Bases de Dades: introduction to SQL (part 1)

Transcription

1 Bases de Dades: introduction to SQL (part 1) Andrew D. Bagdanov Departamento de Ciencias de la Computación Universidad Autónoma de Barcelona Fall, 2010

2 Outline Last week on bases de dades... 1 Last week on bases de dades

3 Embedded databases Embedded databases The NOSQL movement DB archaeology: Repeat my locate *.sqlite experiment on your (or a university) computer. Try to find more examples of embedded databases in applications. Other things you might search for are *.sql or *.db.

4 The nosql movement Embedded databases The NOSQL movement Changing times: I mentioned nosql databases a few times, but never said what that means. Do some searching to discover what the principal ideas are behind the nosql movement.

5 Goals for today Last week on bases de dades... Goals and decisions Outline Begin thinking about database design and how it affects database applications. Introduce the high-level language SQL (Structured Query Language). Familiarize ourselves with some of the main types of SQL statements. Understand the importance of data modeling.

6 Design or use? Last week on bases de dades... Goals and decisions Outline We have a problem of the chicken and the egg... It s hard to teach about database use before we know how a database is designed. But it s hard to design a database before we know something about how it will be used. In this class we will follow an iterated design philosophy. We will design a little (learning as we go), use a DB a little (learning as we go), and then re-design a little to fix any problems we encounter. Pretty similar to the real world (but be careful...)

7 Goals and decisions Outline Standards, standards everywhere! Someone once commented: The great thing about standards is that there are so many to choose from! This is very true in the DBMS world, especially with SQL. There have been many revisions and major releases of the ANSI SQL standard. SQL:1999 is the most broadly supported, SQL:2008 is the latest version. No implementation is complete (there will be some missing features). Every implementation implements custom extensions. Read the manual.

8 Outline Last week on bases de dades... Goals and decisions Outline Data and data centricity When data is central. Why use a database? A simple case study But nonetheless a real one Choosing a data model Expressing the design in SQL Answering questions

9 Data-centric applications Why use a database? The unholy triad Many applications are inherently data-centric: the generate and consume large quantities of data. clients, search engines, reservation systems, product catalogs, you name it: it s all about the data. How should this data be stored in a way that is efficient? How should this data be stored so that it can be retrieved? How should this data be stored so that it can be maintained? How should this data be stored so that it can be flexibly retrieved and maintained in unforeseen ways?

10 Efficiency, standard API Why use a database? The unholy triad DBMS provide answers to all of those questions. They are one of the most researched, advanced, sophisticated and reliable technologies in the world of computer science. Unless you have a very good reason for doing otherwise, a DBMS should be the backbone of any data-centric application. Andy s tenth axiom: Any sufficiently complex data-centric application contains an ad hoc, informally-specified, bug-ridden, slow implementation of half a relational database management system. 1 1 With apologies to Greenspun and his tenth rule of programming

11 Design decisions Last week on bases de dades... Why use a database? The unholy triad Life in the database world is often a zero-sum game: Compromise and cooperation is necessary. Remember that you may have to wear any of these hats at any time...

12 A country profile database A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB Imagine an application (or group of applications) that must routinely deal with information about countries and the languages spoken in them. It could be a GIS application, or a shipping address database, or just about anything. What are the data modeling needs of such applications? What information should we model about countries? Assume for now that we are interested mostly in geopolitical and linguistic information.

13 Thought experiments A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB What types of queries will applications need to perform against this type of database? What language is spoken in a particular (group of) countries? In what countries is a particular language spoken? In how many countries is a language spoken? What countries are part of a geopolitical region?...

14 A first design: mono-tabular A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB Let s assume we want to model the following information about a country: 1 Country name 2 Official ISO country code 3 The continent on which it is 4 The geopolitical region it belongs to 5 Language spoken Hmmm... Already we run into problems.

