Hey Mohit,
from your explanation I don't think you really go for a "Calculated Columne". If you want to have something simple, like a substring of an existing column or just the concatenation of two columns. You can happily go for Open SQL and/or Core Data Services - both just in ABAP - no need for the extra effort of native SQL.
Please find examples in the ABAP language documentation (e.g. ABAP Keyword Documentation for CDS build-in functions, or ABAP Keyword Documentation for Open SQL) - whereas the available features of course depend on your ABAP and HANA releases.
Cheers,
Jasmin