遞減在MySQL停在0

Question

嘿傢伙我有一個MySQL查詢，減少股票每按一下按鈕，

查詢

UPDATE Animals 
    SET Stock = Stock - 1 
WHERE Specie ='{1}' and Country ='{0}'   

問題是，我希望它停止在0和沒有進入負面是有辦法做到這一點？

Answer 1

這應該工作：

 

"UPDATE Animals SET Stock = Stock - 1 WHERE Specie ='{1}' and Country ='{0}' and Stock >0" 
 

Answer 2

UPDATE Animals 
    SET Stock = Stock - 1 
WHERE Specie ='{1}' and Country ='{0}' AND Stock > 0 

所以限制了行，從而具有Stock > 0明確

Answer 3

... SET Stock = IF(Stock < 1, Stock, Stock -1)...

Answer 4

，以防止過度的排序，你可能要檢查，看是否有實際存貨，保留備用，然後通過交易做出決定：

START TRANSACTION; 
SELECT Stock 
    FROM Animals 
    WHERE Specie ='{1}' and Country ='{0}' AND Stock > 0 
    FOR UPDATE; 

如果您沒有獲得記錄，那麼股票是< = 0.如果您獲得了一條記錄，那麼您已經有剩餘的股票數量。

現在更新：

UPDATE Animals SET Stock = Stock - 1 WHERE Specie ='{1}' and Country ='{0}'; 
COMMIT; 

否則，

ROLLBACK; 

你要做到這一點在同一個會話（連接）。