问题
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?
回答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.
回答2:
NexusRex provided a very good solution for deleting with join from the same table.
If you do this:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id AS cid FROM category
INNER JOIN story_category ON category_id=category.id
)
you are going to get an error.
But if you wrap the condition in one more select:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT cid FROM (
SELECT DISTINCT category.id AS cid FROM category
INNER JOIN story_category ON category_id=category.id
) AS c
)
it would do the right thing!!
Explanation: The query optimizer does a derived merge optimization for the first query (which causes it to fail with the error), but the second query doesn't qualify for the derived merge optimization. Hence the optimizer is forced to execute the subquery first.
回答3:
The inner join in your sub-query is unnecessary. It looks like you want to delete the entries in story_category where the category_id is not in the category table.
Do this:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id
FROM category);
Instead of that:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id
FROM category INNER JOIN
story_category ON category_id=category.id);
回答4:
Recently i had to update records in the same table i did it like below:
UPDATE skills AS s, (SELECT id FROM skills WHERE type = 'Programming') AS p
SET s.type = 'Development'
WHERE s.id = p.id;
回答5:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT cid FROM (
SELECT DISTINCT category.id AS cid FROM category INNER JOIN story_category ON category_id=category.id
) AS c
)
回答6:
If you can't do
UPDATE table SET a=value WHERE x IN
(SELECT x FROM table WHERE condition);
because it is the same table, you can trick and do :
UPDATE table SET a=value WHERE x IN
(SELECT * FROM (SELECT x FROM table WHERE condition) as t)
[update or delete or whatever]
回答7:
This is what I did for updating a Priority column value by 1 if it is >=1 in a table and in its WHERE clause using a subquery on same table to make sure that at least one row contains Priority=1 (because that was the condition to be checked while performing update) :
UPDATE My_Table
SET Priority=Priority + 1
WHERE Priority >= 1
AND (SELECT TRUE FROM (SELECT * FROM My_Table WHERE Priority=1 LIMIT 1) as t);
I know it's a bit ugly but it does works fine.
回答8:
You could insert the desired rows' ids into a temp table and then delete all the rows that are found in that table.
which may be what @Cheekysoft meant by doing it in two steps.
回答9:
The simplest way to do this is use a table alias when you are referring parent query table inside the sub query.
Example :
insert into xxx_tab (trans_id) values ((select max(trans_id)+1 from xxx_tab));
Change it to:
insert into xxx_tab (trans_id) values ((select max(P.trans_id)+1 from xxx_tab P));
回答10:
According to the Mysql UPDATE Syntax linked by @CheekySoft, it says right at the bottom.
Currently, you cannot update a table and select from the same table in a subquery.
I guess you are deleting from store_category while still selecting from it in the union.
回答11:
If something does not work, when coming thru the front-door, then take the back-door:
drop table if exists apples;
create table if not exists apples(variety char(10) primary key, price int);
insert into apples values('fuji', 5), ('gala', 6);
drop table if exists apples_new;
create table if not exists apples_new like apples;
insert into apples_new select * from apples;
update apples_new
set price = (select price from apples where variety = 'gala')
where variety = 'fuji';
rename table apples to apples_orig;
rename table apples_new to apples;
drop table apples_orig;
It's fast. The bigger the data, the better.
回答12:
try this
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id
FROM (SELECT * FROM STORY_CATEGORY) sc;
回答13:
Try to save result of Select statement in separate variable and then use that for delete query.
回答14:
how about this query hope it helps
DELETE FROM story_category LEFT JOIN (SELECT category.id FROM category) cat ON story_category.id = cat.id WHERE cat.id IS NULL
回答15:
For the specific query the OP is trying to achieve, the ideal and most efficient way to do this is NOT to use a subquery at all. Here are the LEFT JOIN versions of the OP's two queries:
SELECT s.* FROM story_category s LEFT JOIN category c ON c.id=s.category_id WHERE c.id IS NULL;
DELETE s FROM story_category s LEFT JOIN category c ON c.id=s.category_id WHERE c.id IS NULL;
来源：https://stackoverflow.com/questions/45494/mysql-error-1093-cant-specify-target-table-for-update-in-from-clause