LESSON 3: DATA INTEGRITY AND ANALYTICS
What we will learn today?
- How to add invalid relational data to a database
- How to prevent invalid relational data with foreign keys
- Database nulls
- Combining SQL predicates with AND and OR
- SQL - Order by
- SQL - Count / Sum / Avg
- SQL - Group by
- SQL - HAVING
- SQL - LIMIT
- SQL - like
- SQL - DISTINCT
- SQL - 'IN'
LESSON 7 : JOIN ME, AND TOGETHER WE CAN RULE THE INTERNET AS FATHER AND SON!
Now let's say we want to get the names of customers who have a reservation today.
From what we know now, we could do it like this:
- select customer_id from reservations where date_started = '01/01/2018'
- write down the list of customer ids on paper (e.g. 3, 5, 7)
- select * from customers where id = 3 or id = 5 or id = 7
However, that's stupid. We want the computer to figure that out. That's where a database "join" comes in handy. In real life, if you work with databases, you will be using this thing all of the time.
Now, we have data that spans two tables - we have reservations with a "customer_id" column that refers to the id column in the "customers" table.
SELECT reservations.date_started, customers.firstname, customers.surname from reservations join customers on reservations.customer_id = customer.id where reservation.date_started = '01/01/2018';
User Story: As a staff member, I want to get the list of all the invoices, and the details of the referring reservations.
Create and endpoint
/detailed-invoices from where we can get the list of invoices, together with the details for the reservation that they refer to.
User Story: As a staff member, I want to consult reservations, but including the room and customer information.
Update the exercies 5.* to retreieve the information of the rooms and customers as well.
EXERCISE 8: STRETCH GOAL
User story: As a staff member, I want ot retrieve the reservations and details for rooms and customers, that happened between a given date range.
Create and endpoint to get from
/reservations/details-between/:from_day/:to_day all the reservations and details about customer and room, between a given date range.
sqlite> create table rooms ( id INTEGER PRIMARY KEY AUTOINCREMENT, type_name varchar(50), original_price integer, current_price int, );
sqlite> insert into rooms (1, 'seaview', 50.0, 55.0);
sqlite temp.sqlite < example.sql
EXERCISE 1: Recreate the hotel database and add some entries
- Recreate customer, room and reservation tables
- Insert some example data making sure that the IDs match
LESSON 2: Data integrity
Try this in your database:
INSERT INTO Invoices (reservation_id, total, surcharges, invoice_date_time, paid) values (9999, 35.0, 0, '01/01/2018', 0);
QUESTIONS FOR CLASS:
What's the problem with the above SQL statement?
What happens to the application and the end user if you accidentally do this?
How can you deal with a problem like this? There are 2/3 answers to this question - what are they?
A1: Delete data
A2: Make it impossible to add invalid data in the first place in the code.
A3: Make it impossible to add invalid data in the first place in the database.
LESSON: Deleting records from a table
To fix the invalid data we can first delete the data from the database. This is as good a time as any to tell you about deleting, so here goes:
DELETE FROM [ TABLE ] WHERE id = [ ID ];
EXERCISE: First find the row with select, then delete it
Start with the
SELECT * FROM customers WHERE <...>
Then the delete.
DELETE FROM customers WHERE <...>
LESSON : Foreign Keys
CREATE TABLE customers ( id INTEGER PRIMARY KEY AUTOINCREMENT, title VARCHAR(10), firstname VARCHAR (50), surname VARCHAR (50), email VARCHAR (255) );
CREATE TABLE reservations ( ID INTEGER PRIMARY KEY AUTOINCREMENT, customer_id INTEGER, room_id INTEGER, check_in_date DATETIME NOT NULL, checkout_out_date DATETIME, room_price_per_night REAL, FOREIGN KEY (customer_id) REFERENCES customers(id) );
Now, try running this again:
INSERT INTO reservations (9999, 35, '01/01/2018', '01/01/2018', invoice_date_time, paid) values (9999, 35.0, 0, '01/01/2018', 0);
A foreign key is a constraint.
EXERCISE: Add foreign key for invoices
Deal with imcomplete data - foreign keys
- Add reservations without either costumer or room
- Try to get the information for given reservation
- things should break because there is missing data on the reservation
- Fix the table with the foreign keys
Alter the table to have contrains on the room and customer ids..
ALTER TABLE Reservation (for room)
Deal with incomplete data - not null
Use checkin date as an example
- Add reservations with no check-in date
- Bring up the scenario where the client comes up to the hotel and the reservation is missing the checking date, so the room is being used, because another customer extended the stay.
- Fix table with NOT NULL on all the required columns
ALTER TABLE Reservation (for room)
Go over analysis through querying
get all the reservations we have
get all the reservations we have for room with number 3 -
get all the reservations we have by date -
number of reservations for room number 3 -
number of reservations per room number -
rooms that have more than 10 reservations -
3 most reserver rooms on the database -
search for onn wich is probably misspelled -
list dates we have check-ins on -
clients with surnames in [
list reservations for clients with surnames in [
Silva] - joining in addition to what we already have
list all reservations for non deluxe rooms -
number of reservations for room number 3 on the summer of 2017 -
date >= '21-06-2017' and date < '21-09-2017'
list reservations for clients with surnames not like [
number of reservations for room number 3 on the summer and winter of 2017 -
date >= '21-06-2017' and date < '21-09-2017' or date >= '21-12-2017' and date < '21-04-2018'
Notes for next steps
<=---- what are all the reservations after tomorrow?
combining predicates (