Question

我是SQL新手，但需要为必须与数据库交互的Java程序编写一些SQL语句。我们的Java教科书仅涵盖了非常基本的SQL命令，而且我无法获得更高级的（按我的标准）工作。

情况如下：

数据库有5个表。

Teacher: TeacherID (PK), LastName
Class: ClassID (PK), Description
Room: Building, Room Number, PK is the combo of those two
TeachingAssignments: TeacherID(FK), ClassID(FK)
ClassRoomAssignments: ClassID(FK), Building, Room Number(combo is FK)

我只需要提供完全分配的教师，班级和房间的LastName, ClassID, and Building。即，如果一个班级都有教师和房间分配，那么我需要提供该班级的ID，指定教师的姓氏和指定的建筑物。

我不知道如何继续。

我一直在玩下面的陈述，但它们对我不起作用：

SELECT Teacher.LastName, Class.ClassID, Room.Building 
FROM Teacher, Class, Room, TeachingAssignments, ClassRoomAssignments 
WHERE Teacher.TeacherID = TeachingAssignments.TeacherID 
    AND Room.Building = ClassRoomAssignments.Building
    AND Class.ClassID = TeachingAssignments.ClassID
    AND Class.ClassID = ClassRoomAssignments.ClassID

有人可以帮忙吗？谢谢！

Answer 1

您的问题是您需要为表添加相应的连接。

而不是：

SELECT Teacher.LastName, Class.ClassID, Room.Building 
FROM Teacher, Class, Room, TeachingAssignments, ClassRoomAssignments 
WHERE Teacher.TeacherID = TeachingAssignments.TeacherID 
AND Room.Building = ClassRoomAssignments.Building 
AND Class.ClassID = TeachingAssignments.ClassID 
AND Class.ClassID = ClassRoomAssignments.ClassID

你需要类似的东西

SELECT Teacher.LastName, Class.ClassID, Room.Building 
FROM 
Teacher INNER JOIN TeachingAssignments
  ON Teacher.TeacherID = TeachingAssignments.TeacherID 
INNER JOIN Class
  ON Class.ClassID = TeachingAssignments.ClassID
INNER JOIN ClassRoomAssignments
  ON Class.ClassID = ClassRoomAssignments.ClassID
INNER JOIN Room
  ON Room.Building = ClassRoomAssignments.Building

正如您所看到的，每个INNER Join后面跟着它的各个ON子句，它负责设计要连接的元素。

SQL查询根据其他表中的外观限制结果

1 个答案: