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Introduction Databases SQL Exercising Wrap Up
A (short) Introduction to Databases(v.145 2015-04-22)
Nicola [email protected]
“S.Cecilia” Conservatory - Rome
April 22 2015Service Systems Design Master
Aalborg University in CopenhagenCopenhagen, Denmark
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
Topics we will cover
Just a touch of history
Why do we need databases
What are they, anyway?
How do they work (in a very basic form)
Exercising database design
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
Topics we will NOT cover
(but feel free to ask about them if you feel so inclined, by anymeans :-))
Advanced topics, such as:
search algorithmsdata indexingcomplex multi–table queriessophisticated database functions. . .
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
What are computers for? (1)
Figure: A Leibnitz computingmachine
Back in the old days (I meanthe eighteenth century). . .
. . . computers were conceivedand built for calculus andcomputation
Mathematicians found out thatreality was quite morecomplicate than the abstractmodels they had conceived. . .
. . . so they invented “computingtechnologies”, that iscomputers
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
What are computers for? (2)
Figure: An IBM 7094 (1962)
The last time computers wereused solely for computingpurposes was during World WarII
At the end of WW II,computing technology wasmature enough to enter theindustrial world. . .
. . . and indeed, the computingcapability of computers fadedinto the background
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
What are computers for? (3)
Today, the most useful functionof computers is. . .
. . . their ability to store andorganize data
and this is where databaseskick in.
Computation is still used ofcourse, but it is no longer thecore function (it appears inseveral specific fields in whatare now called “numbercrunching applications”)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
So why do we need databases? (1)
int i = 42;
4238
11432
1923
972397249725
Ram Memory
Addresses
4238
11432
1923
972397249725
Ram Memory
Addresses
As you all know, computersstructure (and store)information in RAM memory
Data maps into memorystraight memory
Memory is characterized by acontent and an address(pointer)
An address can also beconsidered some form ofcontent
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
So why do we need databases? (2)
struct birthday {
int day;
int month;
int year;
};
yearday
monthyear
monthday
972197249727
Ram Memory
Addresses
Ram Memory
Addresses
Furthermore, data in memorycan be structured
That is, it can be organized sothat complex structures arekept physically together in RAMmemory
In the past few decades, thesestructures have become object,in object–oriented programminglingo
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
So why do we need databases? (3)
However, data in RAM memory is not persistent
When the computer shuts off, the data is gone
So: data needs to be persisted
Which translates into: databases
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How many different kinds of databases are there?
You can make databases (that is: save your data) with justabout anything
log filesline oriented delimited text(f. ex. Nicola;Bernardini;Rome;14/08/1956;+4512345678)spreadsheetsdisk file systemsproper relational database
only the last two do not save only data but also relations
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
What are relations for? (1)
If you start collecting data you soon realize that
a) either you repeat parts of the data endlesslyb) or you have to save data relations along with the data itself
and here we get to the golden rule of databases, namely. . .
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
The Golden Rule of Databases
DO NOT REPLICATEDATA.
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
Rationale for the golden rule
You don’t want to replicate data because you might need tochange it in the future: if your data is replicated all over, youneed to change it all over (very error-prone)
while if you keep relationships instead, a single change will besufficient for each piece of datum you have
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
What are relations for? (2)
This is what relations are for. For example:
If you have one or more telephone numbers for each person inyour agenda (which is likely, or make it e-mail addresses, orwhatever)You need to be able to express something like “a person canhave many telphone numbers” or like “this number belongs tothis person”Both sentences basically establish a one–way relation betweenpersons and phone numbers
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How do you represent relationships inside computers? (1)
struct person {
char first_name[256];
char last_name[256];
};
struct number {
struct person *owner; /* <- */
int number;
};
struct person persons[1000];
struct number numbers[10000];
When your data is stored inRAM. . .
. . . you represent relationshipsby memory pointers (also knownas references) (check the arrow)
but what happens when youwant to persist the relationship?(== save it to disk)
this (and some more) is whatrelational databases are for
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How do you represent relationships inside computers? (2)
Before we go on, a question for you.
