Chapter two
nDatabase Schema
- The description of a database. - Includes descriptions of the database structure, data types, and the constraints on the database.
History of Data Models in Network Model
- The first network DBMS was implemented by Honeywell in 1964-65 (IDS System). - Adopted heavily due to the support by CODASYL (Conference on Data Systems Languages) (CODASYL DBTG report of 1971). -Later implemented in a large variety of systems - IDMS (Cullinet - now Computer Associates), DMS 1100 (Unisys), IMAGE (H.P. (Hewlett-Packard)), VAX -DBMS (Digital Equipment Corp., next COMPAQ, now H.P.).
Types of DML
1- High Level or Non-procedural Language 2- Low Level or Procedural Language
Data Manipulation Language (DML) contains :
1- High-Level or Non-procedural Languages: These include the relational language SQL - May be used in a standalone way or may be embedded in a programming language 2- Low Level or Procedural Languages: - These must be embedded in a programming language
Variations of Distributed DBMSs (DDBMSs)
1- Homogeneous DDBMS 2- Heterogeneous DDBMS 3- Federated or Multidatabase Systems - Participating Databases are loosely coupled with high degree of autonomy. 4- Distributed Database Systems have now come to be known as client-server based database systems because: - They do not support a totally distributed environment, but rather a set of database servers supporting a set of clients.
History of Data Models in Hierarchical Model
1- Initially implemented in a joint effort by IBM and North American Rockwell around 1965. Resulted in the IMS family of systems. 2- IBM's IMS product had (and still has) a very large customer base worldwide 3- Hierarchical model was formalized based on the IMS system 4- Other systems based on this model: System 2k (SAS inc.)
DBMS schemas levels
1- Internal schema 2- Conceptual schema 3- External schemas
Database System Utilities
1- Loading data stored in files into a database. Includes data conversion tools. 2- Backing up the database periodically on tape. 3- Reorganizing database file structures. 4- Performance monitoring utilities. 5- Report generation utilities. 6- Other functions, such as sorting, user monitoring, data compression, etc.
Data Independence
1- Logical Data Independence 2- Physical Data Independence 3- When a schema at a lower level is changed, only the mappings between this schema and higher-level schemas need to be changed in a DBMS that fully supports data independence. 4- The higher-level schemas themselves are unchanged. - Hence, the application programs need not be changed since they refer to the external schemas.
User-Friendly DBMS Interfaces
1- Menu-based (Web-based), popular for browsing on the web 2- Forms-based, designed for naïve users used to filling in entries on a form 3- Graphics-based - Point and Click, Drag and Drop, etc. - Specifying a query on a schema diagram 4- Natural language: requests in written English 5- Combinations of the above: - For example, both menus and forms used extensively in Web database interfaces
Other DBMS Interfaces
1- Natural language: free text as a query 2- Speech : Input query and Output response 3- Web Browser with keyword search 4- Parametric interfaces, e.g., bank tellers using function keys. 5- Interfaces for the DBA: - Creating user accounts, granting authorizations - Setting system parameters - Changing schemas or access paths
Disadvantage of Network Model
1- Navigational and procedural nature of processing 2- Database contains a complex array of pointers that thread through a set of records. - Little scope for automated "query optimization"
Disadvantage of Hierarchical Model
1- Navigational and procedural nature of processing 2- Database is visualized as a linear arrangement of records 3- Little scope for "query optimization"
History of Data Models (Additional Material)
1- Network Model 2- Hierarchical Model 3- Relational Model 4- Object-oriented Data Models 5- Object-Relational Models
DBMS Programming Language Interfaces
1- Programmer interfaces for embedding DML in a 2- programming languages: - Embedded Approach: e.g embedded SQL (for C, C++, etc.), SQLJ (for Java) - Procedure Call Approach: e.g. JDBC for Java, ODBC (Open Databse Connectivity) for other programming languages as API's (application programming interfaces) - 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 - Scripting Languages: PHP (client-side scripting) and Python (server-side scripting) are used to write database programs.
History of Data Models in Relational Model
1- Proposed in 1970 by E.F. Codd (IBM), first commercial system in 1981-82. 2- Now in several commercial products (e.g. DB2, ORACLE, MS SQL Server, SYBASE, INFORMIX). 3- Several free open source implementations, e.g. MySQL, PostgreSQL 4- Currently most dominant for developing database applications. 5- SQL relational standards: SQL-89 (SQL1), SQL-92 (SQL2), SQL-99, SQL3, ... 6- Chapters 5 through 11 describe this model in detail
Three-Schema Architecture
1- Proposed to support DBMS characteristics of: - Program-data independence. - Support of multiple views of the data. 2- Not explicitly used in commercial DBMS products, but has been useful in explaining database system organization 3- Defines DBMS schemas at three levels
Clients
1- Provide appropriate interfaces through a client software module to access and utilize the various server resources. 2- Clients may be diskless machines or PCs or Workstations with disks with only the client software installed. 3- Connected to the servers via some form of a network. - (LAN: local area network, wireless network, etc.)
