Databases - Unit 2.pdf
- 2. UNITTWO – DATA MODELLING: ENTITY RELATIONSHIP DIAGRAM •Relational Model •Entity relationship diagram
- 3. RELATIONAL MODEL • The relational data model was introduced by C. F. Codd in 1970. Currently, it is the most widely used data model. The relational model has provided the basis for: • Research on the theory of data/relationship/constraint • Numerous database design methodologies • The standard database access language called Structured Query Language (SQL) • Almost all modern commercial database management systems The relational data model describes the world as “a collection of inter-related relations (or tables).
- 4. FUNDAMENTAL CONCEPT OF RELATIONAL MODEL • Relation A relation, also known as a table or file, is a subset of the Cartesian product of a list of domains characterized by a name. And within a table, each row represents a group of related data values. A row, or record, is also known as a tuple. The columns in a table is a field and is also referred to as an attribute. • Table A database is composed of multiple tables and each table holds the data.
- 5. FUNDAMENTAL CONCEPT OF RELATIONAL MODEL conti.
- 6. FUNDAMENTAL CONCEPT OF RELATIONAL MODEL cont. • Column A database stores pieces of information or facts in an organized way. The principal storage units are called columns or fields or attributes . These house the basic components of data into which your content can be broken down. • Records Records contain fields that are related, such as a customer or an employee. As noted earlier, a tuple is another term used for record.
- 7. FUNDAMENTAL CONCEPT OF RELATIONAL MODEL cont. • Properties of aTable • A table has a name that is distinct from all other tables in the database. •There are no duplicate rows; each row is distinct. • Entries in columns are atomic. The table does not contain repeating groups or multivalued attributes. • Entries from columns are from the same domain based on their data type including: number (numeric, integer, float, smallint,…) character (string) date logical (true or false) • Operations combining different data types are disallowed. •Each attribute has a distinct name.
- 8. FUNDAMENTAL CONCEPT OF RELATIONAL MODEL cont. • Degree The degree is the number of attributes in a table. • Domain Domain is a set of acceptable values that a column is allowed to contain. This is based on various properties and the data type for the column. For example: • The domain of Marital Status has a set of possibilities: Married, Single, Divorced. •The domain of Shift has the set of all possible days: {Mon,Tue,Wed…}. •The domain of Salary is the set of all integer greater than 0 and less than 244. This simple means, the Marital status column will not accept the “attached” as data since it is not part of the acceptable data set.
- 9. ENTITY RELATIONAL MODEL • The entity relationship (ER) data model has existed for over 35 years. It is well suited to data modelling for use with databases because it is fairly abstract and is easy to discuss and explain. ER models are readily translated to relations. ER models, also called an ER schema, are represented by ER diagrams. ER modelling is based on three concepts: • Entity • Attribute • Relationship
- 10. ENTITY, ENTITYTYPE AND ENTITY SET •An entity is an object in the real world with an independent existence that can be differentiated from other objects. An entity might be: •An object with physical existence (e.g., a lecturer, a student, a car) •An object with conceptual existence (e.g., a course, a job, a position)
- 11. ENTITY, ENTITYTYPE AND ENTITY SET cont.
- 12. ENTITY, ENTITYTYPE AND ENTITY SET cont. •EntityType The entity type is a collection of the entity with similar attributes. In the student table for example, we have each row as an entity and they all have common attributes.
- 13. ENTITY, ENTITYTYPE AND ENTITY SET cont. Student E-R representation of Student entity type. NB: We use rectangle to represent an entity type on E-R diagram
- 14. ENTITY, ENTITYTYPE AND ENTITY SET cont. • Types of Entity type 1.Strong entity type 2.Weak entity type Strong entity type (s) are entity types with key attribute.The primary key helps in identifying the strong entity type. Weak entity type(s) cannot be identified separately.They depend on other strong entity for their identity.
- 15. ENTITY, ENTITYTYPE AND ENTITY SET cont.
- 16. ENTITY, ENTITYTYPE AND ENTITY SET cont.
- 17. ENTITY, ENTITYTYPE AND ENTITY SET cont. •Entity set Entity set is a collection of entities of the same entity type at a any point in time.
- 18. ENTITY, ENTITYTYPE AND ENTITY SET cont. • Existence dependency An entity’s existence is dependent on the existence of the related entity. It is existence-dependent if it has a mandatory foreign key (i.e., a foreign key attribute that cannot be null). For example, in a COMPANY database, a Spouse entity is existence -dependent on the Employee entity. Existence dependency
- 19. INDEPENDENT AND DEPENDENT ENTITIES • Independent entity does not rely on another entity for identification. • Dependent entity relies on another entity for identification. Independent entity Dependent entity
- 20. ATTRIBUTES • Each entity is described by a set of attributes (e.g., Employee = (Name, Address, Birthdate (Age), Salary). Each attribute has a name, and is associated with an entity. In an entity relationship diagram, attribute is represented by an oval with a name inside. Name Attribute represented on E-R diagram
- 21. TYPES OF ATTRIBUTES • Simple attributes Simple attributes are those drawn from the atomic value domains; they are also called single-valued attributes. In the STUDENT database, an example of this would be: Age = {23}.
