Question

| postid | ref_postid | title   |
| 1      | 0          | Title 1 | 
| 2      | 1          |         | 
| 3      | 1          |         | 
| 4      | 0          | Title 2 | 
| 5      | 4          |         |

It´s a table for a discussion forum. When ref_postid = 0, that means it is a main post. When ref_postid is not 0 it is a answer and references to a postid, see table example above. (Only main posts has a title.)

I want to select all rows in the table, but the problem is that I want to display the title for the discussion an answer refers to.

Example: Let´s say I want the row where postid = 2, I also want to get “Title 1” which is the name of the discussion.

I tried doing it with CASE but I just got errors. I´m not very good at this. Is it doable with one query? Or will I have to use two queries?

Thanks for your help!

Answer 1

select ifnull(q.title, a.title) title, ... (other columns)
from posts a
left join posts q on a.ref_postid <> 0 and a.ref_postid = q.postid