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DBMS Unit 2 ppt.ppt

The document defines various entity-related concepts in database management systems (DBMS). It states that an entity can represent real-world objects like professors or students in a college database. Entities have attributes that describe their properties. It distinguishes between strong and weak entities, where strong entities have primary keys and weak entities depend on strong entities. It also defines entity sets, attributes, different types of keys like primary keys, foreign keys and composite keys.

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Entity ļ‚— Entity in DBMS can be a real-world object with an existence. ļ‚— For example, in a College database, the entities can be Professor, Students, Courses, etc. ļ‚— Entities has attributes, which can be considered as properties describing it, for example, for Professor entity, the attributes are Professor_Name, Professor_Address, Professor_Salary, etc. The attribute value gets stored in the database.
Example of Entity in DBMS <Professor> ļ‚— Here, Professor_Name, Professor _Address and Professor _Salary are attributes. Professor_ID is the primary key Professor_ID Professor_Name Professor_City Professor_Salary P01 Tom Sydney $7000 P02 David Brisbane $4500 P03 Mark Perth $5000
Types of Entities ļ‚— strong entity ļ‚— The strong entity has a primary key. ļ‚— Weak entities are dependent on strong entity.
Strong Entity ļ‚— The strong entity has a primary key. ļ‚— Strong Entity is represented by a single rectangle āˆ’ ļ‚— Continuing our previous example, Professor is a strong entity here, and the primary key is Professor_ID. Strong Entity
Weak Entity ļ‚— The weak entity in DBMS do not have a primary key and are dependent on the parent entity. It mainly depends on other entities. ļ‚— Weak Entity is represented by double rectangle āˆ’ Weak entity Continuing our previous example, Professor is a strong entity, and the primary key is Professor_ID. However, another entity is Professor_Dependents, which is our Weak Entity.
Example of Strong and Weak Entity ļ‚— The example of strong and weak entity can be understood by the below figure. ļ‚— The Strong Entity is Professor, whereas Dependent is a Weak Entity. ļ‚— ID is the primary key (represented with a line) and Name in Dependent entity is called Partial Key (represented with a dotted line).
Entity Set ļ‚— An entity set is a group of similar entities and these entities can have attributes. ļ‚— In terms of DBMS, an entity is a table or attribute of a table in database, so by showing relationship among tables and their attributes. ļ‚— ER diagram shows the complete logical structure of a database.
Types of Entity Set Entity set can be classified into two categories as shown below: ļ‚—Strong Entity Set ļ‚—Weak Entity Set
Strong Entity Set ļ‚— An entity set that has a primary key using which, entities in the table can be uniquely identified. This kind of entity set is termed as a strong entity set. Strong entity set is also known as a regular entity set. ļ‚— In an ER diagram, the strong entity set is represented by the rectangle. Here, the primary key is underlined with the solid line.
Weak Entity Sets ļ‚— A weak entity set doesnā€™t have any primary key which can identify each entity in a set distinctly. But, for discriminating the entities in a set, the weak entity set is dependent on a particular strong entity set. ļ‚— A weak entity is also said to be existence dependent as for the existence of its entities it has to be dependent on identifying entity set i.e. a particular ā€˜strong entity setā€™. The relation between a weak entity set and a strong entity set is said to be identifying relationship. ļ‚—
Attributes ļ‚— An attribute is a property or characteristic of an entity. An entity may contain any number of attributes. One of the attributes is considered as the primary key. In an Entity-Relation model, attributes are represented in an elliptical shape. ļ‚— Example: Student has attributes like name, age, roll number and many more. To uniquely identify the student, we use the primary key as roll number as it is not repeated. Attributes can also be subdivided into another set of attributes.
