在MySQL中递减到0

Question

大家好，我有一个MySQL查询，可减少每次单击按钮的库存，

询问

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

问题是我希望它停止在0而不是变成负数吗？有没有办法做到这一点？

Answer 1

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

因此限制行明确地使Stock > 0

Answer 2

这应该工作：

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

Answer 3

为了防止订单过多，您可能需要检查是否确实有库存，将其保留为备用，然后通过交易来做出决定：

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

如果没有记录，则库存量<=0。如果有记录，则具有剩余库存量。

现在更新它：

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

除此以外，

 ROLLBACK;

您将需要在同一会话（连接）上执行此操作。

Answer 4

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

Answer 5

如果您想将库存减少1个以上，

这里的'{2}'是指要减少的数量。

UPDATE Animals
  SET Stock = Stock - '{2}'
  WHERE 
     Specie ='{1}' and Country ='{0}' 
     AND Stock - '{2}' >= 0