从左表和全联接右表中选择值

Question

This is a bit difficult to explain, but I'll give my best:

Let's say, I have table A:

event | task | ref_person
------+------+-----------
  1   |  20  |     1
  2   |   9  |     2

And I have table B (containing person ):

id | name
---+-----
 1 | foo
 2 | bar
 3 | jim

What does a MySQL-query look like, that produces this sort of table:

 event | task | person
 ------+------+-------
   1   |  20  |   foo
   1   | NULL |   bar
   1   | NULL |   jim
   2   | NULL |   foo
   2   |   9  |   bar
   2   | NULL |   jim

My current approach is by using a RIGHT JOIN , but this won't get me the event combined with the NULL-value.

This is what my current statement looks like:

SELECT
    a.*,
    b.name

FROM
    a

RIGHT JOIN b
    ON b.id = a.ref_person

ORDER BY 
    a.event,
    b.name

Notice

sqlfiddle seems down, I'll add one as soon as it's up again

Answer 1

You want to do a cross join to get all the rows, then case logic to get the task:

select a.event,
       (case when a.ref_person = b.id then a.task end) as task,
       b.name
from tablea a cross join
     tableb b ;