Conditional GROUP BY and additional columns?
Insert into x (...)
select a,b,c from prq where p_a is null group by a,b,c

INSERT INTO x(a, b, c, x, y)
    SELECT a, b, c, MAX(CAST(NULL AS INTEGER)) AS x, MAX(CAST(NULL AS INTEGER)) AS y
      FROM pqr
     WHERE p_a IS NULL
     GROUP BY a, b, c
    UNION
    SELECT MAX(a) AS a, MAX(b) AS b, MAX(c) AS c, x, y
      FROM pqr
     WHERE p_a IS NOT NULL
     GROUP BY x, y;

INSERT INTO x(a, b, c)
    SELECT DISTINCT a, b, c
      FROM pqr
     WHERE p_a IS NULL;
INSERT INTO x(a, b, c, x, y)
    SELECT MAX(a) AS a, MAX(b) AS b, MAX(c) AS c, x, y
      FROM pqr
     WHERE p_a IS NOT NULL
     GROUP BY x, y;

SELECT a, b, c, x, y
      FROM pqr
     WHERE p_a IS NOT NULL
     GROUP BY a, b, c, x, y;

SELECT DISTINCT a, b, c, x, y
      FROM pqr
     WHERE p_a IS NOT NULL;

declare @p_a datatype

select @p_a= P_a from prq

if(@p_a is null)

select a,b,c from prq group by a,b,c

else

select a,b,c from prq group by x,y