Question

This is how the data is formatted:

item_name | item_serial | sub_group | conc_stuff | other_data | more_data
----------+-------------+-----------+------------+------------+-----------
foo        bar-01-a      widget      b-3          towel        dent
foo        bar-02-a      widget      a-1          42           mouse
foo        bar-03-a      widget      p-1          babel        dolphin
foo3       bar-21-f      widget      f-1          42           marvin
foo3       bar-22-f      widget      x-1          poetry       vogon

I have gotten the query to perform the way I want it to, the problem is I need to return more data.

SELECT item_name, 
       array_to_string(array_agg(conc_stuff), ',') as stuff 
FROM dataset 
WHERE some_selector = 'X' 
GROUP BY item_name 
ORDER BY item_name;

I have tried what seems simple yet logical to me:

SELECT item_name, 
       item_serial, 
       sub_group, 
       array_to_string(array_agg(conc_stuff), ',') as stuff 
FROM dataset 
WHERE some_selector = 'X' 
GROUP BY item_name 
ORDER BY item_name;

I need to return something that looks like this:

item_name | item_serial | sub_group |   stuff
----------+-------------+-----------+-------------
foo        bar-01-a      widget      a-1,b-3,p-1
foo3       bar-21-f      widget      f-1,x-1,g-5
foo6       bar-81-z      widget      r-1,d-8,w-0

instead of just this:

item_name |   stuff
----------+--------------
foo        a-1,b-3,p-1
foo3       f-1,x-1,g-5
foo6       r-1,d-8,w-0

When I try to add additional fields to the query, I get :

ERROR: column "stuff.item_serial" must appear in the GROUP BY clause or be used in an aggregate function

But I dont want to GROUP BY item_serial, I just want it to be returned with the aggregate, right?

Do I need to run a subquery? Im sure this is simple. The if there are multiple methods, which is most efficient? Some of the text I will be concatenating are coordinates (LARGE string of text).

Answer 1

You need to pick one value for each name you have. You can’t have each name only returned once but the item_serial value multiple times.

Picking one value out of those that are there for a group value is done through aggregate functions:

SELECT item_name,
       min(item_serial),
       min(sub_group),
       array_to_string(array_agg(conc_stuff),',') AS stuff
FROM dataset
WHERE sub_group = 'widget'
GROUP BY item_name
ORDER BY item_name;

This picks th4e “first” item_serial and sub_group for each name.

If you want the last value, use max instead.

But the important thing to understand is that you have to pick one value for the non-grouped columns. And you need to tell the DBMS exactly which one it should use by supplying an aggregate function which picks one value.

SQLFiddle example: http://www.sqlfiddle.com/#!1/58009/1