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CITS1402 Project
Gordon Royle
2020 Semester Two
So far in this unit, the labs have been focussed on writing SQL queries learning how the SQL “rowprocessing-machine”
can be used to select, manipulate and summarise data contained in multiple relational
tables.
This project, really a mini-project, is going to focus on some of the other aspects of databases.
A database designer builds the database schema and possibly enters the initial data, but over time the
data evolves as rows are inserted, updated and deleted during the day-to-day use of the database. An
important role of the database designer is to make the database resistant to data corruption caused by
careless users.
This project explores some of the steps that a database designer can take to enhance the long-term
integrity of the database.
The questions may require you to undertake your own research into how certain SQLite features are
implemented. The official documentation is located at https://www.sqlite.org/docs.html/index.
html, and there are numerous SQLite tutorial sites with examples.
Project Rules
For the duration of the project, different (stricter) rules apply for obtaining help from the facilitators
and help1402 for the duration of the project.
1. Absolutely no “pre-marking” requests
Do not show your code to a facilitator and say “Is this right?”
Firstly, this is not fair to the facilitator, who is there to provide general assistance about SQL
and not to judge whether code meets the specifications.
Secondly, from previous experience, such requests often degenerate into the situation where
the facilitator “helps out” with the first line of code, then the student returns five minutes
later and asks for help with the second line of code, and so on, until the final query is mostly
written line-by-line by the facilitator and not the student.
Facilitators are there to gently nudge you in the right direction, not by just “giving the answer”
and supplying code that works, but by making general suggestions on SQL features, reminders
about what concepts might be useful, and advice on how you might investigate and resolve
problems yourself.
2. No validation requests for your submission
Please do not ask the facilitators anything about the mechanics of making a valid submission
such as file names, due dates etc. This is not their job and it leads to awkward situations where
a student submits something that is obviously incorrect, but then claims that “the facilitator
said it was ok”.
1
DRAFT
You are responsible for writing, testing, formatting and submitting your code correctly, and if
you have any doubts about what is required, then please ask on help1402
.
3. Avoid low-quality help1402 posts
Before the mid-semester test, there was a huge spike of activity on help1402. While I encourage
thoughtful questions and try to answer them promptly, the sheer volume of questions almost
overwhelmed me.
Even without help1402, this unit is already consuming far more time than I am meant to
spend on it, so I have to cut back. A lot of my time earlier in semester was spent dealing with
repeated or low-quality questions, so I’d like to eliminate (or at least reduce) these. Ideally
help1402 should be a lower-volume but higher-quality forum.
So before you post, please ensure that: • Your question is actually new
Don’t ask a question that has already been answered in another thread. You can either
monitor help1402 daily so you always know what has been discussed, or use the search
facility.
• You actually need external help
Quite a few posts have asked for confirmation that the output of a SQL query is “correct”,
even though it would be straightforward for the user to check this themselves.
Given access to an actual database, you should normally be able to tell how many rows of
output there should be by using SQLiteStudio to examine the data directly or manually
running a few simpler queries.
So just make sure that you have made reasonable efforts to test your query yourself before
posting to help1402
• Your question is precise
Please don’t post vague or overly general requests for assistance such as: “I tried using
but it didn’t work. Any help”.
All coding starts by forming a logical plan for extracting the required information from
the database. Of course you have to keep the general overall structure of an SQL query
in mind in terms of the sorts of things that SQL can and cannot do, but try to get a clear
idea of what you want to do before you start actually coding it.
While forming the plan, you may notice that you need a table or a value that is not
actually stored in the existing tables, but needs to be computed. This is when you think
about how you can use subqueries to create the table or compute the value.
When it is time to implement your plan in SQL, remember that very few people can
just sit down and code an entire complicated SQL query from first line to last line, partly
because the order in which the keywords occur is not the order in which the actual steps of
the row-processing are conducted. So write and test small portions of the code separately
and then put them together. For example, if counting parking tickets for black and white
cars has to be done for every state, then first write the query for just one state and one
of black options, and then gradually extend it.
Finally, remember that you are in control — you are the coder and the machine is doing
exactly what you tell it to do. If you accidentally tell it to do the wrong thing, then work
out why it is doing the wrong thing (by mentally going through the process) and change
it.
While coding certainly requires experimentation and testing, it should be a systematic
process. In other words, just randomly changing one SQL keyword to another or shuffling
around the lines of code is not an effective method of coding.
