Firebird 2.1 stored procedure to concatenate text on multiple rows

Question

I am trying to write a stored procedure to concatenate multiple rows of text together to return it as a single string. For example:

CREATE TABLE TEST (
 ID INTEGER,
 SEQ INTEGER,
 TEXT VARCHAR(255));

COMMIT;

INSERT INTO TEST (ID, SEQ, TEXT) VALUES (1, 1, "LINE 1");
INSERT INTO TEST (ID, SEQ, TEXT) VALUES (1, 2, "LINE 2");
INSERT INTO TEST (ID, SEQ, TEXT) VALUES (1, 3, "LINE 3");

COMMIT;

SET TERM !!;
CREATE PROCEDURE concat_names (iID INTEGER)
  RETURNS (CONCAT VARCHAR(2000))
AS
DECLARE VARIABLE name VARCHAR(255);
BEGIN
  CONCAT = '';
  FOR SELECT TEXT FROM TEST where id=:iID INTO :name
  DO BEGIN
    CONCAT = CONCAT || name;
  END
END!!
SET TERM ;!!

commit;

However when I run:

select concat from concat_names(1);

It always returns zero rows.

Any ideas?

Answer 1

You forget for SUSPEND. Your proc should look like this:

SET TERM !!;
CREATE PROCEDURE concat_names (iID INTEGER)
  RETURNS (CONCAT VARCHAR(2000))
AS
DECLARE VARIABLE name VARCHAR(255);
BEGIN
  CONCAT = '';
  FOR SELECT TEXT FROM TEST where id=:iID INTO :name
  DO BEGIN
    CONCAT = CONCAT || name;
  END
  SUSPEND;
END!!
SET TERM ;!!

You can achieve the same result without stored proc. Use LIST aggregate function:

SELECT LIST(text, '') FROM TEST where id=:iID 

Second parameter of LIST is a delimiter. If you call LIST with only field name, then comma ',' will be used to separate values.

Answer 2

In the case the field TEST can ben null and you don't want to set to null the whole result it is useful to use:

 CONCAT = CONCAT || coalesce(name,'');

instead of

CONCAT = CONCAT || name;

Answer 3

Without utilizing a Stored Proc and using version Firebird 2.5, the LIST aggregation function will return "Comma-separated string concatenation of non-NULL values in the column"*. Using the aforementioned TEST table, the SQL

SELECT LIST(TEXT)
    FROM TEST

returns

LINE 1,LINE 2,LINE 3

This may be of some interest.

*Taken from the Firebird reference page here