Presentation on theme: "FEN 2014-02-061 Concepts and terminology Operations (relational algebra) Integrity constraints The relational model."— Presentation transcript:
FEN 2014-02-061 Concepts and terminology Operations (relational algebra) Integrity constraints The relational model
Relational Databases All data is organised in tables with atomic values Associations are represented by primary key/foreign key connections Every operation operates on tables and returns tables FEN 2014-02-062 Small exercise (5 min..): Describe step by step how you will retrieve this information: When did Mr Smith attend the “Intro to Computer Science” course, who was the instructor and what grade did he receive?
The Relational Model A sound theoretical data model (Codd, 1970). Based on the mathematical theory of relations, sets and first order predicate logic. De facto standard since the late eighties. Many-many implementations – most SQL-based. FEN 2014-02-063 The Notorious Supplier-Part Database (Date) For instance: Oracle MySQL MS SQL Server PostgreSQL For instance: Oracle MySQL MS SQL Server PostgreSQL
The Relational Model: Concepts FEN 2014-02-064 Central concepts: Tables (relations). Columns (attributes). Type (domain). Rows (tuples). Tuples are unordered. Tuples are unique. A relation is a set (mathematical) of tuples. Primary and foreign keys The Notorious Supplier-Part Database (Date)
The Relational Model Data is organised in a number of tables (relations). Each table has a number (>=1) columns (attributes). Attributes are atomic and defined over some domain. A table holds a number (maybe none) rows (tuples). Tuples are unordered. Tuples are unique (existence of a key is guaranteed). A relation is a set (mathematical) of tuples. FEN 2014-02-065
Attributes and Domains A domain defines the valid value of an attribute. Domains are based on the built-in standard data types (int, char etc.) offered by the DBMS. Theoretically it should be possible to define problem specific domains as CPR numbers, account numbers, IP addresses etc. and complex aggregate (structured) domain as maps, diagrams, pictures, sound bites, video clips etc. More attributes may be defined over the same domain. An attribute may have the value “empty” (not known /not defined for this instance). Empty is notated NULL. FEN 2014-02-066
Properties of a Relation Follows from the fact that relations are (mathematically) sets: Tuples must be unique within a relation (hence a primary key always exists) Tuples are unordered (vertically) Attributes are unordered (horizontally) Attribute values are atomic Note the difference to the usual notion of a table FEN 2014-02-067
Keys A key is a combination of attributes that is: Unique and Minimal An attribute combination that is unique, but not minimal is called a superkey The set of all attributes will always be a superkey, hence a superkey (and a key) always exists. A relation (table) may have several candidate keys. One these is appointed primary key. FEN 2014-02-068 Any primary keys here?
Associations Between Relations Associations are represented by foreign keys. A foreign key is an attribute (combination) that corresponds to an attribute (combination) of the primary key of some other relation. A foreign key references a tuple in another relation and indicates that here is more information about the entity. Foreign key attributes and corresponding primary key attributes must be defined over compatible domains (normallly the same domain). FEN 2014-02-069 Any foreign keys here?
Integrity Constraints FEN 2014-02-0610 Domain constraints Attributes may only hold valid values Entity Integrity Primary key attributes may not hold NULL-values Referential Integrity (foreign key constraint) A foreign key must either be NULL or reference an existing primary key in the other relation Semantic Integrity Constraints depending on the problem domain Any constraints here?
Example: MiniBank Two tables: Customers Accounts Associated: An account belongs to one customer FEN 2014-02-0611 Association Any constraints here? (primary keys – foreign keys) Any constraints here? (primary keys – foreign keys)
Example: MiniBank What happens if: We try to insert a customer with an existing custNo? We try to insert an account with a not existing custNo? Let’s try in MS SQL Server FEN 2014-02-0612
Quering a relational database Database Languages: Data Definition DDL Should provide constructs for defining all the previous (as “create table) Data Manipulation DML (queries, insert, delete, update) procedural (How?) nonprocedural (What?) The Relational Algebra is a procedural DML SQL includes a (sort of) nonprocedural DML FEN 2014-02-0614
The Relational Algebra Data Manipulation in the Relational Model Operates on relations, which are input to the operations is tables and the result is a table Operations Row selection (RESTRICT/SELECT) Column selection (PROJECT) Combining tables (JOIN) Set operations (UNION, INTERSECTION, DIFFERENCE, PRODUCT) More advanced operations (OUTER (LEFT/RIGTH) JOIN) FEN 2014-02-0615
Example: MiniBank Retrieve information about customer number 3: Row selection on custNo = 3 from Customer Retrieve account number, balance and customer number for accounts with a balance between 1000 and 2000: Row selection on 1000 <= balance and balance <= 2000 from Account Column selection on accNo, balance, custNo Retrieve information about customer Tommy and his accounts: Row selection on name = ‘Tommy’ from Customer Join with Account on custNo FEN 2014-02-0617 What if we have more than one customer called ‘Tommy’? And what if we have no ‘Tommy’? What if we have more than one customer called ‘Tommy’? And what if we have no ‘Tommy’?
18 Informal TermsFormal Terms TableRelation ColumnAttribute/Domain RowTuple Values in a columnDomain Table DefinitionSchema of a Relation Definitions - Summary FEN 2014-02-06