牛客网数据库SQL实战42—— 删除emp_no重复的记录，只保留最小的id对应的记录。

牛客网数据库SQL实战42—— 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');

输入描述:

无

输出描述:

无

我的解答

delete from titles_test
where id in(
    select min(id) from titles_test 
    group by emp_no
)

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 “非每个分组最小id对应的所有记录”

我觉得最好的答案

正向思维：找出所有需要删除的记录的id
step1:找出所有出现的频次>1 emp_no 及最小id

SELECT MIN(id) AS id,emp_no FROM titles_test
GROUP BY emp_no HAVING COUNT(emp_no) > 1
+------+--------+
|    1 |  10001 |
|    2 |  10002 |
|    3 |  10003 |
+------+--------+

step2:联合titles_test 找出所有 需要删除的id

SELECT a.id FROM titles_test a ,
    (SELECT MIN(id) AS id,emp_no FROM titles_test
        GROUP BY emp_no HAVING COUNT(emp_no) > 1) AS b
WHERE a.emp_no = b.emp_no AND a.id > b.id
+----+
|  5 |
|  6 |
|  7 |
+----+

step3:根据id删除数据

DELETE FROM titles_test WHERE id IN (
    SELECT id FROM(
                SELECT a.id FROM titles_test a ,
                   (SELECT MIN(id) AS id,emp_no FROM titles_test
                    GROUP BY emp_no HAVING COUNT(emp_no) > 1) AS b
                WHERE a.emp_no = b.emp_no AND a.id > b.id )t
)

逆向思维：找出所有emp_no组中最小的那些id
step1: 找出所有emp_no组中最小的那些id

SELECT MIN(id) FROM titles_test GROUP BY emp_no

step2: 根据id删除数据

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