- 22. TYPES OF ATTRIBUTES • Composite attributes Composite attributes can be divided into subparts. Using our database example, Address may consist of Number, Street and Suburb; Name = Firstname and Lastname. Student Name Address Birthdate Number Street Suburb Firstname Lastname
- 23. salary TYPES OF ATTRIBUTES • Multivalued attributes Multivalued attributes are attributes that have a set of values for each entity. An example of a multivalued attribute from the Employee table below, are the degrees of an employee: BSc, MIT, PhD. It is important to note that, to represent multivalued attributes on relational schema, new tables are created. Employee Name Address salary Degrees Degrees NB: Double ovals are used to represent Multivalued attributes on E-R diagram
- 24. • Derived attributes Derived attributes are attributes that contain values calculated from other attributes. An example of this can be seen the E-R diagram below. Age can be derived from the attribute Birthdate. In this situation, Birthdate is called a stored attribute, which is physically saved to the database. TYPES OF ATTRIBUTES Employee Name Address Birthdate Age Age NB:Dotted oval with name inside represent derived attributes on E-R diagram
- 25. •A key is an attribute or a set of attributes in a relation that identifies a tuple (record) in a relation. •Keys are defined in a table to allow access or storage of data quickly. •They are also used to create relationship between tables. KEYS
- 26. •Primary key •Candidate key •Alternate key •Super key •Composite key •Foreign key •Unique key TYPES OF KEYS
- 27. •Primary key (PK): It helps to identify every record present in a table uniquely. There can be only one primary key in a table. Every value of a primary key must different with no repetition. No two row can have the same primary key.The primary Key field cannot be null. A null value is either unknown or not application. It does not mean blank. TYPES OF KEYS
- 28. •Candidate keys : Keys with attributes that can uniquely identify rows of a table are called candidate keys.There can be more than two candidate keys from a table but they can not be empty or null.The primary is selected from the candidate keys. TYPES OF KEYS
- 29. •Alternate keys : As stated, a table can have multiple choice of a primary key, however, it can choose only one. So, all the keys which did not become the primary key are called alternate keys. TYPES OF KEYS
- 30. •Super key : It is set of attributes that uniquely identifies each tuple of a relation. Super key is the superset of a candidate Key.The primary key of a table is picked from super key set. TYPES OF KEYS
- 31. •Composite key : If a table not have a single attribute that qualifies for a Candidate key, then you have to select two or more columns to make a row Unique. So, EmployeeName+Date of Birth are used as primary key. TYPES OF KEYS
- 32. •Foreign key : It is column of a table that points to the primary key of another table. A foreign key is used to link to tables. It helps to maintain data and referential integrity.The DeptID on employee table is a foreign key. TYPES OF KEYS
- 33. •Unique key :It is same as primary with the difference being the existence of null. Unique key field allows one value as NULL value. TYPES OF KEYS
- 34. It is the association between entities or tables. We will discuss only six relationships in relational model: •One to one relationship (1:1) •One to many or many to one relationship (1:M) •Many to many relationship (M:N) •Unary relationship (Recursive) •Binary relationship •Ternary relationship NB: Diamond symbol is used to represent relationship in an E-R diagram. RELATIONSHIPS Works
- 35. •One to One relationship (1:1): It is used to create a relationship between two tables in which single row of the first table can only be related to one and only one record of the second table. TYPES OF RELATIONSHIPS
- 37. •One to Many relationship (1:M): It is used to create a relationship between two tables. Any single row of the first table can be related to one or more rows of the second table. Meanwhile, the rows of the second table can relate to only one row in the first table. TYPES OF RELATIONSHIPS
- 39. •Many to Many relationship (M:N): In M:N relationship, each record in the first table can relate to more than one record in the second table. Likewise, a record in the second table can relate more than one record in the first table. TYPES OF RELATIONSHIPS
- 40. TYPES OF RELATIONSHIPS • Unary relationship: It describes a relationship in which a table is in relationship with itself. One example is an employee database that keeps track of which employee supervises other employees.
- 41. TYPES OF RELATIONSHIPS • Binary relationship: It describes an association between two entities.
- 42. TYPES OF RELATIONSHIPS • Ternary relationship: It describes an association among three entities.This type of relationship is required when binary relationship is not sufficient to describe the relationshipl