Populate missing value in a column

Gowthami_S Posts 1 Registration date Tuesday September 22, 2015 Status Member Last seen September 22, 2015 - Sep 22, 2015 at 04:53 AM

I need to compare two columns and need to populate the missing values in other column.

Column 1; Column 2; Column 3;
a, b, c, d; a, b, c; ,d;

Comparing Column_1 and Column_2, then populating " ,d" which is missing in Column_2.

Kindly help me on this.