SQL练习42：删除emp_no重复的记录，只保留最小的id对应的记录。

题目链接：牛客网

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

idemp_notitlefrom_dateto_date110001Senior Engineer1986-06-269999-01-01210002Staff1996-08-039999-01-01310003Senior Engineer1995-12-039999-01-01410004Senior Engineer1995-12-039999-01-01

解法 根据题目的要求，可以先用 GROUP BY 和 MIN() 选出每个 emp_no 分组中最小的 id，然后用 DELETE FROM... WHERE ... NOT IN ... 语句删除id不在子查询结果的id。这里要注意子查询结果要添加一个别名，不然报错。

DELETE FROM titles_test
WHERE id NOT IN(SELECT min_id
                FROM
                (SELECT min(id) AS min_id
                 FROM titles_test
                 GROUP BY emp_no) AS t1);