• Your question includes no (or minimal) actual code
As usual, don’t post actual code to help1402, instead giving just a verbal description or
posting a redacted screenshot (i.e., with key parts blurred or otherwise obscured). 2
DRAFT
(Actually, almost everyone is already doing the right thing with obscuring their posted
code, so this is just a reminder to keep doing it properly rather than a change in policy.)
Dodgey Brothers Auto Rentals
Wayne and Arthur Dodgey run a car rental business called Dodgey Brothers Auto Rentals and
want a database to keep a record of their cars, customers and rentals.
They have implemented a SQLite database themselves that is adequate, but after a few months use they
have noticed some problems. Some data is clearly incorrect, while the data in some tables is inconsistent
with the data in others.
You are given the schema of the current database and discuss the requirements with Wayne and Arthur.
The database has four tables, namely Car
, Vehicle
, rental and Customer which have the following
structure:
The table Car has data for types of car
The table stores data about types of car (not individual vehicles).
CREATE TABLE Car (carMake TEXT
,
carModel TEXT
,
carYear INTEGER
,
dailyCost INTEGER
,
kmCost REAL
)
A typical row in this table would be something like:
('Hyundai','i30',2020,30,0.10)
The first three fields describe a type of car, in this case a 2020 Hyundai i30, and the last two fields
indicate that Dodgey Brothers Auto Rentals rents a car of this type for
$30 per day plus
$0.10
per km.
Wayne and Arthur indicate that the combination of make, model and year uniquely determines a car
type, and that the daily and per-km costs depend only on this car type.
If a customer rents a 2020 Hyundai i30 for 3 days and drives 200km, then the cost of this rental will be 3 × 30 + 200 × 0.10 = 110.
The table Vehicle has data for actual vehicles
This table stores data about the individual vehicles in the Dodgey Brothers Auto Rentals fleet.
A typical row in this table would be something like
('Hyundai','i30',2020,'WDCGG5GB8AF429863', 15199)
The fields carMake
, carMode and carYear have the same meaning as in Car while VIN is the car’s Vehicle
Identification Number which is a unique code stamped onto a metal plate and riveted to the car’s frame
by its manufacturer. The code is a 17-digit string containing letters and numbers in a format similar to
the example above. Dodgey Brothers Auto Rentals may have several cars of the same type, but
it is impossible for two different vehicles to have the same VIN.
The odometer field lists the number of kilometres on this vehicle’s odometer, so this particular vehicle
has been driven for a total of 15199 kilometres since it was new.
The table rental has data for each rental
This records the details for each individual rental of a vehicle.
A rental is made by a customer, identified by a unique customer ID. The customer rents a specific vehicle
(identified by the VIN).
A new tuple is entered into the table rental at the time that the customer picks up the vehicle. The fields
odo_out and date_out record the odometer reading on the vehicle, and the date. The fields odo_back
and date_back are set to NULL (because these values will not be known until the car is returned.)
When the car is returned, an UPDATE statement completes the tuple by setting odo_back and date_back
to the actual odometer reading on the car and the actual date that the car is returned.
This rental is now completed and the rental cost can be calculated from the costs for that type of car,
the number of days in the rental (including both the start day and finish day of the rental), and number
of kilometres travelled (the value odo_back - odo_out).
Dates are given in the YYYY-MM-DD string format used by SQLite.
At this stage, the rental desk clerk is meant to update the odometer field in the tuple in the Vehicle
table for this particular car, but sometimes the clerk is busy, puts this off until later, and then forgets
to do it.
The table Customer has data for each customer
This table records the details for each Dodgey Brothers Auto Rentals customer.
Each customer has a unique id, and Dodgey Brothers Auto Rentals only keeps the name and
email address of their customers. An account can be created for a customer before they rent a car.
The tasks
As a database developer, you have been called in to improve the integrity of the database. You will not
be changing any of the column names or data types of the tables, but just adding database features to
improve the integrity and usability of the database.
You are asked to submit four files
ERD.png
DB.sql
DBTrigger.sql
DBView.sql
according to the following specifications:
1. An entity-relationship diagram cssubmit ERD.png (5 marks)
The first task is to get a visual representation of the database. This requires you to “reverse
engineer” the actual database to produce the corresponding entity-relationship diagram.
Do not invent additional entities or attributes in the ERD, but also remember that—in certain
situations—not all of the relations in the ERD will be represented as tables in the database. Video
31 should clarify what is required.
