Database Systems. Lecture Handout 1. Dr Paolo Guagliardo. University of Edinburgh. 21 September 2015

Transcription

1 Database Systems Lecture Handout 1 Dr Paolo Guagliardo University of Edinburgh 21 September 2015 What is a database? A collection of data items, related to a specific enterprise, which is structured and organized so as to be more easily accessed, managed, and updated Database Management System system software for creating and managing databases mediates interaction between end-users (or applications) and the database ensures that data is consistently organized and remains easily accessible

2 The relational model Data is organized in relations (tables) Relational database schema: set of relation names list of attributes (column names) for each table Tables are specified as Name : list, of, attributes Examples of schema Movie : Title, Year, Director, Genre Actor : Name, Movie, Year, Role Account : Number, Branch, CustID, Balance Customer : CustID, Name, City, Address Attributes within a table have different names Tables have different names Example: relational database Customer CustID Name City Address cust1 Renton Edinburgh 2 Wellington Pl cust2 Watson London 221B Baker St cust3 Holmes London 221B Baker St Account Number Branch CustID Balance Edinburgh cust London cust Manchester cust

3 How do we ask questions to a database? Query Languages Commercial : Structured Query Language (SQL) Theoretical : relational calculus, relational algebra, Datalog,... Procedural Specify a sequence of steps to obtain the expected result Declarative Specify what you want not how to get it Queries are typically asked in a declarative way (SQL) DBMSs figure out internally how to translate a query into procedures that are suitable for getting the results Example: declarative vs procedural Q: Find the title of all movies Declarative { title (title,year,director,genre) Movies } Procedural for (t,y,d,g) in Movies: output t

4 Declarative vs procedural Another Example Q: Find remakes (movies with the same title of some older one) Declarative { (title,year2) (title,year1,director1,genre1) Movies, (title,year2,director2,genre2) Movies, year1 < year2 } Procedural for (t1,y1,d1,g1) in Movies: for (t2,y2,d2,g2) in Movies: if t1==t2 and d1==d2 and y1<y2: output (t2,y2) Declarative vs procedural languages Theoretical Languages Declarative Relational calculus, rule-based queries (e.g., Datalog) Procedural Relational algebra Practical Languages Mix of both, but most used are declarative features We will see examples of queries in various query languages

5 Examples of queries (1) Q: Find the name of customers living in Edinburgh We can formulate queries as rules answer(name) Customer(id,name, Edinburgh,addr) While (id,name,city,addr) ranges over the relation Customer, check whether city is Edinburgh and, if yes, output name The above query can be written also as answer(name) Customer(id,name,city,addr), city= Edinburgh Queries like these are called conjunctive queries (we will see why later on) Examples of queries (2) Q: Find the name of customers whose account is overdrawn answer(name) Customer(id,name,city,addr), Account(num,branch,id,bal), bal < 0 While (id,name,city,addr) ranges over Customer and (num,branch,id,bal) ranges over Account, where id is the same in both tuples, check whether bal is less than zero and, if yes, output name Here, we are joining data in Customer with data in Account (by using the same variable id) to retrieve the account data associated with the corresponding customer This is the most common type of queries one asks

6 Examples of queries (3) Q: Find pairs of distinct customers living together We can join a relation with itself by taking new variables and equating those of interest answer(n1,n2) Customer(id1,n1,city1,addr1), Customer(id2,n2,city2,addr2), city1=city2, addr1=addr2, id1 id2, n1 n2 The above rule is equivalent to the following, more succint one: answer(n1,n2) Customer(id1,n1,city,addr), Customer(id2,n2,city,addr), id1 id2, n1 n2 A more complicated example Q: Find customers who have an account in every branch To express every one needs the universal quantifier We use the notation of mathematical logic: { name (num,br,id,bal) Account (id1,name,city,addr,num1,bal1): (id1,name,city,addr) Customer (num1,br,id1,bal1) Account } To see if a customer s name is in the answer, for each branch br check that there is a customer with that name who owns an account in that branch Reminder means for all, means exists is conjunction (logical AND)

7 SQL: Structured Query Language Often pronounced sequel Developed initially at IBM in the 70s Standards: SQL-86, SQL-89, SQL-92 (SQL2), SQL:1999 (SQL3), SQL:2003, SQL-2006, SQL:2008, SQL:2011 The nice thing about standards is that you have so many to choose from. Andrew S. Tanenbaum De-facto standard in the world of relational databases Queries in SQL Follow the basic pattern: SELECT list, of, attributes FROM list, of, relations WHERE conditions SELECT says which attributes go to the query output FROM says from which relations to take tuples WHERE states the conditions for selecting a tuple Example Q: Find the name of customers living in Edinburgh SELECT Name FROM Customer WHERE City= Edinburgh

8 Queries in SQL Another example Q: Find the name of customers whose account is overdrawn SELECT Customer.Name FROM Customer, Account WHERE Account.CustID = Customer.CustID AND Account.Balance < 0 We now specify the relations attributes are coming from, because CustID appears in both Customer and Account Not necessary (redundant) for the other attributes, because there is no ambiguity SELECT Name FROM Customer, Account WHERE Account.CustID = Customer.CustID AND Balance < 0 Joins of relations SELECT Customer.Name, Account.Balance FROM Customer, Account WHERE Account.CustID = Customer.CustID AND Account.Balance < 0 WHERE allows one to join data from different relations Semantics: nested loop over the relations listed in FROM for (id1,name,city,addr) in Customer: for (num,branch,id2,bal) in Account: if id1==id2 and bal < 0: output (name,bal) The join is one of the most fundamental operations in db queries (we will see more examples throughout the course)