Types of Attributes
1. Simple Attributes- ļ‚— Simple attributes are those attributes which can not be divided further. ļ‚— Here, all the attributes are simple attributes as they can not be divided further. ļ‚— Example
2. Composite Attributes- ļ‚— Composite attributes are those attributes which are composed of many other simple attributes. ļ‚— Here, the attributes ā€œNameā€ and ā€œAddressā€ are composite attributes as they are composed of many other simple attributes. ļ‚— Example-
3. Single Valued Attributes- ļ‚— Single valued attributes are those attributes which can take only one value for a given entity from an entity set. ļ‚— Here, all the attributes are single valued attributes as they can take only one specific value for each entity. ļ‚— Example
4. Multi Valued Attributes ļ‚— Multi valued attributes are those attributes which can take more than one value for a given entity from an entity set. ļ‚— Here, the attributes ā€œMob_noā€ and ā€œEmail_idā€ are multi valued attributes as they can take more than one values for a given entity. ļ‚— Example
5. Stored attribute ļ‚— Stored attribute is an attribute which are physically stored in the database. Assume a table called as student. ... We cannot derive value of these attribute using other attributes. So, these attributes are called as stored attribute. ļ‚— Example
5. Derived Attributes ļ‚— Derived attributes are those attributes which can be derived from other attribute(s). ļ‚— Here, the attribute ā€œAgeā€ is a derived attribute as it can be derived from the attribute ā€œDOBā€. ļ‚— Example
6. Null Value Attribute ļ‚— An attribute, which has not any value for an entity is known as null valued attribute. For example, assume Student is an entity and its attributes are Name, Age, Address and Phone no. There may be chance when a student has no phone no. In that case, phone no is called null valued attributes. ļ‚— Example ļ‚— Middle Name ļ‚— E-Mail ID
7. Key Attributes ļ‚— Key attributes are those attributes which can identify an entity uniquely in an entity set. ļ‚— Here, the attribute ā€œRoll_noā€ is a key attribute as it can identify any student uniquely. ļ‚— Example
8. Complex attributes ļ‚— The attributes that are formed by arbitrarily nesting the composite and multivalued attributes are called complex attributes.
Keys in DBMS ļ‚— A key is a set of attributes that can identify each tuple uniquely in the given relation.
Different Types Of Keys in DBMS
ļ‚— A super key is a set of attributes that can identify each tuple(row) uniquely in the given relation(table). ļ‚— A super key is not restricted to have any specific number of attributes. ļ‚— Thus, a super key may consist of any number of attributes. 1. Super Key
ļ‚— Example ļ‚— Consider the following Student schema- ļ‚— Student ( roll , name , age , address , class , section ) ļ‚— Given below are the examples of super keys since each set can uniquely identify each student in the Student table- ļ‚— ( roll , name , age , address , class , section ) ļ‚— ( class , section , roll ) ļ‚— (class , section , roll ) ļ‚— ( name , address )
2. Candidate Key ļ‚— A minimal super key is called as a candidate key. OR ļ‚— A set of minimal attribute(s) that can identify each tuple uniquely in the given relation is called as a candidate key. ļ‚— Example- ļ‚— Consider the following Student schema- ļ‚— Student ( roll , name , age , address , class , section ) ļ‚— Given below are the examples of candidate keys since each set consists of minimal attributes required to identify each student uniquely in the Student table- ļ‚— ( class , section , roll ) ļ‚— ( name , address )
NOTES ļ‚— All the attributes in a candidate key are sufficient as well as necessary to identify each tuple uniquely. ļ‚— Removing any attribute from the candidate key fails in identifying each tuple uniquely. ļ‚— The value of candidate key must always be unique. ļ‚— The value of candidate key can never be NULL. ļ‚— It is possible to have multiple candidate keys in a relation.
3. Primary Key ļ‚— A primary key is a candidate key that the database designer selects while designing the database. OR ļ‚— Candidate key that the database designer implements is called as a primary key.
NOTES ļ‚— The value of primary key can never be NULL. ļ‚— The value of primary key must always be unique. ļ‚— The values of primary key can never be changed i.e. no updation is possible. ļ‚— The value of primary key must be assigned when inserting a record. ļ‚— A relation is allowed to have only one primary key.
4. Alternate Key ļ‚— Candidate keys that are left unimplemented or unused after implementing the primary key are called as alternate keys. OR ļ‚— Unimplemented candidate keys are called as alternate keys.
