Question

I have a query (a view actually) the normally should define a unique mapping from key to value2.

select key, value1, value2
from tbl -- joins are left out here
where (1=1) -- left out here
order by key
;

and usually the result is something like

key       value1  value2
--------------------------
Harry     fish    blue
Sally     dog     green
Willy     dog     red

so I can map key to value2 uniquely.

But as it happens the underlying data may contain junk some day:

key       value1  value2
--------------------------
Harry     fish    blue
Harry     fish    black     -- <<< breaks uniqueness
Sally     dog     green
Willy     dog     red

I would like to add HAVING and/or GROUP BY expressions to identify the nun-unique rows easily. Because I have to do something with value2 in this case I have to aggregate it somehow — my idea is max.

Therefore an example result would be

key       value1  value2   count_value2
----------------------------------------
Harry     fish    blue     2
Sally     dog     green    1
Willy     dog     red      1

But as always, I am completely baffled by GROUP BY and its consorts. Where do I but it? Where and where the aggregations?

Answer 1

something like this?

select 
  key, 
  min(value1) as "a value1", 
  min(value2) as "a value2", 
  max(value2) as "another value2", 
  count(distinct value2) "number of value2"
FROM tbl
group by key
having count(distinct value2) > 1