Overview of Computer Science

Transcription

1 Overview of Computer Science CSC Spring 2013 Database Systems April 15

2 What s a database system? Database: Organized body of related information Database management system (DBMS): A software system that facilitates the creation, querying, update, and administration of an electronic database

3 Database Examples Bank database Account: number, type, owner, balance, etc. University database Student: id, name, birthday, local address, etc. Course: CRN, title, time, instructor, etc. Facebook Google Twitter What type of data to they store?

4 DBMS Examples Commercial DBMS Oracle Microsoft SQL IBM DB2 Sybase PostgreSQL Open Source MySQL Most widely used system, including users such as Twitter and Facebook

5 DBMS Features Data queries, e.g. Balance in your bank account? Courses taught between 9:30 and 10:45? Data updates, e.g. Withdraw $50 from your account Add students to a particular course Data definition, e.g. Define, remove, modify data structures Administration, e.g. User accounts, data security, performance, etc.

6 DBMS Features Data persistency and integrity Over entire life-cycle of data Important for most applications, including data warehousing and business intelligence Logical data model Independence from physical data storage Declarative queries and updates user specifies what answers a query must have DBMS chooses best strategy for answering

7 Relational Data Model A database is a collection of relations (or tables) A relation has a list of attributes (or columns) A relation contains a set of tuples (or rows) duplicates not allowed

8 Examples Students ID Name Birthday GPA 9886 Holly 1/1/ Eddie 7/4/ John 3/22/ Enroll Courses CRN Title CSC Overview of CS CSC Intro to CS MTH Multivariate Calc REL World religions ID CRN

9 Schemas and Instances Schema Metadata: specification of the structure of a table Defined once Does not change later on Instances Actual content conforming to scheme Can change often

10 Examples Schema Student (ID integer, Name string, Birthday date, GPA float) Course (CRN integer, Title string) Enroll (ID integer, CRN integer) Instance { {9886, Holly, 1/1/94, 2.9}, {1234, Eddie, 7/4/93, 3.1},...} { {12134, CSC 101-Overview of CS},...} { {9886,12134},{9886, 65342},...}

11 Predicates Relational algebra: σp(r), p - predicate Rel Op Examples σgpa>3(student): tuples where gpa > 3 in Student table σid=1234(enroll): tuples where ID=1234 in Enroll table Must be able to evaluate a predicate over a single

12 Table Operations Projection: πl(r) Given table and list of columns L, output table with same number rows but with only columns in L Cross product: RxS Given two tables R and S, for each row r in R and each row s in S, output table with row rs Join: R ps Given two tables R and S, create a union of all attributes of R and S, without duplications Union: R S Difference: R-S

13 Basic MySQL Create a database mysql> create database mytable; Use a database mysql> use mytable; Create Student table mysql> create table student(id int, name varchar(40), birthday date, gpa float); Insert data mysql> insert into student values(9886, Holly, , 2.9);

14 Basic MySQL Creating queries mysql> select * from student; mysql> select gpa from student; mysql> select * from student where gpa >= 3; Update a table mysql> update student set gpa = '3.1' where name = Holly ;

15 Database Design How to design a database schema to support a variety of queries? Not a trivial problem The design limits the types of data queries Various models: entity/relationship model, objectoriented design, etc.

16 Entity/Relationship Model Diagram based modeling Entity (rectangle): A collection of things of the same type Relationship (diamond): An association between entities Attribute (oval): Property of an entity or relationship

17 Entity/Relationship Model Diagram based modeling Entity (rectangle): Relationship (diamond): Attribute (oval): ID Name CRN Birthday Students Enroll Courses Title GPA

18 Entity/Relationship Model ID Name CRN Birthday Students Enroll Courses Title GPA Need to add a grades attribute, where? Students? Course? Enroll?

19 Entity/Relationship Model ID Name CRN Birthday Students Enroll Courses Title GPA Grade Need to add a grades attribute, where? Students? different grades for multiple courses Course? different grade for multiple students Enroll? yes!

20 Databases and the Web Enable dynamic webpages Server DBMS such as MySQL Access through server-side PHP scripts Client HTTP or Javascript code calls PHP Image courtesy of Wikipedia