5. Foreign Key ļ‚— An attribute ā€˜Xā€™ is called as a foreign key to some other attribute ā€˜Yā€™ when its values are dependent on the values of attribute ā€˜Yā€™. ļ‚— The attribute ā€˜Xā€™ can assume only those values which are assumed by the attribute ā€˜Yā€™. ļ‚— Here, the relation in which attribute ā€˜Yā€™ is present is called as the referenced relation. ļ‚— The relation in which attribute ā€˜Xā€™ is present is called as the referencing relation. ļ‚— The attribute ā€˜Yā€™ might be present in the same table or in some other table.
NOTES ļ‚— Foreign key references the primary key of the table. ļ‚— Foreign key can take only those values which are present in the primary key of the referenced relation. ļ‚— Foreign key may have a name other than that of a primary key. ļ‚— Foreign key can take the NULL value. ļ‚— There is no restriction on a foreign key to be unique. ļ‚— In fact, foreign key is not unique most of the time. ļ‚— Referenced relation may also be called as the master table or primary table. ļ‚— Referencing relation may also be called as the foreign table.
6. Partial Key ļ‚— Partial key is a key using which all the records of the table can not be identified uniquely. ļ‚— However, a bunch of related tuples can be selected from the table using the partial key.
Example ļ‚— Consider the following schema- ļ‚— Department ( Emp_no , Dependent_name , Relation ) ļ‚— Here, using partial key Emp_no, we can not identify a tuple uniquely but we can select a bunch of tuples from the table.
7. Composite Key ļ‚— A primary key comprising of multiple attributes and not just a single attribute is called as a composite key.
8. Unique Key ļ‚— Unique key is a key with the following properties- ļ‚— It is unique for all the records of the table. ļ‚— Once assigned, its value can not be changed i.e. it is non- updatable. ļ‚— It may have a NULL value.
Example ļ‚— The best example of unique key is Adhaar Card Numbers. ļ‚— The Adhaar Card Number is unique for all the citizens (tuples) of India (table). ļ‚— If it gets lost and another duplicate copy is issued, then the duplicate copy always has the same number as before. ļ‚— Thus, it is non-updatable. ļ‚— Few citizens may not have got their Adhaar cards, so for them its value is NULL.
9. Surrogate Key ļ‚— Surrogate key is a key with the following properties- ļ‚— It is unique for all the records of the table. ļ‚— It is updatable. ļ‚— It can not be NULL i.e. it must have some value. ļ‚— Example- ļ‚— Mobile Number of students in a class where every student owns a mobile phone.
10. Secondary Key ļ‚— Secondary key is required for the indexing purpose for better and faster searching.
Relationships ļ‚— Any association between two entity types is called a relationship. Entities take part in the relationship. It is represented by a diamond shape. ļ‚— For example, A teacher teaches students. Here, "teaches" is a relationship and this is the relationship between a Teacher entity and a Student entity.
Degree of relationship type: ļ‚— The number of entity sets that participate in a relationship set. ļ‚— The degree of a relationship is the number of entity types that participate(associate) in a relationship. ļ‚— E-R diagram, we can simply tell the degree of a relationship i.e the number of an entity type that is connected to a relationship is the degree of that relationship.
Unary Relationship ļ‚— Unary relationship exists when an association is maintained with a single entity.
Binary Relationship: ļ‚—A binary relationship exists when two entities are associated.
Ternary Relationship ļ‚— A ternary relationship exists when there are three entities associated.
Constraints on Relationship Types: ļ‚— Relationship types usually have certain constraints that limit the possible combination of entity that may participate in the relationship instance. ļ‚— Ex: If the company has a rule that each employee must work for exactly one department. The two main types of constraints are cardinality ration and participation constraints.
One-to-One: ļ‚— An entity in A is associated with at most one entity in Band vice versa.
One-to-Many: ļ‚— An entity of entity set A is associated with two or more entities of entity set B. However, each and every entity in entity set B can be associated with almost one entity in entity set A.
Many-to-One: ļ‚— An entity of entity set A is associated with almost one entity of entity set B. However, an entity of entity set B can be associated with two or more entities of entity set A.
Many-to-Many: ļ‚— An entity of entity set A is associated with two or more entities of entity set B. Similarly, an entity of entity set B is associated with two or more entities of entity set A.
Different notations used for ER Diagram
An ER diagram to show student entity details:
An ER diagram to show employee details:
An ER diagram to show employee details:
An ER diagram to show company database:
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