Mysql 错误1093-无法在 FROM 子句中指定更新的目标表

Question

I have a table story_category in my database with corrupt entries. The next query returns the corrupt entries:

SELECT * 
FROM  story_category 
WHERE category_id NOT IN (
    SELECT DISTINCT category.id 
    FROM category INNER JOIN 
       story_category ON category_id=category.id);

I tried to delete them executing:

DELETE FROM story_category 
WHERE category_id NOT IN (
    SELECT DISTINCT category.id 
    FROM category 
      INNER JOIN story_category ON category_id=category.id);

But I get the next error:

#1093 - You can't specify target table 'story_category' for update in FROM clause

How can I overcome this?

转载于:https://stackoverflow.com/questions/45494/mysql-error-1093-cant-specify-target-table-for-update-in-from-clause

Answer 1

Update: This answer covers the general error classification. For a more specific answer about how to best handle the OP's exact query, please see other answers to this question

In MySQL, you can't modify the same table which you use in the SELECT part.
This behaviour is documented at: http://dev.mysql.com/doc/refman/5.6/en/update.html

Maybe you can just join the table to itself

If the logic is simple enough to re-shape the query, lose the subquery and join the table to itself, employing appropriate selection criteria. This will cause MySQL to see the table as two different things, allowing destructive changes to go ahead.

UPDATE tbl AS a
INNER JOIN tbl AS b ON ....
SET a.col = b.col

Alternatively, try nesting the subquery deeper into a from clause ...

If you absolutely need the subquery, there's a workaround, but it's ugly for several reasons, including performance:

UPDATE tbl SET col = (
  SELECT ... FROM (SELECT.... FROM) AS x);

The nested subquery in the FROM clause creates an implicit temporary table, so it doesn't count as the same table you're updating.

... but watch out for the query optimiser

However, beware that from MySQL 5.7.6 and onward, the optimiser may optimise out the subquery, and still give you the error. Luckily, the optimizer_switch variable can be used to switch off this behaviour; although I couldn't recommend doing this as anything more than a short term fix, or for small one-off tasks.

SET optimizer_switch = 'derived_merge=off';

Thanks to Peter V. Mørch for this advice in the comments.

Example technique was from Baron Schwartz, originally published at Nabble, paraphrased and extended here.