1. 题目
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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。 -
一张表
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');
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所需结果
2. 题解
错误写法:
# 错误
DELETE FROM titles_test
WHERE id NOT IN (SELECT MIN(id)
FROM titles_test
GROUP BY emp_no)
以上会出现报错:
# 报错:SQL_ERROR_INFO: "You can't specify target table 'titles_test' for update in FROM clause"
正解:
DELETE FROM titles_test
WHERE id NOT IN (SELECT t.* FROM(
SELECT MIN(id)
FROM titles_test
GROUP BY emp_no) t
)
3. 反思
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问题:同一张表的 UPDATE 操作和 SELECT 不能同时进行
-
解决:在子查询中重命名表格名(必须),再进行 SELECT