You must use ERDPlus.com to prepare your ERD and then use the “Export Image” selection from
the “Menu” button at the top-left of a diagram to save it to a PNG file. The file will be saved
under some generic name like image.png, but you should rename it to ERD.png and submit it as
the first file to cssubmit
.
Include the relevant cardinality and participation constraints according to the specifications above,
using your real-world knowledge of how car rentals work for anything not explicitly specified.
Once again, do not submit anything that is produced by a different ER diagramming tool, or
produced as a figure in Microsoft Word, or drawn in a drawing/painting program, or is handdrawn
and photographed/scanned.
(The reason for this is that there are literally hundreds of diagramming tools / conventions, and it
would be impossible for the markers to know them all.)
2. A database schema cssubmit DB.sql (2 + 2 + 2 = 6 marks as specified below)
You should prepare a file called DB.sql that creates an improved database. It should contain code
to create the four tables Car
, Vehicle
, rental and Customer, with exactly the same attributes
and data types as described above, but with additional features (as described below).
You should only include the DDL statements (the statements that create the tables, views and
triggers) but do not include any statements to insert data into the tables.
Of course, you should test your improved database by populating it your own synthetic (made-up)
sample data, and running various insert, update and delete commands, but do not include this in
your submission.
The additional features you should incorporate into DB.sql are: 5
DRAFT
(a) Key columns (2 marks)
The tables written by Wayne and Arthur Dodgey contain no information about keys, so nothing
prevents the accidental insertion of inconsistent data (for example, two different vehicles
with the same VIN).
Give improved CREATE TABLE statements for the tables Car
, Vehicle and Customer, ensuring
that the uniqueness constraints specified above are enforced by the database.
(b) Referential integrity (2 marks)
One problem for Wayne and Arthur is that the desk clerk often enters a new tuple into rental
in a hurry, and mistypes either the VIN or the customerId. If the VIN is incorrect, then it
is impossible to calculate the cost of a rental, and if the customerId is incorrect, then it is
impossible to know which customer to charge, so this is a major problem.
Give an improved CREATE TABLE rental statement to incorporate referential integrity constraints
ensuring that the VIN and customerId refer to actual vehicles and customers in the
Vehicle and Customer tables.
Wayne and Arthur tell you that a customer is never deleted from the table, but occasionally a
customerId might change (via an UPDATE statement). If this happens, then the tuples in the
rental table for this customer’s previous rentals should automatically be altered to reflect
this change.
For vehicles, Wayne and Arthur tell you that the VIN for a vehicle can never change, and a
vehicle is never deleted from the database.
(c) Data entry validation (2 marks)
A vehicle’s VIN is very important for any and all paperwork, such as lease agreements, insurance
details, servicing schedule etc.
However it is easy to mistype a long sequence of characters, and so we’d like to add some
validation to ensure that anything entered into this field at least has the right format to be a
VIN. • A VIN is a string of exactly 17 characters • Each character in a VIN is a digit or an uppercase letter • A VIN can contain any of the digits 0 to 9 • A VIN can contain any uppercase letter except I, O and
Q
• The 9th character of a VIN is either a digit from
0 to
9 or the letter
X
.
(These are all true facts about a VIN, but in real VINs the 9th character acts a check digit
and must satisfy an equation involving the other 16 characters.)
SQLite implements SQL check constraints. A check constraint is a boolean expression associated
with a single column using the keyword CHECK. Every time the value in that column is
altered (or inserted) the system will check that the boolean expression is still true with the
new value.
For example, consider a table BankAccount for an account where the balance is never allowed
to drop below 0. This could be defined with
CREATE TABLE BankAccount(
accountNumber INTEGER
,
accountBalance REAL CHECK(accountBalance >= 0));
The system will then check the condition when any UPDATE statement is attempted, and prohibit
the operation if the changed value violates the condition.
Add a CHECK constraint to the table Vehicle to ensure that the VIN always meets the basic
requirements above. You may need to look up the documentation for CHECK on sqlite.org
to double-check the exact syntax.
3. A trigger to improve data consistency cssubmit DBTrigger.sql (2 marks)
Wayne and Arthur constantly have problems keeping the odometer fields in Vehicle and rental
consistent.
6
DRAFT
As mentioned previously, when the customer rents a vehicle, a tuple is created in the rental table.
At this point, the clerk checks the actual vehicle’s odometer and enters this value into odo_out
.
When the customer returns the vehicle, the clerk again checks the vehicle’s odometer, and enters
this value into the odo_back field for this rental.
At this point, the clerk is also meant to update the odometer field in the Vehicle table, so that
both Vehicle.odometer and rental.odo_back have the same value.
However, relying on the desk clerk to transfer values correctly when busy helping customers is not
realistic. You advise Wayne and Arthur that having the same data stored in two different places
is poor relational database design. Wayne and Arthur say that they are unwilling to change the
schema because too many other systems rely on it, and ask if you can work around this design flaw
some other way.
You realise that this is an ideal situation for the use of a trigger
.
Write the code for a trigger on the table rental that maintains consistency between the two
odometer fields in the following manner: • When the desk clerk inserts a new tuple into rental, he or she enters the actual values for
the VIN
, customerId and date_out, but enters NULL for the other three values.
The trigger should intercept this operation, look up the odometer reading for this car in the
Vehicle table, and enter this value into the odo_out column for the newly-created tuple in
rental
.
• When the desk clerk updates a tuple in rental (because the customer has returned the car)
he or she updates the date_back and odo_back fields with the current date and the actual
reading on the vehicle’s odometer.
The trigger should intercept this operation and update the correct row of Vehicle with the
new odometer reading.
This ensures that the desk clerk cannot accidentally enter an incorrect odo_out value at the start
of the rental, and cannot forget to update the odometer reading in Vehicle at the end of the rental.
4. A view to improve usability cssubmit DBView.sql (2 marks)
For tax purposes, customers often want a list of all of their rentals together with the cost of each
rental. The necessary SQL command to extract this information in the right format is a little
complicated and too easy for Wayne and Arthur to get wrong.
Write the SQL code that defines a view name CustomerSummary that should behave as though it
were a table with each row containing just the essential information about a completed rental
.
So the view should have the following schema:
CustomerSummary (
customerId INTEGER
,
rental_date_out TEXT
,
rental_date_back TEXT
,
rental_cost REAL);
Write the code to create the view CustomerSummary with the specifications as above.
Wayne and Arthur Dodgey also known as The Dodgey Brothers were characters on the Australian TV Comedy
Show “Australia You’re Standing In It” from the early 1990s. 7
CITS1402 Project
Gordon Royle
2020 Semester Two
So far in this unit, the labs have been focussed on writing SQL queries learning how the SQL “rowprocessing-machine”
can be used to select, manipulate and summarise data contained in multiple relational
tables.
This project, really a mini-project, is going to focus on some of the other aspects of databases.
A database designer builds the database schema and possibly enters the initial data, but over time the
data evolves as rows are inserted, updated and deleted during the day-to-day use of the database. An
important role of the database designer is to make the database resistant to data corruption caused by
careless users.
This project explores some of the steps that a database designer can take to enhance the long-term
integrity of the database.
The questions may require you to undertake your own research into how certain SQLite features are
implemented. The official documentation is located at https://www.sqlite.org/docs.html/index.
html, and there are numerous SQLite tutorial sites with examples.
Project Rules
For the duration of the project, different (stricter) rules apply for obtaining help from the facilitators
and help1402 for the duration of the project.
1. Absolutely no “pre-marking” requests
Do not show your code to a facilitator and say “Is this right?”
Firstly, this is not fair to the facilitator, who is there to provide general assistance about SQL
and not to judge whether code meets the specifications.
Secondly, from previous experience, such requests often degenerate into the situation where
the facilitator “helps out” with the first line of code, then the student returns five minutes
later and asks for help with the second line of code, and so on, until the final query is mostly
written line-by-line by the facilitator and not the student.
Facilitators are there to gently nudge you in the right direction, not by just “giving the answer”
and supplying code that works, but by making general suggestions on SQL features, reminders
about what concepts might be useful, and advice on how you might investigate and resolve
problems yourself.
2. No validation requests for your submission
Please do not ask the facilitators anything about the mechanics of making a valid submission
such as file names, due dates etc. This is not their job and it leads to awkward situations where
a student submits something that is obviously incorrect, but then claims that “the facilitator
said it was ok”.
1
DRAFT
You are responsible for writing, testing, formatting and submitting your code correctly, and if
you have any doubts about what is required, then please ask on help1402
.
3. Avoid low-quality help1402 posts
Before the mid-semester test, there was a huge spike of activity on help1402. While I encourage
thoughtful questions and try to answer them promptly, the sheer volume of questions almost
overwhelmed me.
