Remember the schema from last week about flights and airports:
Airports(_code_,city) FlightCodes(_code_, airlineName) Flights(origin, destination, departure, arrival, _code_) origin → Airports.code destination → Airports.code code → FlightCodes.code
Assume the following two tables are added to this schema: a table listing for each flight the number of available seats and the price per ticket:
AvailableFlights(_flight_, _date_, numberOfFreeSeats, price) flight → Flights.code
And another one listing the passengers that have been booked for each flight, with the price they have paid and a unique booking reference number (an integer, to keep things simple):
Bookings(_reference_, flight, date, passenger, price) (flight, date) → AvailableFlights.(code, date)
Question 1 Create a view that lists booking references, passengers, flight codes, date, and departure and destination cities.
Question 2 Create a trigger on the view defined in quesiton one. This trigger takes care of booking a new passenger to a flight. It is fired by an insertion of a passenger and a flight code to the view, for instance, “book Annie Adams for AF666 on 2016-03-18”. Its effect should be the following:
AvailableFlightswhen booking her; also add a booking reference which is the maximum of the previous references (for all flights) plus one
The airline decides to upgrade its database to keep track of its fleet. This
means adding a table to list the available planes and updating the
AvailableFlights in the following way:
Planes(_regnr_, capacity) AvailableFlights(_flight_, _date_, numberOfFreeSeats, price, plane) (flight, date) → Flights.(code, date) plane → Planes.regnr
We now have duplicated information in the database: we can compute the number
of free seats by subtracting the number of booked seats to the capacity of the
plane. One solution would be to simply remove the
but, as it is often the case with production databases, it is impossible to
update all the systems accessing the database at once to make use of the new
schema, which means that the duplicated field needs to be present in the
database during a transition period.
To make the transition easier, we would like to have the value of the
numberOfFreeSeats automatically updated. In particular, if a flight
becomes full well before its departure date, the airline company would like to
be able to change the airplane for a bigger one. Your job is to create a
trigger that automatically update
numberOfSeats when this happens.
In this exercise, we are creating the following database for a restaurant:
Tables(_number_, seats) Bookings(_name_, _time_, nbpeople, table) table → Tables.number
For the sake of simplicity, we assume that the database only needs to hold
bookings for the current day and that bookings are always done on the hour
time is an integer between 0 and 23).
Finally, tables are always booked for two hours. This means that if table 1 is booked at 19.00, it can’t be booked at 20.00 but it can be booked again at 21.00.
Question 1 Write a view that lists the times at which tables are blocked by a booking. In the example above, where table 1 is booked at 19.00, the view should contain the following rows:
Write a trigger on the table
Bookings that automatically assign a table to
new rows if none is specified. The assigned table should respect the following
In this exercise, we are creating a database for a wiki. A wiki is a website that allows collaborative modification of its content and structure directly from the web browser (Wikipedia).
To make collaborative edition easier, we needs to keep an history of the modifications of each page. To achieve this, we will use a simple model that simply keeps each version of each page as a separate row:
PageRevision(_name_, _date_, author, text)
To make it easier to access the wiki, creat a view
Page(name, last_author, text) that shows only the latest version of each
Question 2 Create a trigger on your newly created view so that when a user tries to update a given page, a new revision is created instead.
Sometimes, pages on the wiki needs to be completely deleted (for instance, if a page contains sensitive information or copyrighted content). In that case, we want to remove all revisions of the page from the database but we still want to remember that the page has existed but has been deleted. To this end, we add the following table to our database:
Write a trigger on the
Page view such that when a page is deleted: