How to display only vacant rooms in my java from a opposed toas show a warning message incase of duplication

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Java

MS-Access

The form is supposed to automatically pick rooms from the db in a table called Roomed and the user will select his preferred room then insert the customers details in the db in a table called Reserves.
Table Roomed has room numbers only.

Table Reserves is where Customers room info are saved.

The task is to display only the vacant rooms in the Table Roomed , to avoid duplication as opposed to show an error/ waring message when one books the same room twice.

Ideas please