Coursehero >> Maryland >> Maple Springs >> CSE 3421

Course Hero has millions of student submitted documents similar to the one below including study guides, homework solutions, papers, exam answer keys and textbook solutions.

Information Course Introduction to Database Management Systems Four Assignments (7%, 3%, 10%, 10%) Mid-term Test (25%) Final Exam (45%) Course Web Site: www.cs.yorku.ca/course/3421 Late Assignment Submission: within 24 hours receives 20% penalty; no submission will be accepted after 24 hours of the due time Instructor: John Li johnli@cs.yorku.ca Database Management Systems 1 Database Management Systems 2 What Is a DBMS? A very large, integrated collection of data. Models real-world enterprise. Entities (e.g., students, courses) Relationships (e.g., Mark is taking COSC3421) Files vs. DBMS Application must stage large datasets between main memory and secondary storage (e.g., buffering, page-oriented access, 32-bit addressing, etc.) Special code for different queries Must protect data from inconsistency due to multiple concurrent users Crash recovery Security and access control 3 Database Management Systems 4 A Database Management System (DBMS) is a software package designed to store and manage databases. Database Management Systems Why Use a DBMS? Why Study Databases? Shift from computation to information at the low end : scramble to webspace (a mess!) at the high end : scientific applications Data independence and efficient access. Reduced application development time. Data integrity and security. Uniform data administration. Concurrent access, recovery from crashes. Datasets increasing in diversity and volume. Digital libraries, interactive video, Human Genome project, EOS project ... need for DBMS exploding DBMS encompasses most of CS OS, languages, theory, A I, multimedia, logic Database Management Systems 5 Database Management Systems 6 Data Models A data model is a collection of concepts for describing data. A schema is a description of a particular collection of data, using the a given data model. The relational model of data is the most widely used model today. Main concept: relation, basically a table with rows and columns. Every relation has a schema, which describes the columns, or fields. Database Management Systems 7 Levels of Abstraction Many views, single conceptual (logical) schema and physical schema. Views describe how users see the data. Conceptual schema defines logical structure Physical schema describes the files and indexes used. View 1 View 2 View 3 Conceptual Schema Physical Schema * Schemas are defined using DDL; data is modified/queried using DML. Database Management Systems 8 Example: University Database Conceptual schema: Students(sid: string, name: string, login: string, age: integer, gpa:real) Courses(cid: string, cname:string, credits:integer) Enrolled(sid:string, cid:string, grade:string) Data Independence * Applications insulated from how data is structured and stored. Logical data independence: Protection from changes in logical structure of data. Physical data independence: Protection from changes in physical structure of data. * One of the most important benefits of using a DBMS! 9 Database Management Systems 10 Physical schema: Relations stored as unordered files. Index on first column of Students. External Schema (View): Course_info(cid:string,enrollment:integer) Database Management Systems Concurrency Control Concurrent execution of user programs is essential for good DBMS performance. Because disk accesses are frequent, and relatively slow, it is important to keep the cpu humming by working on several user programs concurrently. Transaction: An Execution of a DB Program Key concept is transaction, which is an atomic sequence of database actions (reads/writes). Each transaction, executed completely, must leave the DB in a consistent state if DB is consistent when the transaction begins. Users can specify some simple integrity constraints on the data, and the DBMS will enforce these constraints. Beyond this, the DBMS does not really understand the semantics of the data. (e.g., it does not understand how the interest on a bank account is computed). Thus, ensuring that a transaction (run alone) preserves consistency is ultimately the user s responsibility! 11 Database Management Systems 12 Interleaving actions of different user programs can lead to inconsistency: e.g., check is cleared while account balance is being computed. DBMS ensures such problems don t arise: users can pretend they are using a single-user system. Database Management Systems Scheduling Concurrent Transactions DBMS ensures that execution of {T1, ... , Tn} is equivalent to some serial execution T1 ... Tn . Before reading/writing an object, a transaction requests a lock on the object, and waits till the DBMS gives it the lock. All locks are released at the end of the transaction. (Strict 2PL locking protocol.) Idea: If an action of Ti (say, writing X) affects Tj (which perhaps reads X), one of them, say Ti, will obtain the lock on X first and Tj is forced to wait until Ti completes; this effectively orders the transactions. What if Tj already has a lock on Y and Ti later requests a lock on Y? (Deadlock!) Ti or Tj is aborted and restarted! Database Management Systems 13 Ensuring Atomicity DBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a Xact. Idea: Keep a log (history) of all actions carried out by the DBMS while executing a set of Xacts: Before a change is made to the database, the corresponding log entry is forced to a safe location. (WAL protocol; OS support for this is often inadequate.) After a crash, the effects of partially executed transactions are undone using the log. (Thanks to WAL, if log entry wasn t saved before the crash, corresponding change was not