I need help, if possible to design a function:

If Column "AUFNR" is empty, I need to replace the Value in column "Pool2" by
the value in column "AUFNR" for the same pool.
Then I need to multiply the value of column "Owner" for the value in column
"POOL2" * the value of column "Owner" in column "POOL."

Original table
VALID_FR POOL AUFNR POOL2 VALID_TO OWNER
200803 xx-102 xx-104 200803 6.358100
200803 xx-102 xx-109 200803 0.000800
200803 xx-102 Owner1 200803 24.580100
200803 xx-102 Owner2 200803 4.026100
200803 xx-104 Ownerb1 200803 61.136300
200803 xx-104 Ownerb2 200803 38.863700
200803 xx-109 Owner1 200803 68.737600
200803 xx-109 Owner1 200803 31.262400
The function output table should like this.
VALID_FR POOL AUFNR POOL2 VALID_TO OWNER
200803 xx-102 Ownerb1 200803 67.494400
200803 xx-102 Ownerb2 200803 45.221800
200803 xx-102 Owner1 200803 68.738400
200803 xx-102 Owner1 200803 31.263200
200803 xx-102 Owner1 200803 24.580100
200803 xx-102 Owner2 200803 4.026100
200803 xx-104 Ownerb1 200803 61.136300
200803 xx-104 Ownerb2 200803 38.863700
200803 xx-109 Owner1 200803 68.737600
200803 xx-109 Owner1 200803 31.262400