Unit02 dbms

INDEX UNIT-2 PPT SLIDES S.NO Module as per Lecture PPT Session planner No Slide NO ------------------------------------------------------------------------------------------ History of Database Systems L1 L1- 1 to L1- 10 2. DB design and ER diagrams L2 L2- 1 to L2- 10 3. Relationships & sets L3 L3- 1 to L3- 5 Addn features of the ER model L4 L4- 1 to L4- 7 Addn features of the ER model L5 L5- 1 to L5- 6 6. Conceptual design with ER model L6 L6- 1 to L6 -6 7. Large enterprises L7 L7- 1 to L7- 3

History of Database Systems 1950s and early 1960s: Data processing using magnetic tapes for storage Tapes provide only sequential access Punched cards for input Late 1960s and 1970s: Hard disks allow direct access to data Network and hierarchical data models in widespread use Ted Codd defines the relational data model Would win the ACM Turing Award for this work IBM Research begins System R prototype UC Berkeley begins Ingres prototype High-performance (for the era) transaction processing

Magnetic tape unit Magnetic tape Hard disk

History (cont.) 1980s: Research relational prototypes evolve into commercial systems SQL becomes industry standard Parallel and distributed database systems Object-oriented database systems 1990s: Large decision support and data-mining applications Large multi-terabyte data warehouses Emergence of Web commerce 2000s: XML and XQuery standards Automated database administration Increasing use of highly parallel database systems Web-scale distributed data storage systems

Database Design Conceptual design : ( ER Model is used at this stage.) What are the entities and relationships in the enterprise? What information about these entities and relationships should we store in the database? What are the integrity constraints or business rules that hold? A database `schema’ in the ER Model can be represented pictorially ( ER diagrams ). Can map an ER diagram into a relational schema.

Modeling A database can be modeled as: a collection of entities, relationship among entities. An entity is an object that exists and is distinguishable from other objects. Example: specific person, company, event, plant Entities have attributes Example: people have names and addresses An entity set is a set of entities of the same type that share the same properties. Example: set of all persons, companies, trees, holidays

Entity Sets customer and loan customer_id customer_ customer_ customer_ loan_ amount name street city number

Attributes An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set. Domain – the set of permitted values for each attribute Attribute types: Simple and composite attributes. Single-valued and multi-valued attributes Example: multivalued attribute: phone_numbers Derived attributes Can be computed from other attributes Example: age, given date_of_birth Example: customer = ( customer_id, customer_name, customer_street, customer_city ) loan = ( loan_number, amount )

Mapping Cardinality Constraints Express the number of entities to which another entity can be associated via a relationship set. Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types: One to one One to many Many to one Many to many

Mapping Cardinalities One to one One to many Note: Some elements in A and B may not be mapped to any elements in the other set

Mapping Cardinalities Many to one Many to many Note: Some elements in A and B may not be mapped to any elements in the other set

ER Model Basics Entity : Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes . Entity Set : A collection of similar entities. E.g., all employees. All entities in an entity set have the same set of attributes. (Until we consider ISA hierarchies, anyway!) Each entity set has a key . Each attribute has a domain . Employees ssn name lot

ER Model Basics (Contd.) Relationship : Association among two or more entities. E.g., Attishoo works in Pharmacy department. Relationship Set : Collection of similar relationships. An n-ary relationship set R relates n entity sets E1 ... En; each relationship in R involves entities e1 E1, ..., en En Same entity set could participate in different relationship sets, or in different “roles” in same set. lot dname budget did since name Works_In Departments Employees ssn Reports_To lot name Employees subordinate super-visor ssn

Relationship Sets A relationship is an association among several entities Example: Hayes depositor A-102 customer entity relationship set account entity A relationship set is a mathematical relation among n  2 entities, each taken from entity sets {( e 1 , e 2 , … e n ) | e 1  E 1 , e 2  E 2 , …, e n  E n } where ( e 1 , e 2 , …, e n ) is a relationship Example: (Hayes, A-102)  depositor

Relationship Sets (Cont.) An attribute can also be property of a relationship set. For instance, the depositor relationship set between entity sets customer and account may have the attribute access-date

Degree of a Relationship Set Refers to number of entity sets that participate in a relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. Relationship sets may involve more than two entity sets.

Degree of a Relationship Set Example: Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job, and branch Relationships between more than two entity sets are rare. Most relationships are binary. (More on this later.)

Key Constraints Consider Works_In: An employee can work in many departments; a dept can have many employees. In contrast, each dept has at most one manager, according to the key constraint on Manages. Many-to-Many 1-to-1 1-to Many Many-to-1 budget did Departments Additional features of the ER model dname since lot name ssn Manages Employees

Participation Constraints Does every department have a manager? If so, this is a participation constraint : the participation of Departments in Manages is said to be total (vs. partial ). Every Departments entity must appear in an instance of the Manages relationship. lot name dname budget did since name dname budget did since Manages since Departments Employees ssn Works_In

Weak Entities A weak entity can be identified uniquely only by considering the primary key of another ( owner ) entity. Owner entity set and weak entity set must participate in a one-to-many relationship set (one owner, many weak entities). Weak entity set must have total participation in this identifying relationship set. lot name age pname Dependents Employees ssn Policy cost

Weak Entity Sets An entity set that does not have a primary key is referred to as a weak entity set . The existence of a weak entity set depends on the existence of a identifying entity set it must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set Identifying relationship depicted using a double diamond The discriminator ( or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set. The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator.

