postgresql 查询结果 多行变一行

1.SELECT a.id, a.db_name, string_agg(b.username, ",") FROM tbl_company a, tbl_user b WHERE a.id = b.company_id group by a.id

查询字段只能是字符型   结果用“，”隔开

2.SELECT a.id, a.db_name, array_agg(b.username) FROM tbl_company a, tbl_user b WHERE a.id = b.company_id group by a.id

查询字段不分类型  结果为数组{1,2,3}或  {123,123admin}

SELECT a.id, a.db_name, array_to_string(array_agg(b.username), ",") FROM tbl_company a, tbl_user b WHERE a.id = b.company_id group by a.id 

结果是字符串 123,123admin

array_agg(distinct(字段名)) 拼接唯一的字段