通过左联接选择多个

Question

I have a chat table, where users can post message and it is stored in db and read by the intended recipient.

when i send normal chat, its no problem, but when i send multiple or broadcast, i get stock.

table users
id  |  alias
-----------
1   |  mark
2   |  joe

table chat

id  |  uid  | msg   | cmd
-----------------------
1   |  1    | hi    | msg
2   |  2    | hello | msg
3   |  1,2  | msg   | brc <- broadcast

for reading normal one to one message

Select u.alias, c.id cid from chat c left join user u on u.id=c.uid 
where c.uid= 1

now for the broadcast, for a user with id 1, i tried this

Select u.alias, c.id cid from chat c left join user u on u.id=c.uid 
where id in IN (1,2)

for another user with id 2,

Select u.alias, c.id cid from chat c left join user u on u.id=c.uid 
where id in IN (1,2)

I get an empty result

Answer 1

Lets normalize your tables:

table users

id  |  alias
-----------
1   |  mark
2   |  joe

table chat

id  | msg   | cmd
-----------------------
1   | hi    | msg
2   | hello | msg
3   | msg   | brc <- broadcast

table chatrouting

id  | userid| msgid
-----------------------
1   | 1     | 1
2   | 2     | 2
3   | 1     | 3
4   | 2     | 3

then you can get all mark's messages with:

SELECT c.msg
FROM chat c
LEFT JOIN chatrouting cr ON cr.msgid = c.id
LEFT JOIN user u ON u.id = cr.userid
WHERE u.alias = 'mark'

Answer 2

Try to use this:

Select u.alias, c.id cid from chat c 
left join user u on FIND_IN_SET(u.id,'1,2') 
where id in IN (1,2)

Because row looks like 3 | 1,2 | msg | brc <- broadcast So uid in chat table it's not just int, it can have 2 or more integers separate comma. If I right understand.

Answer 3

The reason for this query to fail is that you're testing a single column to match your own concatenation of columns 1,2 . This is generally a bad idea because it's not inline with the paradigm of normalized relational databases. Columns should have singular, atomic values. It's very easy to redesign your 'broadcast' so that you don't have this problem.

Two other problems I notice:

Each of these queries should give an error pointing out that there is an "Ambiguously defined column", but apparantly MySQL accepts this. In any case, you should add a table alias to the id to specify which table you mean.

Also, there is no need at all to use a left join in this case. This would only make sense if it's possible for the id to not exist in the other table, which shouldn't be possible, because it should be guarded by a foreign key constraint.