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题目链接：
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
| 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 |
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_testWHERE id NOT IN(SELECT min_id FROM (SELECT min(id) AS min_id FROM titles_test GROUP BY emp_no) AS t1);
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