同时为两个表创建条件SQL更新语句

Question

我想一次更新两个表。下面的代码似乎工作正常。但是，在某些情况下，bidGroups中没有条目，这意味着整个语句会失败。我如何调整，以便它更新的第一位（watchedItems）并不会尝试第二部分，如果watchedItems.bidGroupID IS NULL

UPDATE watchedItems, bidGroups 
SET watchedItems.won=1, bidGroups.bidGroupQty=bidGroups.bidGroupQty-1 
WHERE watchedItems.id=2 
AND watchedItems.aid=200618152822 
AND bidGroups.bidGroupID=watchedItems.bidGroupID 
AND bidGroups.id=2; 

我想这一点，但语法错了..

UPDATE watchedItems, bidGroups 
SET watchedItems.won=1, 
CASE WHEN watchedItems.bidGroupID IS NOT NULL THEN bidGroups.bidGroupQty=bidGroups.bidGroupQty-1 
ELSE END 
WHERE watchedItems.id=2 
AND watchedItems.aid=200618152822 
AND bidGroups.bidGroupID=watchedItems.bidGroupID 
AND bidGroups.id=2 

Answer 1

尝试LEFT JOIN这样：

UPDATE watchedItems 
LEFT JOIN bidGroups ON watchedItems.bidGroupID = bidGroups.bidGroupID AND bidGroups.id=2 
SET watchedItems.won=1, bidGroups.bidGroupQty=bidGroups.bidGroupQty-1 
WHERE watchedItems.id=2 
AND watchedItems.aid=200618152822; 

......

Answer 2

只要将其设置为当前值，如果您不想更改。 当数值相同时，MySQL通常会通过不更新来优化它。

UPDATE watchedItems 
    LEFT JOIN bidGroups ON bidGroups.bidGroupID=watchedItems.bidGroupID 
     AND bidGroups.id=2 
SET watchedItems.won=1, 
    bidGroups.bidGroupQty= if(watchedItems.bidGroupID IS NULL, 
           bidGroups.bidGroupQty, 
           bidGroups.bidGroupQty-1) 
WHERE watchedItems.id=2 
AND watchedItems.aid=200618152822; 

不过，你不需要在if因为如果watchedItems.bidGroupID IS NULL然后从bidGroups行没有加入和更新为不存在的行被忽略。 所以bidGroups.bidGroupQty=bidGroups.bidGroupQty-1就好了

Answer 3

你需要做一个left outer join就是这样。

update watchedItems wi left outer join bidGroups bg 
on wi.bidGroupID = bg.bidGroupID 
set 
    wi.won = 1, 
    bg.bidGroupQty = bg.bidGroupQty - 1 
where wi.id = 2 
and wi.aid = 200618152822 
and (bg.id = 2 or bg.id is null) 

如果watchedItems.bidGroupId为空，那么它只会在watchedItems更新发现的行，因为没有什么可以加入到bidGroups。如果不是，并且连接在两个表中都获得行，则对两个表的更新都会发生。

只需使用watchedItems, bidGroups相同inner join，所以当没有在bidGroups一个条目加入到，你还没有得到来自watchedItems结果。使用上select而不是update相同join和where条款，你会看到其中的差别：

select wi.won, bg.bidGroupQty 
from watchedItems wi, bidGroups bg 
where wi.id = 2 and wi.aid = 200618152822 
and (bg.id = 2 or bg.id is null) 
and wi.bidGroupID = bg.bidGroupID 

VS：

select wi.won, bg.bidGroupQty 
from watchedItems wi left outer join bidGroups bg 
on wi.bidGroupID = bg.bidGroupID 
where wi.id = 2 and wi.aid = 200618152822 
and (bg.id = 2 or bg.id is null)