '10001', 'Senior Engineer', '1986-06-26', '9999-01-01'),
('6', '10002', 'Staff', '1996-08-03', '9999-01-01'),
('7', '10003', 'Senior Engineer', '1995-12-03', '9999-01-01');

答案

DELETE
FROM titles_test
WHERE id IN (SELECT *
             FROM (SELECT a.id
                   FROM titles_test a
                            INNER JOIN (SELECT emp_no, MIN(id) AS `min_id`
                                        FROM titles_test
                                        GROUP BY emp_no
                                        HAVING COUNT(*) > 1) b ON a.emp_no = b.emp_no AND a.id > b.min_id) c);

DELETE
FROM titles_test
WHERE id NOT IN (SELECT *
                 FROM (SELECT MIN(id)
                       FROM titles_test
                       GROUP BY emp_no) a);