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

问题

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

回答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

来源：https://stackoverflow.com/questions/26363742/how-to-remove-duplicates-which-are-generated-with-array-agg-postgres-function