SubschemasA subschema is the applications programmer’s viewof the data within the database pertinent to the specificapplication. A subschema has access to those areas, settypes, record types, data items, and data aggregates ofinterest in the pertinent application to which it wasdesigned. Naturally, a software system usually hasmore than one programmer assigned and includes morethan one application.This means there are usuallymany different subschemas for each schema.The following are a few of the many reasonssubschemas are used:lllSubschemas provide different views of the datato the user and the programmer, who do not needto know all the data contained in the entiredatabase.Subschemas enhance security factors andprohibit data compromise.Subschemas aid the DBA while assuring dataintegrity.Each data item included in the subschema will beassigned 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 aCALL for data is delivered. It is also where all data to bepicked up by the DBMS must be placed.Schema Data Definition Language (DDL)The schema data definition language (DDL) is usedfor describing a database, which maybe shared by manyprograms written in many languages. This descriptionis in terms of the names and characteristics of the dataitems, data aggregates, records, areas, and sets includedin the database, and the relationships that exist and mustbe maintained between occurrences of those elementsin the database.lData item. A data item is an occurrence of thesmallest unit of named data. It is represented in adatabase by a value.lData aggregate.A data aggregate is anoccurrence of a named collection of data items within arecord. There are two kinds-vectors and repeatinggroups. A vector is a one-dimensional sequence of dataitems, all of which have identical characteristics. Arepeating group is a collection of data that occurs anumber of times within a record occurrence. Thecollection may consist of data items, vectors, andrepeating groups.lRecord. A record is an occurrence of a namedcollection of zero, one, or more data items or dataaggregates. This collection is specified in the schemaDDL by means of a record entry. Each record entry inthe schema for a database determines a type of record,of which there may be an arbitrary number of recordoccurrences (records) in the database. For example,there would be one occurrence of a PAYROLL-RECORD type of record for each employee. Adatabase key is a unique value that identifies a record inthe database to a run unit (program(s)). The value ismade available to the run unit when a record is selectedor stored and maybe used by the run unit to reselect thesame record..Set. A set is an occurrence of a named collectionof records. The collection is specified in the schemaDDL by means of a set entry. Each set entry in theschema for a database determines a type of set, of whichthere may be an arbitrary number of set occurrences(sets) in the database. Each type of set specified in theschema may have one type of record declared as itsowner type of record, and one or more types of recordsdeclared as its member type of record. Each setoccurrence (set) must contain one occurrence of itsdefined owner type of record and may contain anarbitrary number of occurrences of each of its definedmember type of record types. For example, if a settype QUALIFICATIONS was defined as having ownerrecord type EMPLOYEE and member record typesJOB and SKILL, each occurrence of set typeQUALIFICATIONS must contain one occurrence ofrecord type EMPLOYEE, and may contain an arbitrarynumber of occurrences of record types JOB and SKILL.lArea. An area is a named collection of recordsthat need not preserve owner/member relationships. Anarea may contain occurrences of one or more recordtypes, and a record type may have occurrences in morethan one area. A particular record is assigned to a singlearea and may not migrate between areas.lDatabase. 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 aseparate schema for each database. Furthermore, thecontent of each database is assumed to be independent.lProgram. A program is a set or group ofinstructions in a host language such as COBOL or3-23
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