are built on commercial DBMS system on CODASYL model. The DBTG report contained proposals for three distinct database languages: a schema data description language; a subschema data description language; a data manipulation language. Data in the network model is represented by collection of records and relationship among data is represented by links, which can be viewed, as pointers. The records in the database are organized as collection of arbitrary graphs, which allows to have one-to- many as well as many-to-many relationship as shown in the figure 2.2. A record (or, more precisely, record occurrence) is a collection of data items which can be retrieved from a database, or which can be stored in a database as an undivided object. Thus, a DBMS may STORE, DELETE or MODIFY records within a database. In this way, a number of records within a network database are dynamically changed. Relational Model Relational model was first introduced by DR. E.F. Codd, an IBM Database Scientist in 1969. This model has been constantly refined since then. This model is based on 12 rules given by Codd, which are explained in next chapter. It also introduced the concept of normalization, which helps to reduce redundancy to a big extent. This model uses the certain mathematical operations from relational algebra and relational calculus on the relations such as union, intersection, projection, selection etc. Relational Model based databases are so popular and widespread that it is now common to shorten the term Relational DBMS to RDBMS. At present there are so many implementations of the relational model. ORACLE, SQL Server, DB2 etc. are the latest packages, which are based on relational model. A relational database is composed of many relations in the form of two-dimensional tables of rows and columns containing related records. Organizing data into tables, the form in which data is presented to the user and the programmer is also called as logical view of the database. In Relational model, we use the terminology Tuples for records and Attributes for columns. Each attribute has a data type such as float, integer, string etc. The various constraints can also be implemented on the database. These constraints
16 Data Base Management systems Notes Amity Directorate of Distance & Online Education may include domain constraints, key constraints, entity integrity constraints and referential integrity constraints. All these are explained in the next chapter on RDBMS. These constraints ensure that there are no ambiguous and redundant records in the database. The following Table 2 is a relation that describes an employee by empno, ename, job, mgr, hiredate, sal, comm, and dno. ENO ENAME JOB MGR HIREDATE SAL COMM DNO 7369 SMITH CLERK 7902 17-DEC-80 800 20 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30 7566 JONES MANAGER 7839 02-APR-81 2975 20 7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30 7698 BLAKE MANAGER 7839 01-MAY-81 2850 30 Table 2 An example of EMP relation Relational model uses Structured Query Language (SQL) as the primary interface language for interacting the database. By using SQL, we can define the relations and their
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