MySQL全外部联接3个表，按组和计数

Question

My query is only showing a subset of records of people who have messages for me whereas I want to return a list of all users wether they have messages for me or not along with the count (or true/false, 0,1)

SELECT attendee.id, attendee.firstName, attendee.lastName, count(attendee_chat.to) 
from attendee_chat
INNER JOIN attendee ON attendee.id = attendee_chat.to
INNER JOIN chat ON attendee_chat.id = chat.attendee_chat_id
WHERE attendee.id <> 1
GROUP BY attendee_chat.to;

picture is worth a thousand words.

Answer 1

If you want to do a join on records even if they have no matching element(s) in the other relation, you probably want an OUTER JOIN instead of an INNER JOIN.

MySQL: Quick breakdown of the types of joins

Answer 2

Put the attendee table first in the FROM, and then left join to the other tables that may not have records.

SELECT attendee.id, attendee.firstName, attendee.lastName, count(attendee_chat.to) 
from attendee 
LEFT JOIN attendee_chat ON attendee_chat.to = attendee.id
LEFT JOIN chat ON attendee_chat.id = chat.attendee_chat_id
WHERE attendee.id <> 1
GROUP BY attendee_chat.to;

I would avoid doing a FULL OUTER JOIN as it is not necessary and may lead to some unexpected results.

EDIT: Try this. Remove the chat table as you aren't selecting from it anyway, and group on the fields from the primary table attendee , rather than the on tables that may not contain any data.

SELECT attendee.id, attendee.firstName, attendee.lastName, count(attendee_chat.to) 
from attendee 
LEFT JOIN attendee_chat ON attendee_chat.to = attendee.id
WHERE attendee.id <> 1
GROUP BY attendee.id, attendee.firstName, attendee.lastName;

EDIT2: After discussion of requirements that he wants a count of chats that were to him (#1)

SELECT attendee.id, attendee.firstName, attendee.lastName, 
(SELECT count(1) FROM attendee_chat 
   WHERE attendee_chat.from = attendee.id and  attendee_chat.to = 1) as chatcount
 from attendee ;

EDIT3: After comment about external ID

SELECT attendee.id, attendee.firstName, attendee.lastName, attendee.attendee_id,
(SELECT count(1) FROM attendee_chat 
    INNER JOIN attendee a2 ON a2.id = attendee_chat.toid
    WHERE attendee_chat.fromid = attendee.id and a2.attendee_id = 123
) as chatcount
FROM attendee ;

PS I've changed from to fromid and to -> toid as it is dangerous using reserved words as object names ;-) SQL Fiddle here