Question

Hi am I am creating a database which allows users to make a reservation to a restaurant. Below is my data model for the database.

My question is i am a little confused with how i would check for tables that are available on a given night. The restaurant has 15 tables for any night with 4 people to a table (Groups can be 4 – 6 big, groups larger than 4 will take up two tables).

How would i query the database to return the tables which are available on a given night.

Thanks.

EDIT::

This is what i have tried. (Some of it is pseudo as i am not quite sure how to do it)

SELECT tables.table_id
FROM tables
LEFT JOIN table_allocation
ON tables.table_id = table_allocation.table_id
WHERE table_allocation.table_id is NULL;

This returns the well empty rows as it is checking for the none presence of the table. I am not quite sure how i would do the date bit test.

Answer 1

To find TABLE rows that have no TABLE_ALLOCATION rows on a given THEMED_NIGHT.TEME_NIGHT_DATE, you should be able to do something like this:

SELECT *
FROM TABLES
WHERE
    TABLE_ID NOT IN (
        SELECT TABLE_ALLOCATION.TABLE_ID
        FROM
            TABLE_ALLOCATION
            JOIN RESERVATION
                ON TABLE_ALLOCATION.RESERVATION_ID = RESERVATION.RESERVATION_ID
            JOIN THEMED_NIGHT
                ON RESERVATION.THEME_ID = THEMED_NIGHT.THEME_ID
        WHERE
            THEME_NIGHT_NAME = :the_date
    )

In plain English:

• Join TABLE_ALLOCATION, RESERVATION and THEMED_NIGHT and accept only those that are on the given date (:the_date).
• Discard the TABLE rows that are related to the tuples above (NOT IN).
• Those TABLE rows that remain are free for the night.