Database System Concepts and Architecture Relational Model Concepts

Database System Concepts and Architecture

Relational Model Concepts n n relational model represents the database as a collection of relations. Each relation resembles a table of values or, to some extent, a flat file of records. Each row in the table represents a collection of related data values. A row represents a fact that typically corresponds to a real-world entity or relationship Slide 2 - 2

Relational Model Concepts n The table name and column names are used to help to interpret the meaning of the values in each row. Slide 2 - 3

Domains, Attributes, Tuples, and Relations n A domain D is a set of atomic values. n n n each value in the domain is indivisible as far as the formal relational model is concerned. specifying a data type from which the data values forming the domain. Examples : n n Names: The set of character strings that represent names of persons. Employee_ages: Possible ages of employees in a company; each must be an integer value between 15 and 80. Slide 2 - 4

Relation Schema n n A relation schema R, denoted by R(A 1, A 2, . . . , An), is made up of a relation name R and a list of attributes, A 1, A 2, . . . , An. D is called the domain of Ai and is denoted by dom(Ai). The degree (or arity) of a relation is the number of attributes n of its relation schema. Example: n A relation of degree seven. Slide 2 - 5

Relation Schema Cartesian product (denoted by ×) Total number of possible instances or tuples is : Slide 2 - 6

Characteristics of Relations n Ordering of Tuples in a Relation n n A relation is not sensitive to the ordering of tuples. In a file, records are physically stored on disk (or in memory), so there always is an order among the records. Slide 2 - 7

Characteristics of Relations n Ordering of Values within a Tuple and an Alternative Definition of a Relation. n n An n-tuple is an ordered list of n values, so the ordering of values in a tuple—and hence of attributes in a relation schema—is important. At a more abstract level, the order of attributes and their values is not that important as long as the correspondence between attributes and values is maintained. Slide 2 - 8

Characteristics of Relations n Values and NULLs in the Tuples. n n n Each value in a tuple is an atomic value; No composite values are allowed NULL represent the values that may be unknown or may not apply to a tuple. Slide 2 - 9

Characteristics of Relations n Interpretation (Meaning) of a Relation n n The relation schema can be interpreted as a declaration or a type of assertion or predict. For example, n n The schema of the STUDENT relation asserts that, in general, a student entity has a Name, Ssn, Home_phone, Address, Office_phone, Age, and Gpa. The predicate STUDENT (Name, Ssn, . . . ) is true for the five tuples in relation STUDENT Slide 2 - 10

Data Models n n Recall: a major characteristic of DB approach is Data Abstraction: n n n Suppression of details of data organization and storage and the highlighting of the essential features to better understand data. Different users will perceive data at their preferred level of detail. A Data Model enables Data Abstraction Slide 2 - 11

Data Models n Data Model: n A set of concepts to describe n n The structure of a database The operations for manipulating these structures The constraints that the database should obey Data Model Structure and Constraints: n n Constructs are used to define the database structure Constructs typically include n n elements (and their data types) groups of elements (e. g. entity, record, table) relationships among such groups Constraints specify some restrictions on valid data; these constraints must be enforced at all times Slide 2 - 12

Data Models (continued) n Data Model Operations: n n These operations are used for specifying database retrievals and updates by referring to the constructs of the data model. Operations on the data model may include n basic model operations n n (e. g. generic insert, delete, update) user-defined operations n (e. g. compute_student_gpa, update_inventory) Slide 2 - 13

Categories of Data Models n Conceptual (high-level, semantic) data models: n Provide concepts that are close to the way many users perceive data. n n Physical (low-level, internal) data models: n n (Also called entity-based or object-based data models. ) Provide concepts that describe details of how data is stored in the computer. Implementation (representational) data models: n Provide concepts that fall between the above two, used by many commercial DBMS implementations (e. g. relational data models used in many commercial systems). Slide 2 - 14

Meta-data/ Schema Slide 2 - 15

Schemas versus Instances n Database Schema: n n n Schema Diagram: n n The description of a database. Includes descriptions of the database structure, data types, and the constraints on the database. An illustrative display of (most aspects of) a database schema. Schema Construct: n A component of the schema or an object within the schema, e. g. , STUDENT, COURSE. Slide 2 - 16

Schemas versus Instances n Database State: n n The actual data stored in a database at a particular moment in time. This includes the collection of all the data in the database. Also called Database Instance (or Occurrence or Snapshot). n The term instance is also applied to individual database components, e. g. record instance, table instance, entity instance Slide 2 - 17

Database Schema vs. Database State n Database State: n n Initial Database State: n n Refers to the content of a database at a moment in time. Refers to the database state when it is initially loaded into the system. Valid State: n A state that satisfies the structure and constraints of the database. Slide 2 - 18

Database Schema vs. Database State (continued) n Distinction n n The database schema changes very infrequently. The database state changes every time the database is updated. n Schema is also called Intension. n State is also called Extension. Slide 2 - 19

Example of a Database Schema Slide 2 - 20

Example of a Database State STUDENT Name Student_number Class Major Smith 17 1 CS Brown 8 2 CS Slide 2 - 21

Three-Schema Architecture n Proposed to support DBMS characteristics of: n n n Program-data independence. Support of multiple views of the data. Not explicitly used in commercial DBMS products, but has been useful in explaining database system organization Slide 2 - 22

Three-Schema Architecture n Defines DBMS schemas at three levels: n Internal schema at the internal level to describe physical storage structures and access paths (e. g indexes). n n Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users. n n Typically uses a physical data model. Uses a conceptual or an implementation data model. External schemas at the external level to describe the various user views. n Usually uses the same data model as the conceptual schema. Slide 2 - 23

