Question

I have a simple messaging schema where each message has a senderFK and a recipientFK, each mapping to a user record. For the current user, I need to get all distinct threads. A distinct thread is the either the senderFK or the recipientFK for the user who has either sent or received a message.

So, for example:

This query gets all users who sent a message to the current user:

SELECT DISTINCT senderFK AS threadID FROM Messages
WHERE recipientFK = 'usr_developer'

This query gets all users who have received a message from the current user:

SELECT DISTINCT recipientFK AS threadID FROM Messages
WHERE senderFK = 'usr_developer'

How would I combine these queries into a single table that has a distinct row for each user and a single column (threadID)?

Answer 1

Something like this should work:

SELECT recipientFK as UserId, senderFK as ThreadID
 FROM Messages
UNION
SELECT senderFK as UserID, recipientFK AS threadID
 FROM Messages

This method leaves out user filters based on your comment ...a distinct row for each user ...