Question

Insert into x (...)
select a,b,c from prq where p_a is null group by a,b,c

Now, my requirement is that, if p_a is null then i have to group by a,b,c and select only a,b,c column.

But if p_a is not null then, i have to group by two new column x,y and also select these two columns.
These x,y columns are NULLABLE in table x.

Answer 1

Table X must have at least five columns whose names, we can presume, are a, b, c, x, y.

If you are doing a single INSERT, then you’ll need to insert into all five columns. If you are doing multiple INSERT operations, you can insert into 3 and then 5 (or vice versa) columns. You may have to do some juggling with the NULL values in the select-list of the first alternative. I’m assuming that the columns x and y are INTEGER for definiteness – choose the appropriate type.

1st Alternative

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;

You could replace the GROUP BY a, b, c clause with a DISTINCT in front of a in the select-list of the first part of the UNION. In most SQL DBMS, you must list all the non-aggregate columns from the select-list in the GROUP BY clause. Using the MAX means that you have aggregates for x and y in the first half of the UNION and for a, b and c in the second half of the UNION.

2nd Alternative

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;

As discussed before, you need aggregates on the columns not in the GROUP BY list.

3rd Alternative

If you meant that you must group by x and y as well as a, b and c, then the second half of the UNION (or the second SELECT) simplifies to:

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

Or you can use DISTINCT again:

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