Schema DDL and DML

The relationship between a schema DDL and a DML is the relationship between declaration and procedure. The DDL declarations impose a discipline over the executable code and are to some extent substitutes for procedures written in the DML and the host language.

To specify the relationship between DDL declarations and DML commands, a set of basic data manipulation functions must be defined that is independent of the DML and the host language. Specific commands provided by a particular DML must be resolved into those basic functions. The resolution is defined by the implementor of the DML.

The basic data manipulation functions assumed in these specifications include the functions required to: . Select records l Present records to the run unit . Add new records and relationships . Change existing records and relationships . Remove existing records and relationships

Schema and Storage Schema

The concept of separate schema and storage schema allows the separation of the logical description of the entire database from the storage description of the same. This concept is significant from the following points of view:

1. A database administrator may design a schema structure consisting of logical record relationships that sensibly match the totality of applications under implementation or likely to be implemented.

2. Efficiency considerations are separated from the logical description by specifying the storage environment and schema to storage schema mappings in the storage schema. Tuning may be carried out by changing the storage schema without alteration to the schema, subschemas, and programs.

The storage schema describes the representation of stored data in device independent terms. The database may, therefore, be stored on any combination of storage media that is supported by a particular implementation.

The database administrator may allocate media and devices with differing characteristics to suit the command's operational requirements, without alteration to the storage schema.

Database Management System Selection

When selecting a DBMS, the primary consideration should be to select a technology that will support the long-term DBMS needs. The work of identifying the needs of the command should be done in a very careful and thorough manner. The ultimate goal is to make the best choice for the command.

One of the best ways of identifying the needs of the command is to conduct interviews with the users. The results of the interviews will identify areas of concern to them, such as: 

. How fast can data be accessed? 

. How easy is it to retrieve and manipulate the data? 

. How fast and easy is it to develop quality applications? 

l Will the redundancy of data be reduced?

. Will it provide for the management and accurate identity of all the data elements?

Once the needs of the command have been identified, it is time to prepare the presentation for management. A first step in the preparation of the presentation is to describe how the needs of the command will be addressed by the DBMS. Develop specific examples to illustrate how each item identified would be handled in the database environment.

After receiving permission from management to continue, you can start the selection process. Since all DBMS software is not the same, you must look at the

quality of the product and the ability of the vendor to continue to enhance the product in the future. All of the decisions should be based on the features currently available or in a beta testing environment. The goal is not to find the perfect DBMS, but to identify and recommend the best of those available that will meet the command's needs.

This selection criteria applies whether the DBMS is

going to be used on a mainframe computer or a microcomputer system. However, the microcomputer system has a few added concerns that must be met. The most important of these concerns are:

The capability of receiving downloaded data from a mainframe or another microcomputer

The ability to be used on a network

The ability to enable quick and easy setup of screen formats

The ability to handle a maximum number of records, fields, tables, and so on, and the size of each

The ability of the software to accept files in multiple formats and to transport files and records out to other microcomputer packages

LOGICAL STRUCTURE OF A DATABASE MANAGEMENT SYSTEM

To decide on the logical structure to use with a DBMS, you will need to first perform a comprehensive review of the data. Determine what data is needed, where the data comes from, and how the data will be used. Look at the reports needed. Find out what transactions and displays are most appropriate for collecting and manipulating the data. At this point, you can determine what programs and queries will need to be developed and decide on the best organization for the data.

The programmer will be the one to decide the organization of the database and to define the physical structures of the database management system. Programs to process the transactions and to generate the anticipated management information and decision support reports are mapped out, using the pseudocode and flowcharts. Once the programs are mapped out, they are written, tested, and implemented, creating the database management system.

SUMMARY

As a communications specialist, you need to know a lot more about computers than just how to operate them. You must have a good working knowledge of computer software. You should be able to explain and demonstrate how to use systems and applications software to both communications specialists and end-user personnel. You will need to interact with the users to determine their requirements and needs. We covered how to determine system software and hardware requirements and the importance of calculating memory requirements. We discussed the parts of a database management system (DBMS) and the selection process for a DBMS, including those for personal computers.