牛客在线编程练习：SQL42_简单-优快云博客

本文链接：https://blog.youkuaiyun.com/weixin_42623730/article/details/113631482

SQL练习：SQL42_简单

题目： 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');

在这里插入图片描述
题解

DELETE FROM titles_test 
WHERE id NOT IN (
    SELECT * # 同表不支持DELETE或UPDATE子查询结果,所以要将子查询结果再select一次
    FROM (
        SELECT MIN(id) # 选出最小id
        FROM titles_test 
        GROUP BY emp_no
    ) AS a # 作为一个派生表来进行上一级的查询,必须起别名
);