MySQL 错误代码 : 1093. 您无法在 FROM 子句中指定要更新的目标表 'xyz'

Question

我正在尝试针对下表编写查询

Table A
========
ID
A1


Table B
=========
ID
B1

查询 1:

update A
set A.A1 =(
select count (distinct b1)
from B
join A on A.ID = B.ID
Group by B1

给我一​​个错误-

MySQL Error Code: 1093. you can't specify target table for update in FROM clause

因此我尝试了以下查询:

查询 2:

update A
set A1 = (
  select count from (
    select count( distinct B1) as count
        from B 
        join A on A.ID = B.ID
        group by ID
  ) s
);

上面的查询给我错误

Error Code: 1242. Subquery returns more than 1 row

我试过如下创建暂存表并且它有效。

查询 3:

insert into stg
(
par1,
par2
)
select ID, count( distinct B1) from B group by ID;


update A
set A1 =  ( select par2 from stg where A.pm = stg.par1 );

有没有更好的方法来做同样的事情？

谢谢

Answer 1

如果您只是计数而没有引用计数，则跳过 GROUP BY

改变。

update A 
JOIN (select count( distinct B1) as count FROM B JOIN A on A.ID = B.ID) B
set A.A1=B.count;