Fetch implementations from MySQL?

Question

MySQL

Hello!
There are 5 tables with relationships,

Organizations
Users
friends
Messages

fields for Mesages :

id
user_id not null
organization_id
text

if the message is sent from an Organization , then the fields are filled in
user_id - user ID (message sender)
organization_id - organization ID (in which the message was posted)
if the message is sent by User -then the
user_id fields are filled - the user id (sender of the message)
organization_id - will be NULL
I do not know how to get all the messages of a specific user, his friends and organizations in which he is a member.
Of course, you can cycle, but this is not good, tell me where to dig?

Answer 1

www.mysql.ru/docs/man/UNION.html