A | B | C
a1| b1| c1
a1| b2| c2
a1| b3| c3
a2| b1| c2
a2| b2| c5

a1 | (c1,b1) ; (c2,b2);(c3;b3)
a2 | (c2,b1) ; (c5,b2)

An error occurred when executing the SQL command:
CREATE FUNCTION cut_semicolon(text) RETURNS text AS $$
BEGIN
  RETURN SUBSTRING($1 FROM 4)

ERROR: unterminated dollar-quoted string at or near "$$
BEGIN
  RETURN SUBSTRING($1 FROM 4)"
  Position: 53

CREATE FUNCTION cut_semicolon(text) RETURNS text AS $$
                                                    ^

Execution time: 0.24s
(Statement 1 of 7 finished)

0 rows affected
END executed successfully

Execution time: 0.22s
(Statement 2 of 7 finished)

An error occurred when executing the SQL command:
$$ LANGUAGE 'plpgsql' IMMUTABLE

ERROR: unterminated dollar-quoted string at or near "$$ LANGUAGE 'plpgsql' IMMUTABLE"
  Position: 1

$$ LANGUAGE 'plpgsql' IMMUTABLE
^

Execution time: 0.22s
(Statement 3 of 7 finished)

An error occurred when executing the SQL command:
CREATE FUNCTION concat_semicolon(text, text) RETURNS text AS $$
BEGIN
  RETURN $1 || ' ; ' || $2

ERROR: unterminated dollar-quoted string at or near "$$
BEGIN
  RETURN $1 || ' ; ' || $2"
  Position: 62

CREATE FUNCTION concat_semicolon(text, text) RETURNS text AS $$
                                                             ^

Execution time: 0.22s
(Statement 4 of 7 finished)

0 rows affected
END executed successfully

Execution time: 0.22s
(Statement 5 of 7 finished)

An error occurred when executing the SQL command:
$$ LANGUAGE 'plpgsql' IMMUTABLE

ERROR: unterminated dollar-quoted string at or near "$$ LANGUAGE 'plpgsql' IMMUTABLE"
  Position: 1

$$ LANGUAGE 'plpgsql' IMMUTABLE
^

Execution time: 0.22s
(Statement 6 of 7 finished)

An error occurred when executing the SQL command:
CREATE AGGREGATE concat_semicolon(
  BASETYPE=text,
  SFUNC=concat_semicolon,
  STYPE=text,
  FINALFUNC=cut_semicolon,
  INITCOND=''
)

ERROR: SQL command "CREATE AGGREGATE concat_semicolon(
  BASETYPE=text,
  SFUNC=concat_semicolon,
  STYPE=text,
  FINALFUNC=cut_semicolon,
  INITCOND=''
)" not supported.

Execution time: 0.23s
(Statement 7 of 7 finished)


5 statements failed.
Script execution finished
Total script execution time: 1.55s

select A, concat(concat(concat(C, ',' ) , cast(B as varchar)), ',')
from  my_table
group by A,B,C


-- Is it ok to group by all A,B, C - since I can't group by A alone, which removes the related "C" columns--

gives -:
a1 c1b1b2b3
a2 c2b1b2

a1 c1,b1;c2,b2;c2,b3
a2 c2,b1;c5,b2

SELECT
  a,
  STRING_AGG('(' || c || ',' || b || ')', ' ; ')
FROM
  tbl
GROUP BY
  a;

CREATE FUNCTION cut_semicolon(text) RETURNS text AS '
BEGIN
  RETURN SUBSTRING($1 FROM 4);
END;
' LANGUAGE 'plpgsql' IMMUTABLE;


CREATE FUNCTION concat_semicolon(text, text) RETURNS text AS '
BEGIN
  RETURN $1 || '' ; '' || $2;
END;
' LANGUAGE 'plpgsql' IMMUTABLE;

CREATE AGGREGATE concat_semicolon(
  BASETYPE=text,
  SFUNC=concat_semicolon,
  STYPE=text,
  FINALFUNC=cut_semicolon,
  INITCOND=''
);

SELECT
  a,
  CONCAT_SEMICOLON('(' || c || ',' || b || ')')
FROM
  tbl
GROUP BY
  a;

SELECT
  a,
  GROUP_CONCAT(CONCAT('(', c, ',', b, ')') SEPARATOR ' ; ')
FROM
  tbl
GROUP BY
  a;