Grouping by multiple columns in SQL - mysql

Question

I am trying to bring through the site.Site_Name, for each hive.hiveno and it's max(hiverdg.invdate). Running the code below doesn't work because site.Site_Name is not aggrigated. If I add site.Site_Name to the Group By, the code runs, but the ouput displays the results repeated, once for each site.Site_Name
select site.Site_Name ,hive.hiveno, max(hiverdg.invdate)
from hiverdg
inner join hive
on hiveRdg.hive_Link = hive.hive_Link
inner join Customer
on customer.Customer_Link = hive.Customer_Link
inner join site
on site.Customer_Link = customer.Customer_Link
where
(hiverdg.xtype = 'N'
and customer.CustomerName = 'Cust1')
or
(hiverdg.xtype = 'A'
and customer.CustomerName = 'Cust1')
group by hive.hiveno

Answer 1

The easiest way to do this, with your query, is the substring_index()/group_concat() trick:
select substring_index(group_concat(s.Site_Name order by rdg.invdate desc separator '|'
), '|', 1
) as SiteName,
h.hiveno, max(rdg.invdate)
from hiverdg rdg inner join
hive h
on rdg.hive_Link = h.hive_Link inner join
Customer c
on c.Customer_Link = h.Customer_Link inner join
site s
on s.Customer_Link = c.Customer_Link
where rdg.xtype in ('N', 'A') and c.CustomerName = 'Cust1')
group by h.hiveno;
I also made the following changes to your query:
Introduced table aliases, to make the query easier to write and to read.
Changed the where to use in, simplifying the logic.