applied to database!) Database Management Systems 14 The Log The following actions are recorded in the log: Ti writes an object: the old value and the new value. Log record must go to disk before the changed page! Databases make these folks happy ... End users and DBMS vendors DB application programmers E.g. smart webmasters Ti commits/aborts: a log record indicating this action. Log records chained together by Xact id, so it s easy to undo a specific Xact (e.g., to resolve a deadlock). Log is often duplexed and archived on stable storage. All log related activities (and in fact, all CC related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS. Database Management Systems 15 Database administrator (DBA) Designs logical /physical schemas Handles security and authorization Data availability, crash recovery Database tuning as needs evolve Must understand how a DBMS works! Database Management Systems 16 Structure of a DBMS These layers must consider concurrency control and recovery Summary DBMS used to maintain, query large datasets. Benefits include recovery from system crashes, concurrent access, quick application development, data integrity and security. Levels of abstraction give data independence. A DBMS typically has a layered architecture. DBAs hold responsible jobs and are well-paid! DBMS R&D is one of the broadest, most exciting areas in CS. 17 Database Management Systems 18 A typical DBMS has a Query Optimization layered architecture. and Execution The figure does not Relational Operators show the concurrency Files and Access Methods control and recovery components. Buffer Management This is one of several Disk Space Management possible architectures; each system has its own variations. DB Database Management Systems Overview of Database Design Conceptual design: (ER Model is used at this stage.) The Entity-Relationship Model 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. 19 Database Management Systems Database 20 Management Systems ER Model Basics ssn name name lot ER Model Basics (Contd.) name ssn Employees lot Works_In since did dname budget Departments ssn lot Employees Employees supervisor subordinate Entity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes. Reports_To 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. Database Management Systems 21 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. Database Management Systems 22 Key Constraints ssn since name lot did dname budget 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 did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) name ssn Employees lot Manages since did dname budget Departments 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. Database Management Systems Employees Manages Departments 1-to-1 1-to Many Many-to-1 Many-to-Many Works_In since 23 Database Management Systems 24 name ssn lot 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-tomany relationship set (one owner, many weak entities). Weak entity set must have total participation in this identifying relationship set. ISA (`is a ) Hierarchies hourly_wages hours_worked Employees name ssn lot cost pname age Employees Policy Dependents vAs in C++, or other PLs, ISA contractid attributes are inherited. vIf we declare A ISA B, every A Contract_Emps Hourly_Emps entity is also considered to be a B entity. 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. Database Management Systems 26 Database Management Systems 25 ssn name lot Employees 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. Conceptual Design Using the ER Model until Monitors 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? started_on pid Projects pbudget since did dname budget Departments Sponsors * Aggregation vs. ternary relationship: v Monitors is a distinct relationship, with a descriptive attribute. v Also, can say that each sponsorship is monitored by at most one employee. 27 Constraints in the ER Model: A lot of data semantics can (and should) be captured. But some constraints cannot be captured in ER diagrams. Database Management Systems 28 Database Management Systems 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 setvalued). 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). Database Management Systems 29 Entity vs. Attribute (Contd.) name from lot Works_In4 to did dname budget Departments Works_In4 does not allow an employee to work in a department for two or more periods. ssn Employees 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. Database Management Systems ssn name lot Works_In4 did dname budget Departments Employees from Duration to 30 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. Database Management Systems Binary vs. Ternary Relationships ssn name lot Covers Policies policyid ssn name lot cost pname age pname age dbudget did Manages2 dname budget Departments name ssn Employees name ssn Employees lot lot since since dname did budget Departments ISA Manages2 Managers dbudget This fixes the problem! 31 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? Database Management Systems Employees Dependents Bad design Employees Purchaser Dependents Beneficiary Better design policyid Policies cost 32 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? Database Management Systems 33 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. Database Management Systems 34 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 nary 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. Database Management Systems 36 Database Management Systems 35

Find millions of documents here - Study Guides, Homework Solutions, Papers, Exam Answer Keys and more. Course Hero has millions of course related materials that will enable you to learn better, faster and get an A in all your courses.
Below is a small sample set of documents: