conversions  between  subschema  PICTURES  and  any schema data type. 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: 3-26