Question

我有下面的SQL语句，但我遇到了一个问题，我还没弄明白如何正确地订购WHERE语句。

当我运行这个语句时，它应该返回0，因为对于categoryid为2没有问题，并且只要我取出或者useranswers.level = 5就可以工作，因为还有其他类别id＆＃39; s这是第5级。

所以我的问题是我需要在哪里提出或声明？

SELECT COUNT(*) FROM useranswers INNER JOIN users on useranswers.userid = users.id 
INNER JOIN kumiquestions on useranswers.questionid = kumiquestions.id 
WHERE login = 'chris3spice' 
and kumiquestions.categoryid = 2 and useranswers.level = 4 or useranswers.level = 5;

Answer 1

使用括号对相关条件进行分组。

Answer 2

逻辑运算符具有优先权 - ＆＃34;和＆＃34;最高的，然后＆＃34;或＆＃34;。所以，你需要把paretheses，像这样

and kumiquestions.categoryid = 2 and (useranswers.level = 4 or useranswers.level = 5);

Answer 3

两种解决方案：

SELECT COUNT(*) FROM useranswers INNER JOIN users on useranswers.userid = users.id 
INNER JOIN kumiquestions on useranswers.questionid = kumiquestions.id 
WHERE login = 'chris3spice' 
and kumiquestions.categoryid = 2 and (useranswers.level = 4 or useranswers.level = 5);

OR

SELECT COUNT(*) FROM useranswers INNER JOIN users on useranswers.userid = users.id 
INNER JOIN kumiquestions on useranswers.questionid = kumiquestions.id 
WHERE login = 'chris3spice' 
and kumiquestions.categoryid = 2 and useranswers.level IN (4, 5);

两者在逻辑上都是相同的。

Answer 4

你可以写：

SELECT COUNT(*) FROM useranswers INNER JOIN users on useranswers.userid = users.id 
INNER JOIN kumiquestions on useranswers.questionid = kumiquestions.id 
WHERE login = 'chris3spice' 
and kumiquestions.categoryid = 2 and (useranswers.level = 4 or useranswers.level = 5);

如果您没有明确使用括号，则根据OR的位置自动完成分组，如下所示：

SELECT COUNT(*) FROM useranswers INNER JOIN users on useranswers.userid = users.id 
INNER JOIN kumiquestions on useranswers.questionid = kumiquestions.id 
WHERE (login = 'chris3spice' 
and kumiquestions.categoryid = 2 and useranswers.level = 4) or (useranswers.level = 5);

这就是为什么当你没有使用括号时你没有得到count（*）= 0。

MySQL AND OR语句顺序

4 个答案: