Databases/Tables checking if they exist

How is it done. I was thinking, that trying to make a database/table and if it failed then it already existed but then i thought it might overwrite instead so i figured i would ask before i tried.

If you try and make one that exists then it will not allow you to do so and will generate an error message.

You can use SHOW DATABASES or SHOW TABLES to see all your databases or all your tables in the currently selected database.