Subschemas A subschema is the applications programmer’s view of the data within the database pertinent to the specific application. A subschema has access to those areas, set types, record types, data items, and data aggregates of interest  in  the  pertinent  application  to  which  it  was designed.  Naturally,  a  software  system  usually  has more  than  one  programmer  assigned  and  includes  more than  one  application. This  means  there  are  usually many different subschemas for each schema. The   following   are   a   few   of   the   many   reasons subschemas are used: l l l Subschemas provide different views of the data to the user and the programmer, who do not need to  know  all  the  data  contained  in  the  entire database. Subschemas  enhance  security  factors  and prohibit data compromise. Subschemas  aid  the  DBA  while  assuring  data integrity. Each data item included in the subschema will be assigned a location in the user working area (UWA). The UWA is conceptually a loading and unloading zone, where all data provided by the DBMS in response to a CALL for data is delivered. It is also where all data to be picked up by the DBMS must be placed. Schema Data Definition Language (DDL) The schema data definition language (DDL) is used for describing a database, which maybe shared by many programs written in many languages. This description is in terms of the names and characteristics of the data items,  data  aggregates,  records,  areas,  and  sets  included in the database, and the relationships that exist and must be maintained between occurrences of those elements in the database. l Data item.  A data item is an occurrence of the smallest  unit  of  named  data.  It  is  represented  in  a database by a value. l   Data  aggregate. A   data   aggregate   is   an occurrence of a named collection of data items within a record.   There   are   two   kinds-vectors   and   repeating groups.  A  vector  is  a  one-dimensional  sequence  of  data items,  all  of  which  have  identical  characteristics.  A repeating  group  is a collection of data that occurs a number   of   times   within   a   record   occurrence.   The collection   may   consist   of   data   items,   vectors,   and repeating  groups. l Record.  A  record  is  an  occurrence  of  a  named collection  of  zero,  one,  or  more  data  items  or  data aggregates. This collection is specified in the schema DDL by means of a record entry. Each record entry in the schema for a database determines a type of record, of which there may be an arbitrary number of record occurrences  (records)  in  the  database.  For  example, there  would  be  one  occurrence  of  a  PAYROLL- RECORD   type   of   record   for   each   employee.   A database key is a unique value that identifies a record in the  database  to  a  run  unit  (program(s)).  The  value  is made available to the run unit when a record is selected or stored and maybe used by the run unit to reselect the same record. . Set. A set is an occurrence of a named collection of  records.  The  collection  is  specified  in  the  schema DDL  by  means  of  a  set  entry.  Each  set  entry  in  the schema for a database determines a type of set, of which there  may  be  an  arbitrary  number  of  set  occurrences (sets) in the database. Each type of set specified in the schema  may  have  one  type  of  record  declared  as  its owner type of record, and one or more types of records declared  as  its  member  type  of  record.  Each  set occurrence  (set)  must  contain  one  occurrence  of  its defined  owner  type  of  record  and  may  contain  an arbitrary number of occurrences of each of its defined member  type  of  record  types.  For  example,  if  a  set type QUALIFICATIONS was defined as having owner record  type  EMPLOYEE  and  member  record  types JOB   and   SKILL,   each   occurrence   of   set   type QUALIFICATIONS  must  contain  one  occurrence  of record type EMPLOYEE, and may contain an arbitrary number of occurrences of record types JOB and SKILL. l Area. An area is a named collection of records that  need  not  preserve  owner/member  relationships.  An area  may  contain  occurrences  of  one  or  more  record types, and a record type may have occurrences in more than one area. A particular record is assigned to a single area and may not migrate between areas. l Database. A database consists of all the records, sets, and areas that are controlled by a specific schema. If  a  facility  has  multiple  databases,  there  must  be  a separate  schema  for  each  database.  Furthermore,  the content of each database is assumed to be independent. l  Program.  A  program  is  a  set  or  group  of instructions  in  a  host  language  such  as  COBOL  or 3-23