使用INNER JOIN显示多个结果

Question

I have these three MySQL tables:

companies                    services                      auxiliary
id_company name_company      id_service  name_service      id_company id_service   

1          Google            1           Search            1          1   
2          Yahoo             2           Calendar          1          2  
                             3           Mail              1          3
                             4           Maps              1          4
                                                           2          1
                                                           2          3

I am wondering if I could display with only one query (I'm using php):

Google: Search, Calendar, Mail, Maps.
Yahoo: Search, Mail.

Now what I am doing is display the companies, and for each company I make another MySQL query to display the services. Is it possible to do that with only one query? Thank you.

Answer 1

The answer to your question is: "Yes".

Oh, you want to know how to do it as well. The key is to join the tables together and then aggregate at the company level. I am going to assume that you really want the output as two columns (company name, services), rather than one string with them concatenated together.

The SQL is:

select c.name_company, group_concat(s.name_service separator ', ') as services
from auxiliary a join
     companies c
     on a.id_company = c.id_company join
     services s
     on a.id_service =  s.id_service
group by c.id_company, c.name_company;

Answer 2

Yes, you could. Although in my opinion the approach you described seems more natural to me. (Having one query/cursor for the companies, and another one for the services)

You could use one single query to prepare following list:
Google, Search
Google, Calendar
Google, Mail
Google, Maps
Yahoo, Search
Yahoo, Mail
...
(I have left out the ids for simplicity)
Then you run through the list, every time the company changes you begin a new result line. I find this a bit more complicated than the approach you described.