sql9

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');

注意几点：1.题目看清楚意思，别写到一半自己会错意了。

                   2.DELETE from 语句没有(delete * from，没有*).

                   3.group by 是函数可以对列去重，效果和select distinct（列名一样）.

                   4.where 是连接条件的,where 后面必须跟列名

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