Weak Entity Sets (Cont.) We depict a weak entity set by double rectangles. We underline the discriminator of a weak entity set with a dashed line. payment_number – discriminator of the payment entity set Primary key for payment – ( loan_number, payment_number )

Weak Entity Sets (Cont.) Note: the primary key of the strong entity set is not explicitly stored with the weak entity set, since it is implicit in the identifying relationship. If loan_number were explicitly stored, payment could be made a strong entity, but then the relationship between payment and loan would be duplicated by an implicit relationship defined by the attribute loan_number common to payment and loan

More Weak Entity Set Examples In a university, a course is a strong entity and a course_offering can be modeled as a weak entity The discriminator of course_offering would be semester (including year) and section_number (if there is more than one section) If we model course_offering as a strong entity we would model course_number as an attribute. Then the relationship with course would be implicit in the course_number attribute

ISA (`is a’) Hierarchies Overlap constraints : Can Joe be an Hourly_Emps as well as a Contract_Emps entity? ( Allowed/disallowed ) Covering constraints : Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Reasons for using ISA: To add descriptive attributes specific to a subclass. To identify entitities that participate in a relationship. Contract_Emps name ssn Employees lot hourly_wages ISA Hourly_Emps contractid hours_worked As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity.

Aggregation Used when we have to model a relationship involving (entitity sets and) a relationship set . Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships. Aggregation vs. ternary relationship : Monitors is a distinct relationship, with a descriptive attribute. Also, can say that each sponsorship is monitored by at most one employee . budget did pid started_on pbudget dname until Departments Projects Sponsors Monitors lot name ssn since Employees

Aggregation Consider the ternary relationship works_on , which we saw earlier Suppose we want to record managers for tasks performed by an employee at a branch

Aggregation (Cont.) Relationship sets works_on and manages represent overlapping information Every manages relationship corresponds to a works_on relationship However, some works_on relationships may not correspond to any manages relationships So we can’t discard the works_on relationship Eliminate this redundancy via aggregation Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity

Aggregation (Cont.) Eliminate this redundancy via aggregation Treat relationship as an abstract entity Allows relationships between relationships Abstraction of relationship into new entity Without introducing redundancy, the following diagram represents: An employee works on a particular job at a particular branch An employee, branch, job combination may have an associated manager

Conceptual Design Using the ER Model Design choices: Should a concept be modeled as an entity or an attribute? Should a concept be modeled as an entity or a relationship? Identifying relationships: Binary or ternary? Aggregation? Constraints in the ER Model: A lot of data semantics can (and should) be captured. But some constraints cannot be captured in ER diagrams.

Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? Depends upon the use we want to make of address information, and the semantics of the data: If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic).

Entity vs. Attribute (Contd.) Works_In4 does not allow an employee to work in a department for two or more periods. Similar to the problem of wanting to record several addresses for an employee: We want to record several values of the descriptive attributes for each instance of this relationship. Accomplished by introducing new entity set, Duration. Works_In4 from to budget Departments name Departments ssn lot Employees Works_In4 name Employees ssn lot dname did dname budget did Duration from to

Entity vs. Relationship First ER diagram OK if a manager gets a separate discretionary budget for each dept. What if a manager gets a discretionary budget that covers all managed depts? Redundancy : dbudget stored for each dept managed by manager. Misleading: Suggests dbudget associated with department-mgr combination. Manages2 name dname budget did Employees Departments ssn lot dbudget since dname budget did Departments Manages2 Employees name ssn lot since Managers dbudget ISA This fixes the problem!

Binary vs. Ternary Relationships If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, first diagram is inaccurate. What are the additional constraints in the 2nd diagram? age pname Dependents Covers age pname Dependents Purchaser Bad design Better design name Employees ssn lot Policies policyid cost Beneficiary policyid cost Policies name Employees ssn lot

Binary vs. Ternary Relationships (Contd.) Previous example illustrated a case when two binary relationships were better than one ternary relationship. An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers , and has descriptive attribute qty . No combination of binary relationships is an adequate substitute: S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S. How do we record qty ?

Summary of Conceptual Design Conceptual design follows requirements analysis , Yields a high-level description of data to be stored ER model popular for conceptual design Constructs are expressive, close to the way people think about their applications. Basic constructs: entities , relationships , and attributes (of entities and relationships). Some additional constructs: weak entities , ISA hierarchies , and aggregation . Note: There are many variations on ER model.

Summary of ER (Contd.) Several kinds of integrity constraints can be expressed in the ER model: key constraints , participation constraints , and overlap/covering constraints for ISA hierarchies. Some foreign key constraints are also implicit in the definition of a relationship set. Some constraints (notably, functional dependencies ) cannot be expressed in the ER model. Constraints play an important role in determining the best database design for an enterprise.

Summary of ER (Contd.) ER design is subjective . There are often many ways to model a given scenario! Analyzing alternatives can be tricky, especially for a large enterprise. Common choices include: Entity vs. attribute, entity vs. relationship, binary or n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation. Ensuring good database design: resulting relational schema should be analyzed and refined further. FD information and normalization techniques are especially useful.

Unit02 dbms

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