Question

I have two tables, one called “pinned” and one called “unread”

“pinned” looks like this:

+---------+-------+
|pinned_by|post_id|
+---------+-------+
|2        |3      |
+---------+-------+
|2        |5      |
+---------+-------+

“unread” looks like this:

+---------+-------+
|unread_by|post_id|
+---------+-------+
|2        |5      |
+---------+-------+
|2        |10     |
+---------+-------+

I want to select this from the two tables:

+-------+------+------+
|post_id|unread|pinned|
+-------+------+------+
|3      |0     |1     |
+-------+------+------+
|5      |1     |1     |
+-------+------+------+
|10     |1     |0     |
+-------+------+------+

How can I do this? The values that come through for pinned and unread could be 1/0, 1/null, true/false, etc. I don’t really care as long as I could differentiate which post_id’s came from unread, which came pinned, and which came from both. I’m using MySQL. The _by columns all have 2 in this example, but will vary in the actual implementation. The thought is that a where unread_by=2 and where pinned_by=2 will be included somehow.

Thanks

Answer 1

Presuming that there is a table of Posts, you can do:

Select P.post_Id
    , Max( Case When UR.unread_by Is Not Null Then 1 Else 0 End ) As unread
    , Max( Case When P2.pinned_by Is Not Null Then 1 Else 0 End ) As pinned
From Posts As P
    Left Join Unread As UR
        On UR.post_Id = P.post_Id
    Left Join Pinned As P2
        On P2.post_id = P.post_id
Group By P.post_id