DBMS Server
1- Provides database query and transaction services to the clients 2- Relational DBMS servers are often called SQL servers, query servers, or transaction servers 3- Applications running on clients utilize an Application Program Interface (API) to access server databases via standard interface such as: - ODBC: Open Database Connectivity standard - JDBC: for Java programming access
what is the meaning of Low Level or Procedural Language ?
1- Retrieve data one record-at-a-time; 2- Constructs such as looping are needed to retrieve multiple records, along with positioning pointers.
State is also called
extension.
Initial Database State
Refers to the database state when it is initially loaded into the system.
nData Model Structure and Constraints:
- Constructs are used to define the database structure - Constructs typically include elements (and their data types) as well as groups of elements (e.g. entity, record, table), and relationships among such groups - Constraints specify some restrictions on valid data; these constraints must be enforced at all times
explain Conceptual (high-level, semantic) data models
- Provide concepts that are close to the way many users perceive data. - (Also called entity-based or object-based data models.)
Distinction
- The database schema changes very infrequently. - The database state changes every time the database is updated.
Data Model Operations:
- 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 basic model operations (e.g. generic insert, delete, update) and user-defined operations (e.g. compute_student_gpa, update_inventory)
Classification of DBMSs
1- Based on the data model used - Legacy: Network, Hierarchical. - Currently Used: Relational, Object-oriented, Object-relational - Recent Technologies: Key-value storage systems, NOSQL systems: document based, column-based, graph-based and key-value based. Native XML DBMSs. 2- Other classifications - Single-user (typically used with personal computers)vs. multi-user (most DBMSs). - Centralized (uses a single computer with one database) vs. distributed (multiple computers, multiple DBs)
Two Tier Client-Server Architecture
1- Client and server must install appropriate client module and server module software for ODBC or JDBC 2- A client program may connect to several DBMSs, sometimes called the data sources. 3- In general, data sources can be files or other non-DBMS software that manages data. 4- See Chapter 10 for details on Database Programming
Centralized DBMS
1- Combines everything into single system including- DBMS software, hardware, application programs, and user interface processing software. 2- User can still connect through a remote terminal - however, all processing is done at centralized site.
Three Tier Client-Server Architecture
1- Common for Web applications 2- Intermediate Layer called Application Server or Web Server: - 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. 3- Three-tier Architecture Can Enhance Security: - Database server only accessible via middle tier - Clients cannot directly access database server - Clients contain user interfaces and Web browsers - The client is typically a PC or a mobile device connected to the Web
Categories of Data Models
1- Conceptual (high-level, semantic) data models 2- Physical (low-level, internal) data models 3- Implementation (representational) data models 4- Self-Describing Data Models
Cost considerations for DBMSs
1- Cost Range: from free open-source systems to configurations costing millions of dollars 2- Examples of free relational DBMSs: MySQL, PostgreSQL, others 3- Commercial DBMS offer additional specialized modules, e.g. time-series module, spatial data module, document module, XML module - These offer additional specialized functionality when purchased separately - Sometimes called cartridges (e.g., in Oracle) or blades 4- Different licensing options: site license, maximum number of concurrent users (seat license), single user, etc.
DBMS Languages
1- Data Definition Language (DDL) 2- Data Manipulation Language (DML) 3- Data Definition Language (DDL) 4- Data Manipulation Language (DML)
Other Tools ... ?
1- Data dictionary / repository: - Used to store schema descriptions and other information such as design decisions, application program descriptions, user information, usage standards, etc. - Active data dictionary is accessed by DBMS software and users/DBA. - Passive data dictionary is accessed by users/DBA only. 2- Application Development Environments and CASE (computer-aided software engineering) tools: - Examples: - PowerBuilder (Sybase) - JBuilder (Borland) - JDeveloper 10G (Oracle)
DBMS Interfaces
1- Stand-alone query language interfaces - Example: Entering SQL queries at the DBMS interactive SQL interface (e.g. SQL*Plus in ORACLE) 2- Programmer interfaces for embedding DML in programming languages 3- User-friendly interfaces - Menu-based, forms-based, graphics-based, etc. 4- Mobile Interfaces:interfaces allowing users to perform transactions using mobile apps
Advantage of Network Model
1- Network Model is able to model complex relationships and represents semantics of add/delete on the relationships. 2- Can handle most situations for modeling using record types and relationship types. 3- Language is navigational; uses constructs like FIND, FIND member, FIND owner, FIND NEXT within set, GET, etc. - Programmers can do optimal navigation through the database.
