如何使用case语句来比较多列 [英] How to use case statement to compare multiple columns

问题描述

你好，

我有2张桌子：

1）桌面课程（id，姓名）

2）桌子预订（id，姓名）

大学的经理可以保留任何课程，

确保课程保留在哪里，然后系统应插入本课程的ID和名称预订表，
预订课程后
，

i想要显示课程名称附近的栏目：

if（id，name ）到课程表包含在预订表的（ID，名称）然后'课程保留'否则'不保留'

i尝试不在声明，但它不适用于多个列

谢谢

Hello,
I have 2 tables :
1)table course (id,name)
2)table reservation (id,name)
The manager in the university can reserve any course,
sure where a course is reserved then the id and the name of this course should be inserted by the system in the reservation table,
after reserve courses,
i want to show a column near to the name of the course :
if (id,name) to the course table are contained in the (id,name) to the reservation table then 'Course Reserved' else 'Not Reserved'
i tried not in statement but it don't work on multiple columns
thanks

推荐答案

请阅读我对这个问题的评论。



我不确定你的要求，但试试这个：

Please, read my comments to the question.

I'm not sure your requirements, but try this:

SELECT c.ID AS CourseID, CASE WHEN r.ID IS NULL THEN 'Not reserved' ELSE 'Reserved' END AS ReservationID
FROM Course AS c LEFT JOIN Reservation AS r ON c.ID = r.ID

你可以与 COALESCE 函数类似的效果：

You can achive similar effect with COALESCE function:

SELECT c.ID AS CourseID, COALESCE(r.ID, 'Not reserved') AS ReservationID
FROM Course AS c LEFT JOIN Reservation AS r ON c.ID = r.ID

请阅读这些文章：

SQL连接的可视化表示 [ ^ ]

CASE（SQL） [ ^ ]

COALESCE（SQL） [ ^ ]

Please, read these articles:
Visual Representation of SQL Joins[^]
CASE (SQL)[^]
COALESCE (SQL)[^]

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