How to concatenate strings of a string field in a PostgreSQL 'group by' query?

Question

I am looking for a way to concatenate the strings of a field within a group by query. So for example, I have a table:

ID   COMPANY_ID   EMPLOYEE
1    1               


        

Answer 1

According to version PostgreSQL 9.0 and above you can use the aggregate function called string_agg. Your new SQL should look something like this:

SELECT company_id, string_agg(employee, ', ')
    FROM mytable GROUP BY company_id;

Answer 2

You can also use format function. Which can also implicitly take care of type conversion of text, int, etc by itself.

create or replace function concat_return_row_count(tbl_name text, column_name text, value int)
returns integer as $row_count$
declare
total integer;
begin
    EXECUTE format('select count(*) from %s WHERE %s = %s', tbl_name, column_name, value) INTO total;
    return total;
end;
$row_count$ language plpgsql;


postgres=# select concat_return_row_count('tbl_name','column_name',2); --2 is the value

Answer 3

How about using Postgres built-in array functions? At least on 8.4 this works out of the box:

SELECT company_id, array_to_string(array_agg(employee), ',')
FROM mytable
GROUP BY company_id;

Answer 4

I claim no credit for the answer because I found it after some searching:

What I didn't know is that PostgreSQL allows you to define your own aggregate functions with CREATE AGGREGATE

This post on the PostgreSQL list shows how trivial it is to create a function to do what's required:

CREATE AGGREGATE textcat_all(
  basetype    = text,
  sfunc       = textcat,
  stype       = text,
  initcond    = ''
);

SELECT company_id, textcat_all(employee || ', ')
FROM mytable
GROUP BY company_id;

Answer 5

Use STRING_AGG function for PostgreSQL and Google BigQuery SQL:

SELECT company_id, STRING_AGG(employee, ', ')
FROM employees
GROUP BY company_id;

Answer 6

As from PostgreSQL 9.0 you can use the aggregate function called string_agg. Your new SQL should look something like this:

SELECT company_id, string_agg(employee, ', ')
FROM mytable
GROUP BY company_id;