Question

我有以下查询从2个表格中收集一些信息（Students和Subjects）

SELECT 
    'StudentName' As Student,
    'Class'   As Class,
    'Subject' As Subject
    DATEDIFF( HOUR, hoursSpent.ClassStartTime, hoursSpent.ClassEndTime ) As HoursSpent
  FROM (
        SELECT stu.StudentName from Students s

        INNER JOIN Subjects  sub
        WHERE stu.StudentId = sub.StudentId
       ) hoursSpent

我有另一个表与布局中Students表完全相同的表称为OldStudents。

我的问题是我想要合并来自Students和OldStudents的数据并显示为一个但不知道我该怎么做。

我尝试了这个，但这不起作用：

SELECT 
    'StudentName' As Student,
    'Class'   As Class,
    'Subject' As Subject
    DATEDIFF( HOUR, hoursSpent.ClassStartTime, hoursSpent.ClassEndTime ) As HoursSpent
  SELECT(
            FROM (
                SELECT stu.StudentName from Students s
                INNER JOIN Subjects  sub
                WHERE stu.StudentId = sub.StudentId
               ) hoursSpent,
          FROM (
                SELECT stu.StudentName from Students s
                INNER JOIN Subjects  sub
                WHERE stu.StudentId = sub.StudentId
               ) hoursSpent )

Answer 1

基本上，您需要有两个相同的查询，每个表一个，并使用UNION（或UNION ALL连接它们：

SELECT 
    'StudentName' As Student,
    'Class'   As Class,
    'Subject' As Subject
    DATEDIFF( HOUR, hoursSpent.ClassStartTime, hoursSpent.ClassEndTime ) As HoursSpent
  FROM (
        SELECT stu.StudentName from Students s

        INNER JOIN Subjects  sub
        WHERE stu.StudentId = sub.StudentId
       ) hoursSpent
UNION ALL
SELECT 
    'StudentName' As Student,
    'Class'   As Class,
    'Subject' As Subject
    DATEDIFF( HOUR, hoursSpent.ClassStartTime, hoursSpent.ClassEndTime ) As HoursSpent
  FROM (
        SELECT stu.StudentName from OldStudents s

        INNER JOIN Subjects  sub
        WHERE stu.StudentId = sub.StudentId
       ) hoursSpent

UNION会使结果明显，UNION ALL不会。

Answer 2

您可以尝试合并（合并）Students和OldStudents，然后将结果加入到查询的其余部分：

WITH AllStudents
AS (
  SELECT * FROM Students
  UNION
  SELECT * FROM OldStudents
)
SELECT 
    'StudentName' As Student,
    'Class'   As Class,
    'Subject' As Subject
    DATEDIFF( HOUR, hoursSpent.ClassStartTime, hoursSpent.ClassEndTime ) As HoursSpent
  FROM (
        SELECT stu.StudentName from AllStudents stu
        INNER JOIN Subjects  sub
        WHERE stu.StudentId = sub.StudentId
       ) hoursSpent

Answer 3

如果您只想将一组数据添加到另一组，请使用UNION运算符：

SELECT * FROM TableA
UNION
SELECT * FROM TableB

每个SELECT语句必须具有相同的列数

SQL查询组合来自两个表的数据

3 个答案: