How to get distinct array elements with postgres?

问题

I have an array with duplicate values in postgres. For example:

SELECT cardinality(string_to_array('1,2,3,4,4', ',')::int[]) as foo
=> "foo"=>"5"

I would like to get unique elements, for example:

SELECT cardinality(uniq(string_to_array('1,2,3,4,4', ',')::int[])) as foo
=> -- No function matches the given name and argument types. You might need to add explicit type casts.

Can I get unique elements of an array in postgres without using UNNEST ?

回答1:

For integer arrays use intarray extension:

create extension if not exists intarray;
SELECT cardinality(uniq(string_to_array('1,2,3,4,4', ',')::int[])) as foo

or the function

create or replace function public.array_unique(arr anyarray)
    returns anyarray
    language sql
as $function$
    select array_agg(distinct a)
    from (
        select unnest(arr) a 
    ) alias
$function$;

for any array.

回答2:

I prefer this syntax (about 5% faster)

create or replace function public.array_unique(arr anyarray)
returns anyarray as $body$
    select array( select distinct unnest($1) )
$body$ language 'sql';

using:

select array_unique(ARRAY['1','2','3','4','4']);

回答3:

Going off of @klin's accepted answer, I modified it to remove nulls in the process of choosing only the distinct values.

create or replace function public.array_unique_no_nulls(arr anyarray)
returns anyarray
language sql
as $function$
select array_agg(distinct a)
from (
    select unnest(arr) a 
) alias
where a is not null
$function$;

来源：https://stackoverflow.com/questions/40636406/how-to-get-distinct-array-elements-with-postgres