我怎样才能完成这项工作

Question

我怎样才能使这个有效？

我有两张桌子：

当然：
课程ID，姓名
前提条件：
课程ID，先决条件

此结果的查询是什么：

+----------+------------+----------+------------+
|Course ID |Name        |Prereq ID |Prereq Name |
|1         |Intro into B|          |            |
|2         |Biology     |1         |Intro into B|
|3         |Genetics    |1         |Intro into B|
+----------+------------+----------+------------+

Answer 1

您应该可以使用左join将这两个表拉到一起，例如：

Select c.courseid, c.coursename, p.prere1id, p.preqname 
from course as c 
left join prereq as p 
on c.courseid = p.courseid

Answer 2

SELECT c.courseId, c.courseName, p.prereqId, pc.courseName
FROM course as c
     // joins prerequisites
     LEFT JOIN prereq as p on p.courseId=c.courseId
     // joins course data to prerequisites
     LEFT JOIN course as pc on p.prereqId=pc.courseId