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            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

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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

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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

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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

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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

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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

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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

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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

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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

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PostgreSQL 9 0 or later   Recent versions of Postgres  since late 2010  have the string agg expression  delimiter  function which will do exactly what the question asked for  even letting you specify the delimiter string   SELECT company id  string agg employee        FROM mytable GROUP BY company id    Postgres 9 0 also added the ability to specify an ORDER BY clause in any aggregate expression  otherwise  the order is undefined  So you can now write   SELECT company id  string agg employee       ORDER BY employee  FROM mytable GROUP BY company id    Or indeed   SELECT string agg actor name       ORDER BY first appearance    PostgreSQL 8 4 or later   PostgreSQL 8 4  in 2009  introduced the aggregate function array agg expression  which concatenates the values into an array  Then array to string   can be used to give the desired result   SELECT company id  array to string array agg employee         FROM mytable GROUP BY company id    string agg for pre-8 4 versions   In case anyone comes across this looking for a compatibilty shim for pre-9 0 databases  it is possible to implement everything in string agg except the ORDER BY clause   So with the below definition this should work the same as in a 9 x Postgres DB   SELECT string agg name        AS semi colon separated names FROM things    But this will be a syntax error   SELECT string agg name       ORDER BY name  AS semi colon separated names FROM things  -- gt  ERROR  syntax error at or near  ORDER    Tested on PostgreSQL 8 3   CREATE FUNCTION string agg transfn text  text  text      RETURNS text AS                 BEGIN             IF  1 IS NULL THEN                 RETURN  2              ELSE                 RETURN  1     3     2              END IF          END             LANGUAGE plpgsql IMMUTABLE COST 1   CREATE AGGREGATE string agg text  text        SFUNC string agg transfn      STYPE text      Custom variations  all Postgres versions   Prior to 9 0  there was no built-in aggregate function to concatenate strings  The simplest custom implementation  suggested by Vajda Gabo in this mailing list post  among many others  is to use the built-in textcat function  which lies behind the    operator    CREATE AGGREGATE textcat all    basetype      text    sfunc         textcat    stype         text    initcond              Here is the CREATE AGGREGATE documentation   This simply glues all the strings together  with no separator  In order to get a      inserted in between them without having it at the end  you might want to make your own concatenation function and substitute it for the  textcat  above  Here is one I put together and tested on 8 3 12   CREATE FUNCTION commacat acc text  instr text  RETURNS text AS      BEGIN     IF acc IS NULL OR acc      THEN       RETURN instr      ELSE       RETURN acc            instr      END IF    END     LANGUAGE plpgsql    This version will output a comma even if the value in the row is null or empty  so you get output like this   a  b  c    e    g   If you would prefer to remove extra commas to output this   a  b  c  e  g   Then add an ELSIF check to the function like this   CREATE FUNCTION commacat ignore nulls acc text  instr text  RETURNS text AS      BEGIN     IF acc IS NULL OR acc      THEN       RETURN instr      ELSIF instr IS NULL OR instr      THEN       RETURN acc      ELSE       RETURN acc            instr      END IF    END     LANGUAGE plpgsql

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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

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Use STRING AGG function for PostgreSQL and Google BigQuery SQL   SELECT company id  STRING AGG employee        FROM employees GROUP BY company id

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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

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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

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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

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

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