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

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            Anna
2    1            Bill
3    2            Carol
4    2            Dave

and I wanted to group by company_id to get something like:

COMPANY_ID   EMPLOYEE
1            Anna, Bill
2            Carol, Dave

There is a built-in function in mySQL to do this group_concat

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;

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;

If you are on Amazon Redshift, where string_agg is not supported, try using listagg.

SELECT company_id, listagg(EMPLOYEE, ', ') as employees
FROM EMPLOYEE_table
GROUP BY company_id;

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;

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;

Following up on Kev's answer, using the Postgres docs:

First, create an array of the elements, then use the built-in array_to_string function.

CREATE AGGREGATE array_accum (anyelement)
(
 sfunc = array_append,
 stype = anyarray,
 initcond = '{}'
);

select array_to_string(array_accum(name),'|') from table group by id;

As already mentioned, creating your own aggregate function is the right thing to do. Here is my concatenation aggregate function (you can find details in French):

CREATE OR REPLACE FUNCTION concat2(text, text) RETURNS text AS '
    SELECT CASE WHEN $1 IS NULL OR $1 = \'\' THEN $2
            WHEN $2 IS NULL OR $2 = \'\' THEN $1
            ELSE $1 || \' / \' || $2
            END; 
'
 LANGUAGE SQL;

CREATE AGGREGATE concatenate (
  sfunc = concat2,
  basetype = text,
  stype = text,
  initcond = ''

);

And then use it as:

SELECT company_id, concatenate(employee) AS employees FROM ...

This latest announcement list snippet might be of interest if you'll be upgrading to 8.4:

Until 8.4 comes out with a super-effient native one, you can add the array_accum() function in the PostgreSQL documentation for rolling up any column into an array, which can then be used by application code, or combined with array_to_string() to format it as a list:

http://www.postgresql.org/docs/current/static/xaggr.html

I'd link to the 8.4 development docs but they don't seem to list this feature yet.

Use STRING_AGG function for PostgreSQL and Google BigQuery SQL:

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

I'm using Jetbrains Rider and it was a hassle copying the results from above examples to re-execute because it seemed to wrap it all in JSON. This joins them into a single statement that was easier to run

select string_agg('drop table if exists "' || tablename || '" cascade', ';') 
from pg_tables where schemaname != $$pg_catalog$$ and tableName like $$rm_%$$

I found this PostgreSQL documentation helpful: http://www.postgresql.org/docs/8.0/interactive/functions-conditional.html.

In my case, I sought plain SQL to concatenate a field with brackets around it, if the field is not empty.

select itemid, 
  CASE 
    itemdescription WHEN '' THEN itemname 
    ELSE itemname || ' (' || itemdescription || ')' 
  END 
from items;

Following yet again on the use of a custom aggregate function of string concatenation: you need to remember that the select statement will place rows in any order, so you will need to do a sub select in the from statement with an order by clause, and then an outer select with a group by clause to aggregate the strings, thus:

SELECT custom_aggregate(MY.special_strings)
FROM (SELECT special_strings, grouping_column 
        FROM a_table 
        ORDER BY ordering_column) MY
GROUP BY MY.grouping_column

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