SQL query on multiple tables, one being a junction table

Question

I have a fairly simple database consisting of 4 tables:

Table 1: USERS
Columns:  userID, user_name

Table 2: GROUPS
Columns:  groupID, group_name

Table 3 (Junction Table): GROUP_MATRIX
Columns:  userID, groupID

Table 4: Messages
Columns: messageID, message, userID, groupID

I'd like to do a query for all messages with results in the following format:

user_name, message

I formed the query like so:

SELECT USERS.user_name, MESSAGES.message
FROM GROUP_MATRIX
JOIN USERS on GROUP_MATRIX.userID = USERS.userID
JOIN MESSAGES on GROUP_MATRIX.userID = MESSAGES.userID

It sort of works, but I'm getting some duplicate results back. It seems that if a user_ID shows up in the GROUP_MATRIX more than once, that is when I get duplicate results for that user. Clearly I don't understand the JOIN's I'm trying to do, can someone help me understand what I'm doing wrong?

Answer 1

Use the keyword DISTINCT in the SELECT clause like this:

SELECT DISTINCT USERS.user_name, MESSAGES.message
FROM GROUP_MATRIX
JOIN USERS on GROUP_MATRIX.userID = USERS.userID
JOIN MESSAGES on GROUP_MATRIX.userID = MESSAGES.userID;

Answer 2

Since you only want user_name and messages you don't want to use groups and junction table.

Select 
   us.user_name, msg.message 
   from Messages as msg 
   LEFT JOIN USERS as us ON (msg.userID = us.userID)
   groupby msg.messageID