How would you create a persistent telephone book. . .with a filesystem?
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How do you represent relationships inside computers? (3)
Current relational database can do much more than that
Within databases, you
add a progressive number to each data structurethis number is guaranteed (by the database engine) to beuniqueit is called the primary key
Primary keys act like persistent pointers
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How do you represent relationships inside computers? (4)
Relationships are expressed with further indexes, called foreignkeys
Theoreticians have established that with only three kinds ofrelationships you can describe any type of relation among data
These relationships are. . .
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
How do you represent relationships inside computers? (5)
one–to–one (f.ex: a husband has only one wife)
one–to–many (f.ex: a football team has many players)
many–to–many (f.ex: a student has many subjects)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
One–to–one relationships
you establish one–to–onerelationships by:
creating a first table with anauto–generated primary keycreating a second table with a(not auto–generated) foreignkey which points to aninstance of the first table
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
One–to–many relationships
you establish one–to–manyrelationships by:
creating a first table with anauto–generated primary keycreating a second table withan auto–generated primarykey and a foreign key whichpoints to an instance of thefirst table
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
Many–to–many relationships
you establish many–to–many relationships by:
creating two separate tables with an auto–generated primarykeyscreating a third table with two foreign keys each pointing toone instance of one of the first two tables
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up Introduction Types Relations
A real–world example
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up SELECT
A structured query language (SQL) (1)
Of course, structuring data is not enough to be able to use itefficiently
You also need to have a way to create, browse, view, update,delete data
Such a language exists: it is called SQL (spelled: sequel –which stands precisely for Structured Query Language)
With minor changes, SQL can be used over a multitude ofdifferent databases (sqlite, MySql, PostgreSQL, Oracle,Informix, etc.)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up SELECT
A structured query language (SQL) (2)
SQL is a text–based language: you type some instruction on adatabase console and the database will reply with an answer
Of course it can also be scripted and put into a file
SQL has several commonly used statements:
“SELECT”, “INSERT”, “UPDATE”, “DELETE”,“CREATE” and “DROP”
“SELECT” is the most commonly used, as it is the statementthat allows you to make queries (it does not change thedatabase)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up SELECT
The “SELECT” statement (1)
Here is what the “SELECT” statement looks like:
select "column1"
[,"column2",etc]
from "tablename"
[where "condition"];
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up SELECT
The “SELECT” statement (2)
Some “SELECT” examples:
select first, last, city from employees where
first like ’Er%’; find all first names that begin with ‘Er’and display first name, last name and city of operationselect * from employees where first = ’Eric’; findall first names that match the name ‘Eric’ exactly and displayall data from themselect last, city, age from employees where age >
30; find all employees which are older than 30 years of ageand display last name, city and ageselect last, city, age from employees where (age >
30) and (last like ’%s’); find all employees which areolder than 30 years of age and have a last name that ends in‘s’ and display last name, city and age. . . let’s do some examples live. . .
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
Live examples
Please download the employees.sqlite from Moodle
If you have a Mac or a Linux laptop, you may open a terminaland type sqlite3 employees.sqlite in the folder whereyou downloaded the db
if you have Windows, you may find a pre–compiled binary athttps://www.sqlite.org/download.html
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
Walk-through exercise
Let’s imagine reverse–engineering. . . FaceBook at leastpartially :-)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
DIY Exercise (30 minutes)
1 Pick your own data set and/or application
2 Design it in terms of a relational database:
1 Establish tables2 Establish relationships among tables, making sure that no data
is replicated
3 Discuss your design
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
What have we covered
1 What are databases and why do they exist at all
2 How they are conceived
3 What are data relationships
4 An introduction to the SQL language
5 How to design proper database structures (with someexercising)
Nicola Bernardini [email protected] Databases
Introduction Databases SQL Exercising Wrap Up
THANK YOU!
Nicola Bernardini [email protected] Databases
