一次兩個表的條​​件SQL更新語句

Question

我想一次更新兩個表。 下面的代碼似乎工作正常。 但是，在某些情況下，bidGroups中沒有條目，這意味着整個語句將失敗。 如何調整它以便更新第一位（watchedItems）並且如果watchedItems.bidGroupID IS NULL則不嘗試第二部分

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;

我試過這個，但語法錯了..

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

你需要做一個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為空，那么它只會更新在發現的行watchedItems ，因為沒有什么可以加入到bidGroups 。 如果不是，並且連接在兩個表中都獲得了行，那么對兩個表的更新都會發生。

僅使用watchedItems, bidGroups與inner join watchedItems, bidGroups相同，因此當bidGroups中bidGroups要加入的條目時，您也無法從watchedItems獲得結果。 在select而不是update上使用相同的join和where子句，您將看到差異：

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

以這種方式嘗試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

如果你不想改變，只需將其設置為當前值。 並且MySQL通常在值相同時不更新它來優化它。

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因為如果watchedItems.bidGroupID IS NULL則不會加入bidGroups中的行，並忽略該不存在的行的更新。 所以bidGroups.bidGroupQty=bidGroups.bidGroupQty-1就好了