SQL練習42：洗掉emp_no重復的記錄，只保留最小的id對應的記錄，
題目鏈接：牛客網
題目描述
洗掉emp_no重復的記錄，只保留最小的id對應的記錄，
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');
洗掉后titles_test表為
| 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
根據題目的要求，可以先用 GROUP BY 和 MIN() 選出每個 emp_no 分組中最小的 id，然后用 DELETE FROM... WHERE ... NOT IN ... 陳述句洗掉id不在子查詢結果的id，這里要注意子查詢結果要添加一個別名，不然報錯，
DELETE FROM titles_test
WHERE 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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