Create array in SELECT

Maybe I'm missing something, but this should do it:

SELECT a, 
       count(*) as cnt,
       array_agg(b) as all_values
FROM your_table
GROUP BY a

This should do the trick:

SELECT a
     , sum(ab_ct)::int AS ct_total
     , count(*)::int   AS ct_distinct_b
     , array_agg(b || ', ' || ab_ct::text) AS b_arr
FROM  (
    SELECT a, b, count(*) AS ab_ct
    FROM   tbl
    GROUP  BY a, b
    ORDER  BY a, ab_ct DESC, b  -- append "b" to break ties in the count
    ) t
GROUP  BY a
ORDER  BY ct_total DESC;

Returns:

• ct_total: total count of b per a.
• ct_distinct_b: count of distinct b per a.
• b_arr: array of b plus frequency of b, sorted by frequency of b.

Ordered by total count of b per a.

Alternatively, you can use an ORDER BY clause within the aggregate call in PostgreSQL 9.0 or later. Like:

SELECT a
     , sum(ab_ct)::int AS ct_total
     , count(*)::int   AS ct_distinct_b
     , array_agg(b || ', ' || ab_ct::text ORDER BY a, ab_ct DESC, b) AS b_arr
FROM  (
    SELECT a, b, count(*) AS ab_ct
    FROM   tbl
    GROUP  BY a, b
    ) t
GROUP  BY a
ORDER  BY ct_total DESC;

May be clearer. But it's typically slower. And sorting rows in a subquery works for simple queries like this one. More explanation:

• How to apply ORDER BY and LIMIT in combination with an aggregate function?