Use a specific database depending on condition

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

CASE 
    WHEN @开发者_StackOverflow中文版dbnum = 1 THEN USE Db1
        ELSE USE DefaultDb
END

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You can do it with an IF:

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