The three-schema architecture Slide 2 - 24

Three-Schema Architecture n Mappings among schema levels are needed to transform requests and data. n n Programs refer to an external schema, and are mapped by the DBMS to the internal schema for execution. Data extracted from the internal DBMS level is reformatted to match the user’s external view (e. g. formatting the results of an SQL query for display in a Web page) Slide 2 - 25

Data Independence n Logical Data Independence: n n The capacity to change the conceptual schema without having to change the external schemas and their associated application programs. Physical Data Independence: n n The capacity to change the internal schema without having to change the conceptual schema. For example, the internal schema may be changed when certain file structures are reorganized or new indexes are created to improve database performance Slide 2 - 26

Data Independence (continued) n n When a schema at a lower level is changed, only the mappings between this schema and higherlevel schemas need to be changed in a DBMS that fully supports data independence. The higher-level schemas themselves are unchanged. n Hence, the application programs need not be changed since they refer to the external schemas. Slide 2 - 27

Just Smile Slide 2 - 28

DBMS Languages n Data Definition Language (DDL) n Data Manipulation Language (DML) Slide 2 - 29

DBMS Languages n Data Definition Language (DDL): n n n Used by the DBA and database designers to specify the conceptual schema of a database. In many DBMSs, the DDL is also used to define internal and external schemas (views). In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas. Slide 2 - 30

DBMS Languages n Data Manipulation Language (DML): n n n Used to specify database retrievals and updates Stand-alone DML commands can be applied directly (called a query language). DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language), such as COBOL, C, C++, or Java. n A library of functions can also be provided to access the DBMS from a programming language Slide 2 - 31

Types of DML n High Level or Non-procedural Language: n n n May be used in a standalone way or may be embedded in a programming language Are “set”-oriented and specify what data to retrieve rather than how to retrieve it. Also called declarative languages. E. g. SQL relational language Low Level or Procedural Language: n n These MUST be embedded in a programming language Retrieve data one record-at-a-time; Constructs such as looping are needed to retrieve multiple records, along with positioning pointers. E. g. DL/1 (developed for Hierarchical Model) Slide 2 - 32

DBMS Interfaces n Stand-alone query language interfaces n n n Example: Entering SQL queries at the DBMS interactive SQL interface (e. g. SQL*Plus in ORACLE) Programmer interfaces for embedding DML in programming languages User-friendly interfaces n Menu-based, forms-based, graphics-based, etc. Slide 2 - 33

DBMS Programming Language Interfaces n Programmer interfaces for embedding DML in a programming languages: n n n Embedded Approach: e. g embedded SQL (for C, C++, etc. ), SQLJ (for Java) Procedure Call Approach: e. g. JDBC for Java, ODBC for other programming languages Database Programming Language Approach: e. g. ORACLE has PL/SQL, a programming language based on SQL; language incorporates SQL and its data types as integral components Slide 2 - 34

Database System Utilities n To perform certain functions such as: n n n n Loading data stored in files into a database. Includes data conversion tools. Backing up the database periodically on tape. Reorganizing database file structures. Report generation utilities. Performance monitoring utilities. Other functions, such as sorting, user monitoring, data compression, etc. Data dictionary / repository: n Used to store schema descriptions and other information such as design decisions, application program descriptions, user information, usage standards, etc Slide 2 - 35

Typical DBMS Component Modules Schemas, mappings, Constraints, File sizes, Statistics. . etc Low level I/O & buffer mgmt operations Slide 2 - 36

DBMS Architectures n Centralized DBMS n Client/Server DBMS n n 2 -tier 3 -tier Slide 2 - 37

Centralized DBMS Architectures n Combines everything into single system including: n n n DBMS software hardware application programs & user interface processing software User can still connect through a remote terminal n However, all processing is done at centralized site. Slide 2 - 38

A Physical Centralized Architecture Slide 2 - 39

Client/Server Client n n Provides appropriate interfaces through a client software module to access and utilize the various server resources. Applications running on clients utilize an Application Program Interface (API) to access server databases via standard interface such as: n n ODBC: Open Database Connectivity standard JDBC: for Java programming access Server n n n Provides database query and transaction services to the clients Relational DBMS servers are often called SQL servers, query servers, or transaction servers Client and server must install appropriate client module and server module software for ODBC or JDBC Slide 2 - 40

Logical two-tier client server architecture Slide 2 - 41

Traditional Two-Tier Client-Server © Pearson Education Limited 1995, 2005 Slide 2 - 42

Three Tier Client-Server Architecture n n Common for Web applications Intermediate Layer called Application Server or Web Server: n n n Stores the web connectivity software and the business logic part of the application used to access the corresponding data from the database server Acts like a conduit for sending partially processed data between the database server and the client. Three-tier Architecture Can Enhance Security: n n Database server only accessible via middle tier Clients cannot directly access database server Slide 2 - 43

Three-Tier Client-Server © Pearson Education Limited 1995, 2005 Slide 2 - 44

Three-tier client-server architecture Slide 2 - 45

Integrity, Referential Integrity, and Foreign Keys n n The entity integrity constraint states that no primary key value can be NULL. A tuple in one relation that refers to another relation must refer to an existing tuple in that relation. n Dno of EMPLOYEE gives the department number for which each employee works; hence, its value in every EMPLOYEE tuple must match the Dnumber value of some tuple in the DEPARTMENT relation. Slide 2 - 46

Primary and Foreign Keys Slide 2 - 47

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