In a table, there is a colume which data is retrieve from the same/other table. How can I do it??

For example, a table (t1) here
col1 col2 col3
A 1 A1
C 2 A2
R 10 A10
col3 is the combination of col1 & col2. col3 may be at the same table (t1) or the other table (t2).
How do it