Need help joining tables

[chestertb]

Hi All,

How easily one's brain forgets. It's been quite a while since I used JOIN in mySQL and I'm struggling to remember...

I have two tables.

The first, 'attendees', lists members who attended a particular event as follows;
date = the event date
mrn = the member record number

It might look like

date..mrn
2012-08-24..1504
2012-08-24..1621
2012-08-21..3455
2012-08-24..8891
2012-08-24..1110
2012-09-24..1631
2012-09-24..4455
2012-08-24..6329

The second, 'friends', lists members who are friends of an individual member as follows
mrn = the member's record number
frn = the record number of a member who is a friend of the member in mrn

it might look like

mrn..frn
1515..1631
1681..6695
1515..7841
1515..6200

I want to look at all members who attended an event on, say, 2012-08-24, and determine if any of them were friends of member 1515.

If no attendees were friends, then the query should return 0 rows.
If at least one attendee is a friend of 1515, then the query should return more than 0 rows.

For this query, I don't actually need the data... I just need a yes/no answer on whether friends of 1515 attended, so I figured I'd get that from the number of rows returned. In the case above, there would be 1 row.

I'm not even coming close to figuring out how to construct that query so any help will be greatly appreciated.

Cheers
CTB

[NogDog]

Assuming no silly mistakes on my part:

SELECT COUNT(*) AS num_friends_attending
FROM attendees
INNER JOIN friends ON attendees.mrn = friends.frn
WHERE friends.mrn = 1515 AND attendees.date='2012-08-24'

Off hand, I don't know if that would be more or less efficient than using a sub-query, e.g.:

SELECT COUNT(*) AS num_friends_attending
FROM attendees
WHERE date='2012-08-24' AND mrn IN(
  SELECT frn FROM friends WHERE mrn=1515
)

[chestertb]

Once again, you're a lifesaver!
That works perfectly.
Thanks you.