Even without help1402, this unit is already consuming far more time than I am meant to
spend on it, so I have to cut back. A lot of my time earlier in semester was spent dealing with
repeated or low-quality questions, so I’d like to eliminate (or at least reduce) these. Ideally
help1402 should be a lower-volume but higher-quality forum.
So before you post, please ensure that: • Your question is actually new
Don’t ask a question that has already been answered in another thread. You can either
monitor help1402 daily so you always know what has been discussed, or use the search
facility.
• You actually need external help
Quite a few posts have asked for confirmation that the output of a SQL query is “correct”,
even though it would be straightforward for the user to check this themselves.
Given access to an actual database, you should normally be able to tell how many rows of
output there should be by using SQLiteStudio to examine the data directly or manually
running a few simpler queries.
So just make sure that you have made reasonable efforts to test your query yourself before
posting to help1402
• Your question is precise
Please don’t post vague or overly general requests for assistance such as: “I tried using
All coding starts by forming a logical plan for extracting the required information from
the database. Of course you have to keep the general overall structure of an SQL query
in mind in terms of the sorts of things that SQL can and cannot do, but try to get a clear
idea of what you want to do before you start actually coding it.
While forming the plan, you may notice that you need a table or a value that is not
actually stored in the existing tables, but needs to be computed. This is when you think
about how you can use subqueries to create the table or compute the value.
When it is time to implement your plan in SQL, remember that very few people can
just sit down and code an entire complicated SQL query from first line to last line, partly
because the order in which the keywords occur is not the order in which the actual steps of
the row-processing are conducted. So write and test small portions of the code separately
and then put them together. For example, if counting parking tickets for black and white
cars has to be done for every state, then first write the query for just one state and one
of black options, and then gradually extend it.
Finally, remember that you are in control — you are the coder and the machine is doing
exactly what you tell it to do. If you accidentally tell it to do the wrong thing, then work
out why it is doing the wrong thing (by mentally going through the process) and change
it.
While coding certainly requires experimentation and testing, it should be a systematic
process. In other words, just randomly changing one SQL keyword to another or shuffling
around the lines of code is not an effective method of coding.
• Your question includes no (or minimal) actual code
As usual, don’t post actual code to help1402, instead giving just a verbal description or
posting a redacted screenshot (i.e., with key parts blurred or otherwise obscured). 2
DRAFT
(Actually, almost everyone is already doing the right thing with obscuring their posted
code, so this is just a reminder to keep doing it properly rather than a change in policy.)
Dodgey Brothers Auto Rentals
Wayne and Arthur Dodgey run a car rental business called Dodgey Brothers Auto Rentals and
want a database to keep a record of their cars, customers and rentals.
They have implemented a SQLite database themselves that is adequate, but after a few months use they
have noticed some problems. Some data is clearly incorrect, while the data in some tables is inconsistent
with the data in others.
You are given the schema of the current database and discuss the requirements with Wayne and Arthur.
The database has four tables, namely Car
, Vehicle
, rental and Customer which have the following
structure:
The table Car has data for types of car
The table stores data about types of car (not individual vehicles).
CREATE TABLE Car (carMake TEXT
,
carModel TEXT
,
carYear INTEGER
,
dailyCost INTEGER
,
kmCost REAL
)
A typical row in this table would be something like:
('Hyundai','i30',2020,30,0.10)
The first three fields describe a type of car, in this case a 2020 Hyundai i30, and the last two fields
indicate that Dodgey Brothers Auto Rentals rents a car of this type for
$30 per day plus
$0.10
per km.
Wayne and Arthur indicate that the combination of make, model and year uniquely determines a car
type, and that the daily and per-km costs depend only on this car type.
If a customer rents a 2020 Hyundai i30 for 3 days and drives 200km, then the cost of this rental will be 3 × 30 + 200 × 0.10 = 110.
The table Vehicle has data for actual vehicles
This table stores data about the individual vehicles in the Dodgey Brothers Auto Rentals fleet.
A typical row in this table would be something like
('Hyundai','i30',2020,'WDCGG5GB8AF429863', 15199)
The fields carMake
, carMode and carYear have the same meaning as in Car while VIN is the car’s Vehicle
Identification Number which is a unique code stamped onto a metal plate and riveted to the car’s frame
by its manufacturer. The code is a 17-digit string containing letters and numbers in a format similar to
the example above. Dodgey Brothers Auto Rentals may have several cars of the same type, but
it is impossible for two different vehicles to have the same VIN.
