How to create a mutually exclusive FreeBSD port?

I have created multiply FreeBSD ports of a program, that supports different database flavours (e.g. PostgreSQL, MariaDB, ...). That part is working perfectly fine. Now I want to create a dependency to all the different program-database ports, but only one should be selected. I know how to add dependencies to a port with BUILD_DEPENDS and RUN_DEPENDS, but don't know how to make a list mutual exclusive?