SQL query that checks another table's data

Question

I'm not too good with the more complicated SQL queries like JOIN, so I'm clueless in this aspect.

I've got three tables:

FORUM_Topics
FORUM_Threads
FORUM_Replies

A forum topic is the highest level.

A forum thread is inside a topic.

A forum reply is inside a thread.

Replies don't directly say what topic they're in, only what thread. The thread then says what topic.

So it looks sort of like this:

FORUM_Topics
ID = 1
Name = A topic

FORUM_Threads
ID = 1
TopicID = 1
Name = A forum thread

FORUM_Replies
ID = 1
ThreadID = 1
Name = A forum reply

If I wanted to see how many threads in a topic, it's as simple as:

$threads = mysql_query("SELECT * FROM FORUM_Threads WHERE TopicID = $ID");

but how would I check how many replies in a topic?

Answer 1

select FORUM_Replies.* from FORUM_Replies
inner join FORUM_Threads on FORUM_Replies.ThreadID = FORUM_Threads.ID
where FORUM_Threads.TopicID = $ID

You might want to get result from both Replies and Threads.

select FORUM_Replies.Name as ReplyName, FORUM_Threads.Name as ForumName from FORUM_Replies
inner join FORUM_Threads on FORUM_Replies.ThreadID = FORUM_Threads.ID
where FORUM_Threads.TopicID = $ID