Update multiple columns with data retrieved from multiple rows of same table

Question

For the following data:

PGM      INC        EXC
First    A          AA
First    B          BB
First    C          CC
First    D          DD

Second   E          EE
Second   F          FF
Second   G          GG
Second   H          HH

How can we sequentially read column data of PGM='First' and update into Columns of PGM='Second'

PGM      INC        EXC
Second   E          AA
Second   F          BB
Second   G          CC
Second   H          DD

Here as we can see PGM='Second' has mapped the same data of PGM='First' for the 4 rows. How can this be achieved in a single update query of such multiple records.

Please guide.

Answer 1

The basic idea is to use the row_number window function to assign 1, 2, 3 etc in order to the two sets. You can then join these to figure out which value from set 1 should apply to which value in set 2.

Here's a lowest common denominator method that works on Postgres/Oracle/SQL Server. I don't have access to DB2 to try it.

if there aren't enough values in First, then the end values for second will be overwritten with null

Update
    test
Set
    Exc = (
        Select
            f.Exc
        From (
            Select
                Exc,
                row_number() over (order by inc) as rn
            From
                test
            Where
                Pgm = 'First'
        ) as f
        inner join (
            Select
                Inc,
                row_number() over (order by inc) as rn
            From
                test
            Where
                Pgm = 'Second'
        ) as s
            On f.rn = s.rn
        Where
             test.inc = s.inc
    )
Where
    test.Pgm = 'Second';

Example Fiddle