Question

SELECT ModuleID 
  FROM tblActivity
 Where (Day = 2 And Day = 3 )

在这里，我试图显示星期二（2）和星期三（3）的ModuleID。所有信息都存储在同一个表中。当我执行它时，它只显示模块ID作为列，而下面没有数据。

Answer 1

你应该使用OR而不是AND。您也可以使用IN，如下所示

SELECT ModuleID 
  FROM tblActivity
Where Day IN (2,3)

Answer 2

where语句由ROW评估，因此请考虑以下示例：

ModuleId       Day
1               1
2               2
3               3
4               4
5               2
6               2
7               3

要参加这组数据的请求，ModuleId的结果为2,3,5,6和7

当数据库评估ROW所处的条件时，所以当你说(Day = 2 And Day=3)时，它会像这样测试每一行：

1               1   is day=2 - false AND is day=3 - false
2               2   is day=2 - TRUE  AND is day=3 - FALSE | TRUE and FALSE = FALSE
3               3   is day=2 - false AND is day=3 - TRUE  | FALSE and TRUE = FALSE
4               4   is day=2 - false AND is day=3 - false    and so on for next lines
5               2   is day=2 - false AND is day=3 - false
6               2   is day=2 - false AND is day=3 - false
7               3   is day=2 - false AND is day=3 - false

这就是你必须使用OR

的原因

select ModuleID
  from tblActivity
 where days = 2 or days = 3

将评估为

1               1   is day=2 - false OR is day=3 - false > FALSE
2               2   is day=2 - TRUE  OR is day=3 - FALSE | TRUE OR FALSE = TRUE
3               3   is day=2 - false OR is day=3 - TRUE  | FALSE OR TRUE = TRUE
4               4   is day=2 - false OR is day=3 - false    and so on for next lines
5               2   is day=2 - false OR is day=3 - false
6               2   is day=2 - false OR is day=3 - false
7               3   is day=2 - false OR is day=3 - false

请在此处查看：http://sqlfiddle.com/#!9/a7c5b0/1

如评论中所述，您可以使用IN运算符：

select ModuleID
  from tblActivity
 where days IN (2, 3)

请在此处查看：http://sqlfiddle.com/#!9/a7c5b0/2

如何根据一个列值显示同一个表中的多个数据

2 个答案: