php – 有条件地插入到MySQL中 – 不存在

当客户还没有购买产品的记录时,我正在尝试执行INSERT INTO查询.

我已经尝试了下面的sql,但似乎没有用.

$queryAddPurchase = "INSERT INTO purchase (UserID,Product,Price) 
              VALUES ('$userID','$product','$price')
              WHERE NOT EXISTS(SELECT Product
                                       FROM purchase
                       WHERE Product = '$product'
                       AND UserID = '$userID')";

我收到的错误如下：

You have an error in your sql Syntax; check the manual that corresponds to your MysqL server version for the right Syntax to use near ‘WHERE NOT EXISTS(SELECT Product FROM purchase ‘ at line 3

任何建议将不胜感激！

假设用户可能只购买每种产品中的一种(永远和所有产品).

ALTER TABLE purchase ADD UNIQUE KEY (`UserID`,`Product`); -- run this just once. this changes the table

INSERT IGnorE INTO purchase (UserID,Price) VALUES ('$userID','$price');

请注意,这会阻止他多次购买任何产品,这可能不是理想的结果.