Union of arrays as aggregate function

Question

I have the following input:

name  | count | options
-----------------------
user1 | 3     | ['option1', 'option2']
user1 | 12    | ['option2', 'option3']
user2 | 2     | ['option1', 'option3']
user2 | 1     | []

I want the following output:

name  | count | options
-----------------------
user1 | 12    | ['option1', 'option2', 'option3']
user2 | 2     | ['option1', 'option3']

I am grouping by name. For each group, the count should be aggregated as the max and the options should be aggregated as the union. I am having troubles figuring out how do the the latter.

Currently, I have this query:

with data(name, count, options) as (
    select 'user1', 12, array['option1', 'option2']::text[]
    union all
    select 'user1', 12, array['option2', 'option3']::text[]
    union all
    select 'user2', 2, array['option1', 'option3']::text[]
    union all
    select 'user2', 1, array[]::text[]
)
select name, max(count)
from data
group by name

http://rextester.com/YTZ45626

I know this can be easily done by defining a custom aggregate function, but I want to do this via a query. I understand the basics of unnest() the array (and array_agg() the results later on), but cannot figure out how to inject this in my query.

Answer 1

You can use an implicit lateral join using unnest(options) in the FROM list, and then using array_agg(distinct v) to create an array with the options:

with data(name, count, options) as (
    select 'user1', 12, array['option1', 'option2']::text[]
    union all
    select 'user1', 12, array['option2', 'option3']::text[]
    union all
    select 'user2', 2, array['option1', 'option3']::text[]
    union all
    select 'user2', 1, array[]::text[]
)
select name, array_agg(distinct v)  -- the 'v' here refers to the 'f(v)' alias below
from data, unnest(options) f(v)
group by name;
┌───────┬───────────────────────────┐
│ name  │         array_agg         │
├───────┼───────────────────────────┤
│ user1 │ {option1,option2,option3} │
│ user2 │ {option1,option3}         │
└───────┴───────────────────────────┘
(2 rows)