我的数据库中有一个表story_category,其中包含损坏的条目。下一个查询将返回损坏的条目:

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

我试图删除它们执行:

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

但我得到了下一个错误:

#1093-不能在FROM子句中为更新指定目标表“story_category”

我如何克服这一问题?


当前回答

如果Priority列值在表中大于等于1,并且在WHERE子句中使用同一表上的子查询,以确保至少有一行包含Priority=1(因为这是在执行更新时要检查的条件),则我会这样做:


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);

我知道它有点难看,但它确实很好。

其他回答

最简单的方法是在子查询中引用父查询表时使用表别名。

例子:

insert into xxx_tab (trans_id) values ((select max(trans_id)+1 from xxx_tab));

将其更改为:

insert into xxx_tab (trans_id) values ((select max(P.trans_id)+1 from xxx_tab P));

如果从前门进来时有些东西不起作用,那么从后门走:

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;

速度很快。数据越大越好。

NexusRex提供了一个非常好的解决方案,可以从同一个表中删除带有联接的内容。

如果您这样做:

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
)

你会得到一个错误。

但如果您将条件包装为一个以上的选择:

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
)

它会做正确的事情!!

解释:查询优化器对第一个查询进行派生合并优化(这会导致它失败并出现错误),但第二个查询不符合派生合并优化的条件。因此,优化器被迫首先执行子查询。

就问题而言,您希望删除story_category中不存在于类别中的行。

以下是用于标识要删除的行的原始查询:

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

将NOT IN与JOIN原始表的子查询组合在一起似乎不太复杂。这可以用不存在和相关子查询以更直接的方式表示:

select sc.*
from story_category sc
where not exists (select 1 from category c where c.id = sc.category_id);

现在很容易将其转换为delete语句:

delete from story_category
where not exists (select 1 from category c where c.id = story_category.category_id);    

这个quer可以在任何MySQL版本上运行,也可以在我知道的大多数其他数据库中运行。

DB Fiddle演示:

-- set-up
create table story_category(category_id int);
create table category (id int);
insert into story_category values (1), (2), (3), (4), (5);
insert into category values (4), (5), (6), (7);

-- your original query to identify offending rows
SELECT * 
FROM  story_category 
WHERE category_id NOT IN (
    SELECT DISTINCT category.id 
    FROM category INNER JOIN 
       story_category ON category_id=category.id);
| category_id |
| ----------: |
|           1 |
|           2 |
|           3 |
-- a functionally-equivalent, simpler query for this
select sc.*
from story_category sc
where not exists (select 1 from category c where c.id = sc.category_id)
| category_id |
| ----------: |
|           1 |
|           2 |
|           3 |
-- the delete query
delete from story_category
where not exists (select 1 from category c where c.id = story_category.category_id);

-- outcome
select * from story_category;
| category_id |
| ----------: |
|           4 |
|           5 |

根据@CheekySoft链接的Mysql UPDATE语法,它就在底部。

当前,无法更新表并从子查询中的同一表中进行选择。

我猜您正在从store_category中删除,同时仍在联盟中从中进行选择。