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)