Use a specific database depending on condition

Question

Can I somehow use a specific database given a specific condition? To clarify I will give a naive example:

CASE 
    WHEN @dbnum = 1 THEN USE Db1
        ELSE USE DefaultDb
END

Answer 1

You can do it with an IF:

IF @dbnum = 1
    USE Db1;
ELSE
    USE DefaultDb;