Mysql SELECT inside UPDATE

问题:

UPDATE forms SET

pos = (SELECT MIN(pos)-1 FROM forms)

WHERE id=$id

This doesn't work, error message:

**You can't specify target table 'form' for update in FROM clause**

I hope it's clear: I want to get the minimal element-1 from the same table and assign it to pos

回答1:

Consp is right that it's not supported. There's a workaround, however:

UPDATE forms SET
pos = (SELECT MIN(pos)-1 FROM (SELECT * FROM forms) AS x)
WHERE id=$id

A version that is probably faster:

UPDATE forms 
SET pos = (SELECT pos-1 FROM (SELECT MIN(pos) AS pos FROM forms) AS x)
where id=$id

回答2:

Your problem is stated plainly in the MySQL manual:

Currently, you cannot update a table and select from the same table in a subquery.

You'll want to use a transaction. Turn AutoCommit off, begin a transaction, then do a SELECT MIN(pos)-1 FROM forms FOR UPDATE, take that result, do the update with it, then commit your transaction.

回答3:

I think that you can not use a subquery inside an update statement, but any way there are workarounds for it ...

Here is a Quotation from the following site:

"dev.mysql.com"

“Currently, you cannot delete from a table and select from the same table in a sub-query ”

回答4:

You could also try:

START TRANSACTION;
SET @newMin := MIN(pos)-1 FROM forms;
UPDATE forms SET pos=@newMin WHERE id='$id';
COMMIT;