【385天】我愛刷題系列144（2018.02.25）

@(一隻心中無碼的程序員)專欄

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SQL習題039

1

題目描述
刪除emp_no重複的記錄，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');

本題思路以下：先用 GROUP BY 和 MIN() 選出每一個 emp_no 分組中最小的 id，而後用 DELETE FROM ... WHERE ... NOT IN ... 語句刪除 「非每一個分組最小id對應的全部記錄」

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