The odometer field lists the number of kilometres on this vehicle’s odometer, so this particular vehicle
has been driven for a total of 15199 kilometres since it was new.
The table rental has data for each rental
This records the details for each individual rental of a vehicle.
A rental is made by a customer, identified by a unique customer ID. The customer rents a specific vehicle
(identified by the VIN).
A new tuple is entered into the table rental at the time that the customer picks up the vehicle. The fields
odo_out and date_out record the odometer reading on the vehicle, and the date. The fields odo_back
and date_back are set to NULL (because these values will not be known until the car is returned.)
When the car is returned, an UPDATE statement completes the tuple by setting odo_back and date_back
to the actual odometer reading on the car and the actual date that the car is returned.
This rental is now completed and the rental cost can be calculated from the costs for that type of car,
the number of days in the rental (including both the start day and finish day of the rental), and number
of kilometres travelled (the value odo_back - odo_out).
Dates are given in the YYYY-MM-DD string format used by SQLite.
At this stage, the rental desk clerk is meant to update the odometer field in the tuple in the Vehicle
table for this particular car, but sometimes the clerk is busy, puts this off until later, and then forgets
to do it.
The table Customer has data for each customer
This table records the details for each Dodgey Brothers Auto Rentals customer.
Each customer has a unique id, and Dodgey Brothers Auto Rentals only keeps the name and
email address of their customers. An account can be created for a customer before they rent a car.
The tasks
As a database developer, you have been called in to improve the integrity of the database. You will not
be changing any of the column names or data types of the tables, but just adding database features to
improve the integrity and usability of the database.
You are asked to submit four files
ERD.png
DB.sql
DBTrigger.sql
DBView.sql
according to the following specifications:
1. An entity-relationship diagram cssubmit ERD.png (5 marks)
The first task is to get a visual representation of the database. This requires you to “reverse
engineer” the actual database to produce the corresponding entity-relationship diagram.
Do not invent additional entities or attributes in the ERD, but also remember that—in certain
situations—not all of the relations in the ERD will be represented as tables in the database. Video
31 should clarify what is required.
You must use ERDPlus.com to prepare your ERD and then use the “Export Image” selection from
the “Menu” button at the top-left of a diagram to save it to a PNG file. The file will be saved
under some generic name like image.png, but you should rename it to ERD.png and submit it as
the first file to cssubmit
.
Include the relevant cardinality and participation constraints according to the specifications above,
using your real-world knowledge of how car rentals work for anything not explicitly specified.
Once again, do not submit anything that is produced by a different ER diagramming tool, or
produced as a figure in Microsoft Word, or drawn in a drawing/painting program, or is handdrawn
and photographed/scanned.
(The reason for this is that there are literally hundreds of diagramming tools / conventions, and it
would be impossible for the markers to know them all.)
2. A database schema cssubmit DB.sql (2 + 2 + 2 = 6 marks as specified below)
You should prepare a file called DB.sql that creates an improved database. It should contain code
to create the four tables Car
, Vehicle
, rental and Customer, with exactly the same attributes
and data types as described above, but with additional features (as described below).
You should only include the DDL statements (the statements that create the tables, views and
triggers) but do not include any statements to insert data into the tables.
Of course, you should test your improved database by populating it your own synthetic (made-up)
sample data, and running various insert, update and delete commands, but do not include this in
your submission.
The additional features you should incorporate into DB.sql are: 5
DRAFT
(a) Key columns (2 marks)
The tables written by Wayne and Arthur Dodgey contain no information about keys, so nothing
prevents the accidental insertion of inconsistent data (for example, two different vehicles
with the same VIN).
Give improved CREATE TABLE statements for the tables Car
, Vehicle and Customer, ensuring
that the uniqueness constraints specified above are enforced by the database.
(b) Referential integrity (2 marks)
One problem for Wayne and Arthur is that the desk clerk often enters a new tuple into rental
in a hurry, and mistypes either the VIN or the customerId. If the VIN is incorrect, then it
is impossible to calculate the cost of a rental, and if the customerId is incorrect, then it is
impossible to know which customer to charge, so this is a major problem.
Give an improved CREATE TABLE rental statement to incorporate referential integrity constraints
ensuring that the VIN and customerId refer to actual vehicles and customers in the
Vehicle and Customer tables.
