How to remove duplicates, which are generated with array_agg postgres function

Question

Does anyone an idea how to rewrite following SQL query to generate results, that would contains only one occurrence of name? (results grouped by user).

The query

SELECT array_to_string(array_agg(CONCAT(u.firstname, ' ', u.lastname)), ', ')
FROM log_has_item logitem
  INNER JOIN log log ON log.id = logitem.log_id
  INNER JOIN worker u ON log.worker_id = u.id
WHERE logitem.company_id = 1

Executable query is avaiable on sqlfiddle.com. Click on Run SQL button and you will result, which contains Frantisek Smith twice

Answer 1

You can use the distinct keyword inside array_agg:

SELECT ARRAY_TO_STRING(ARRAY_AGG(DISTINCT CONCAT(u.firstname, ' ', u.lastname)), ', ')
FROM log_has_item logitem
  INNER JOIN log log ON log.id = logitem.log_id
  INNER JOIN worker u ON log.worker_id = u.id
WHERE logitem.company_id = 1

SQLFiddle with this example

Answer 2

No need to go all round the houses with ARRAY_TO_STRING(ARRAY_AGG( when a simple STRING_AGG will do as follows (code available on the fiddle here):

--
-- Simplified
--

SELECT
  DISTINCT
  STRING_AGG
  (
    DISTINCT CONCAT(w.firstname, ' ', w.lastname), ', '
  ) AS "The workers"
FROM log_item li
INNER JOIN log l ON li.log_id = l.id 
INNER JOIN worker w ON l.worker_id = w.id
WHERE li.company_id = 1;

Result:

                The workers
Frantisek Smith, Peter Duff