15 Second run: mono-tabular A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB There are few countries in the world where only ONE language is spoken: 1 Country name 2 Official ISO country code 3 The continent on which it is 4 The geopolitical region it belongs to 5 Language spoken 6 Is language official? 7 Percentage of population speaking language

16 The data definition Last week on bases de dades... A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB Data (relations, actually) are represented in as tables in DBMS. Table columns represent the attributes, rows are instances of data. A description of the structure all tables is called a database schema. Our table description (each row corresponds to a column in DB): Column Name Type Null? Default Code char(3) no Name char(52) no Continent enum no Asia Region char(26) no Language char(30) no IsOfficial enum no F Percentage float(4,1) no 0.0

17 Some sqlite meta-commands A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB This is a small sample, the.help command is very useful: Command.read <file>.schema <table>.dump <table>.quit.help Action execute sequence of SQL from file describe table structure dump SQL representing DB quit sqlite3 CLI get help on meta commands

18 Our schema in SQL A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB CREATE TABLE Country ( Code char(3) NOT NULL default, Name char(52) NOT NULL default, Continent enum NOT NULL default Asia, Region char(26) NOT NULL default, Language char(30) NOT NULL default, IsOfficial enum NOT NULL default F, Percentage float NOT NULL default 0.0 ); This SQL statement will create the Country table. Table structure expressed in a Data Definition Language (DDL). The table is created in the current database.

19 Inserting from data A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB Each row in a table must be INSERT-ed. A row corresponds to a datum, or to a single element in the relation (using the set-theoretic formulation of relations). INSERT INTO Country VALUES( AFG, Afghanistan, Southern and Central Asia, Asia, Balochi, F, 0.9);...

20 SQL syntax diagrams A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB SQL has a lot of syntax. Too much, some might say... The sqlite reference manual uses has nice syntax diagrams to help (

21 A sample session Last week on bases de dades... A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB The SQL file used in this example is on the course website 2. 09:09:20> sqlite3 country.sqlite SQLite version Enter ".help" for instructions Enter SQL statements terminated with a ";" sqlite>.read mono_country.sql sqlite> 2

22 SELECT-ing data Last week on bases de dades... A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB The main way to retrieve data (rows) is through the SELECT statement. Your SQL reference will fall open to the select page. General form: SELECT <> FROM <> WHERE <>; Each of the <> can be very, very complex. Simple examples: SELECT * FROM Country; /* ALL columns, ALL rows */ /* In which countries is Spanish spoken? */ SELECT Code,Name FROM Country WHERE Language= Spanish ;

23 More complex queries A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB /* In which countries is Spanish an official language? */ SELECT Code,Name FROM Country WHERE Language= Spanish AND IsOfficial= T ; /* We can COUNT things too. */ SELECT count(*) FROM Country WHERE Language= Spanish ; /* On what continents is Spanish spoken? */ SELECT Continent FROM Country where Language= Spanish ;

24 Key points Last week on bases de dades... A real-world problem Expressing in SQL Coming to grips with SQL syntax A complete DB Things to take home: Data-centric applications Andy s Tenth Axiom Basic sqlite interaction (meta-commands versus SQL statements). The CREATE TABLE, INSERT and SELECT SQL statements (basic versions). SQL syntax diagrams. Next Week: Data Definition Language: specifying structure and constraints on tables. Refining the design of our case study: normalization primary keys. More complex queries.

25 : lecture 2 A few exercises to do at home. Please come to the next problem session prepared to discuss your findings (items indicated in BOLD will be collected for grading): 1 DB creation: Download the mono_country.zip file from the course website. Duplicate my experiments with creating the Country table and each of the sample queries I showed in the course. 3 2 Redundancy: there is a LOT of redundancy in our first design of the database (this is called an unnormalized database ). In particular, all of the country information is repeated for each language in the country. What problems might this cause for the application programmer and the DB administrator? How might you fix this problem of redundancy? 3

26 (TO BE COLLECTED 19 October) 3 Do it yourself: design (just design, do not implement) a data structure implementing the information about countries used in this lecture. What operations must supported on to implement all of the queries we examined today? What are the advantages and disadvantages of implementing your own data structures versus using a DBMS. 4 Distinct attributes: Write queries to determine: The number of distinct languages spoken. The number of distinct regions in which Spanish is spoken. The countries where Spanish is NOT an official language, but is spoken by more than 50 of the population. 5 Inserting new rows: if you search for the countries where Catalan is spoken, you will note that there are some missing entries (France and Italy, at least). Write INSERT statements to insert these missing entries into the DB. Show the new results of search for Catalan-speaking countries.