Answer 1

If you want to connect your c++ code with MySQL, you can use a connector like: https://dev.mysql.com/doc/connector-cpp/1.1/en/connector-cpp-examples-complete-example-1.html

If it isn't MySQL, find a similar connector for it. Most popular SQL database vendors come with documentation explaining how to connect various programming languages with them.
- PostgrSQL: https://www.postgresql.org/docs/7.2/libpqplusplus.html
- SQL Server: https://docs.microsoft.com/bs-latn-ba/azure/sql-database/sql-database-develop-cplusplus-simple?view=sqlallproducts-allversions For SQL Server, consider also using Visual Studio and keeping more of your code-base Microsoft-oriented. For example, I'd use c# instead of c++ unless you have lots of c++ code already or other requirements tying you to c++.
- Oracle: https://www.oracle.com/webfolder/technetwork/tutorials/obe/db/11g/r2/prod/appdev/oci/oci.htm

Do a similar search for No-SQL database vendors if that's what you're using.