如何实现sql查询以基于2列进行外部联接

Question

I have Table x containing Courses done by Users but another Table y containing courses assigned to users. I need to retrieve all courses with null row if user doesn't complete the Course

[Table M] [Table X] [Table Y]
UserId UserId Cid CourseId
1 1 1 1 2
2

I need

[Table Z]

[UserId] [CourseId] [F]
1 1 done
1 2 not done
2 2 not done
2 1 not done

Answer 1

You could do something like this

SQL Fiddle Example

SELECT tm.userid,
  ty.courseid,
  CASE WHEN tx.cid IS NULL THEN 'not done' ELSE 'done' END AS 'F'
FROM tablem tm
JOIN tabley ty ON 1=1
LEFT JOIN tablex tx ON tm.userid = tx.userid AND ty.courseid = tx.cid
ORDER BY tm.USERID