Question

我希望通过子选择删除。这个问题的答案并没有解决我的问题：MySQL Error 1093 - Can't specify target table for update in FROM clause。

我找到了一种做法的hackish方式，并希望获得反馈，看看这是否是执行DELETE with SUBSELECT的最佳解决方案 -

DELETE FROM main_history WHERE id NOT IN (
   SELECT * from (
     SELECT max(id) FROM main_history h GROUP BY instance_id, xpath
   ) x
)

基本上，我只是添加第二个（无意义的）子选择来解决mysql错误，如果我只使用子选择执行正常的DELETE，我会收到该错误：

DELETE FROM main_history WHERE id NOT IN (
  SELECT max(id) FROM main_history h GROUP BY instance_id, xpath
);

您无法指定目标表＆＃39; main_history＆＃39;用于FROM子句中的更新

这是解决上述问题的好方法吗？

更新：子选择查询比连接查询执行得更好。以下是我的发现：

# 12m Subselect
DELETE FROM main_history WHERE id NOT IN (
 SELECT * from (
  SELECT max(id) FROM main_history h GROUP BY instance_id, xpath
 ) x
)

# 18m Join
Delete m1 from main_history m1 left join
(
  SELECT max(id) id FROM main_history h GROUP BY instance_id, xpath
) m2 on m1.id = m2.id
Where m2.id is null;

Answer 1

正确的方法是使用删除连接：

Delete m1 from main_history m1 left join
(
  SELECT max(id) id FROM main_history h GROUP BY instance_id, xpath
) m2 on m1.id = m2.id
Where m2.id is null;

Answer 2

您可以使用这样的派生表，然后MySQL无法从SELECT中看到表名：

DELETE FROM main_history WHERE id NOT IN (
   SELECT * FROM (
      SELECT * from (SELECT max(id) FROM main_history h
      GROUP BY instance_id, xpath) AS x
   ) AS delrows
);

使用子选择

2 个答案: