php - mySQl，查询 : How to customize selection from database?

Question

我有四张 table :

courses, allocate_rooms, rooms, departments 

查询:

SELECT
    courses.`name`,
    courses.`code`,
    allocate_rooms.`start`,
    allocate_rooms.`end`,
    rooms.room_number
FROM
    departments
JOIN courses ON departments.id = courses.department_id
LEFT JOIN allocate_rooms ON allocate_rooms.course_id = courses.id
LEFT JOIN rooms ON allocate_rooms.room_id = rooms.id
WHERE
    departments.id = 1

从查询中，我必须将选择 View 设置为 rooms.room_number、allocate_rooms.start-allocate.rooms_end；如果没有与该类(class)相关的数据，我必须显示“尚未安排”。

Eg: R.No: 301,12:00-12:30; (if the course related data is there otherwise it will show "Not Scheduled Yet"

如何重写上面的查询？如果有人帮助我找到解决方案。

Answer 1

我不确定这是否是您所需要的，但请尝试以下操作:

您可以使用 DECODE 来获取您需要的内容。

SELECT
    c.name name
   ,c.code code
   ,decode(ar.start,NULL,'Not Schedule Yet') START
   ,decode(ar.end,NULL,'Not Schedule Yet') END
   ,r.room_number roomnum
FROM
    departments d
   ,courses c
   ,allocate_rooms ar
   ,rooms r
WHEREd.id = c.id
AND ar.course_id = c.id
AND ar.room_id = r.room_id
AND d.id = 1;