History of Data Models in Object-oriented Data Models
1- Several models have been proposed for implementing in a database system. 2- One set comprises models of persistent O-O Programming Languages such as C++ (e.g., in OBJECTSTORE or VERSANT), and Smalltalk (e.g., in GEMSTONE). 3- Additionally, systems like O2, ORION (at MCC - then ITASCA), IRIS (at H.P.- used in Open OODB). 4- Object Database Standard: ODMG-93, ODMG-version 2.0, ODMG-version 3.0. 5- Chapter 12 describes this model.
Advantage of Hierarchical Model
1- Simple to construct and operate 2- Corresponds to a number of natural hierarchically organized domains, e.g., organization ("org") chart 3- Language is simple: - Uses constructs like GET, GET UNIQUE, GET NEXT, GET NEXT WITHIN PARENT, etc.
Basic 2-tier Client-Server Architectures
1- Specialized Servers with Specialized functions - Print server - File server - DBMS server - Web server - Email server 2- Clients can access the specialized servers as needed
History of Data Models in Object-Relational Models
1- The trend to mix object models with relational was started with Informix Universal Server. 2- Relational systems incorporated concepts from object databases leading to object-relational. 3- Exemplified in the versions of Oracle, DB2, and SQL Server and other DBMSs. 4- Current trend by Relational DBMS vendors is to extend relational DBMSs with capability to process XML, Text and other data types. 5- The term "Object-relational" is receding in the marketplace.
Other Considerations for DBMSs
1- Type of access paths within database system - E.g.- inverted indexing based (ADABAS is one such system).Fully indexed databases provide access by any keyword (used in search engines) 2- General Purpose vs. Special Purpose - E.g.- Airline Reservation systems or many others-reservation systems for hotel/car etc. Are special purpose OLTP (Online Transaction Processing Systems)
Data Definition Language (DDL):
1- Used by the DBA and database designers to specify the conceptual schema of a database. 2- In many DBMSs, the DDL is also used to define internal and external schemas (views). 3- In some DBMSs, separate storage definition language (SDL) and view definition language (VDL) are used to define internal and external schemas. - SDL is typically realized via DBMS commands provided to the DBA and database designers
Data Manipulation Language (DML):
1- Used to specify database retrievals and updates 2- DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language), such as COBOL, C, C++, or Java. - A library of functions can also be provided to access the DBMS from a programming language 3- Alternatively, stand-alone DML commands can be applied directly (called a query language).
Three-Schema Architecture ????
4- Mappings among schema levels are needed to transform requests and data.? - 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)
Schema Construct
A component of the schema or an object within the schema, e.g., STUDENT, COURSE.
what is the meaning of Data Models ?
A set of concepts to describe the structure of a database, the operations for manipulating these structures, and certain constraints that the database should obey.
Valid State
A state that satisfies the structure and constraints of the database.
Schema Diagram
An illustrative display of (most aspects of) a database schema.
what is the meaning of High Level or Non-procedural Language ?
Are "set"-oriented and specify what data to retrieve rather than how to retrieve it. For example, the SQL relational language - Also called declarative languages.
explain Self-Describing Data Models
Combine the description of data with the data values. Examples include XML, key-value stores and some NOSQL systems.
explain Physical (low-level, internal) data models
Provide concepts that describe details of how data is stored in the computer. These are usually specified in an ad-hoc manner through DBMS design and administration manuals
explain Implementation (representational) data models
Provide concepts that fall between the above two, used by many commercial DBMS implementations (e.g. relational data models used in many commercial systems).
Database State
Refers to the content of a database at a moment in time.
what is the meaning of Logical Data Independence ?
The capacity to change the conceptual schema without having to change the external schemas and their associated application programs.
what is the meaning of Physical Data Independence ?
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
Schema is also called
intension.
Conceptual schema
the conceptual level to describe the structure and constraints for the whole database for a community of users. - Uses a conceptual or an implementation data model.
External schemas
the external level to describe the various user views. - Usually uses the same data model as the conceptual schema.
Internal schema
the internal level to describe physical storage structures and access paths (e.g indexes). - Typically uses a physical data model.