一次两个表的条​​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，这意味着整个语句将失败。 如何调整它以便更新第一位（watchedItems）并且如果watchedItems.bidGroupID IS NULL则不尝试第二部分

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;

我试过这个，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

你需要做一个left outer join 。

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，那么它只会更新在发现的行watchedItems ，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了行，那么对两个表的更新都会发生。

仅使用watchedItems, bidGroups与inner join watchedItems, bidGroups相同，因此当bidGroups中bidGroups要加入的条目时，您也无法从watchedItems获得结果。 在select而不是update上使用相同的join和where子句，您将看到差异：

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)

Answer 2

以这种方式尝试LEFT JOIN：

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 3

如果你不想改变，只需将其设置为当前值。 并且MySQL通常在值相同时不更新它来优化它。

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;

但是，您不需要if因为如果watchedItems.bidGroupID IS NULL则不会加入bidGroups中的行，并忽略该不存在的行的更新。 所以bidGroups.bidGroupQty=bidGroups.bidGroupQty-1就好了