Wayne and Arthur tell you that a customer is never deleted from the table, but occasionally a
customerId might change (via an UPDATE statement). If this happens, then the tuples in the
rental table for this customer’s previous rentals should automatically be altered to reflect
this change.
For vehicles, Wayne and Arthur tell you that the VIN for a vehicle can never change, and a
vehicle is never deleted from the database.
(c) Data entry validation (2 marks)
A vehicle’s VIN is very important for any and all paperwork, such as lease agreements, insurance
details, servicing schedule etc.
However it is easy to mistype a long sequence of characters, and so we’d like to add some
validation to ensure that anything entered into this field at least has the right format to be a
VIN. • A VIN is a string of exactly 17 characters • Each character in a VIN is a digit or an uppercase letter • A VIN can contain any of the digits 0 to 9 • A VIN can contain any uppercase letter except I, O and
Q
• The 9th character of a VIN is either a digit from
0 to
9 or the letter
X
.
(These are all true facts about a VIN, but in real VINs the 9th character acts a check digit
and must satisfy an equation involving the other 16 characters.)
SQLite implements SQL check constraints. A check constraint is a boolean expression associated
with a single column using the keyword CHECK. Every time the value in that column is
altered (or inserted) the system will check that the boolean expression is still true with the
new value.
For example, consider a table BankAccount for an account where the balance is never allowed
to drop below 0. This could be defined with
CREATE TABLE BankAccount(
accountNumber INTEGER
,
accountBalance REAL CHECK(accountBalance >= 0));
The system will then check the condition when any UPDATE statement is attempted, and prohibit
the operation if the changed value violates the condition.
Add a CHECK constraint to the table Vehicle to ensure that the VIN always meets the basic
requirements above. You may need to look up the documentation for CHECK on sqlite.org
to double-check the exact syntax.
3. A trigger to improve data consistency cssubmit DBTrigger.sql (2 marks)
Wayne and Arthur constantly have problems keeping the odometer fields in Vehicle and rental
consistent.
6
DRAFT
As mentioned previously, when the customer rents a vehicle, a tuple is created in the rental table.
At this point, the clerk checks the actual vehicle’s odometer and enters this value into odo_out
.
When the customer returns the vehicle, the clerk again checks the vehicle’s odometer, and enters
this value into the odo_back field for this rental.
At this point, the clerk is also meant to update the odometer field in the Vehicle table, so that
both Vehicle.odometer and rental.odo_back have the same value.
However, relying on the desk clerk to transfer values correctly when busy helping customers is not
realistic. You advise Wayne and Arthur that having the same data stored in two different places
is poor relational database design. Wayne and Arthur say that they are unwilling to change the
schema because too many other systems rely on it, and ask if you can work around this design flaw
some other way.
You realise that this is an ideal situation for the use of a trigger
.
Write the code for a trigger on the table rental that maintains consistency between the two
odometer fields in the following manner: • When the desk clerk inserts a new tuple into rental, he or she enters the actual values for
the VIN
, customerId and date_out, but enters NULL for the other three values.
The trigger should intercept this operation, look up the odometer reading for this car in the
Vehicle table, and enter this value into the odo_out column for the newly-created tuple in
rental
.
• When the desk clerk updates a tuple in rental (because the customer has returned the car)
he or she updates the date_back and odo_back fields with the current date and the actual
reading on the vehicle’s odometer.
The trigger should intercept this operation and update the correct row of Vehicle with the
new odometer reading.
This ensures that the desk clerk cannot accidentally enter an incorrect odo_out value at the start
of the rental, and cannot forget to update the odometer reading in Vehicle at the end of the rental.
4. A view to improve usability cssubmit DBView.sql (2 marks)
For tax purposes, customers often want a list of all of their rentals together with the cost of each
rental. The necessary SQL command to extract this information in the right format is a little
complicated and too easy for Wayne and Arthur to get wrong.
Write the SQL code that defines a view name CustomerSummary that should behave as though it
were a table with each row containing just the essential information about a completed rental
.
So the view should have the following schema:
CustomerSummary (
customerId INTEGER
,
rental_date_out TEXT
,
rental_date_back TEXT
,
rental_cost REAL);
Write the code to create the view CustomerSummary with the specifications as above.
Wayne and Arthur Dodgey also known as The Dodgey Brothers were characters on the Australian TV Comedy
Show “Australia You’re Standing In It” from the early 1990s. 7