A subschema lets the user have access to different areas of applications in which the user designed. The areas that are included in an application are set, types, record types, data items, and data aggregates. Schemas may have many different subschemas’ that are all very different. There are several different reasons for subschema’s. One reason for a subschema is to let a user or programmer see different views without having to see all the data contained in the database. Another reason would be to improve security so that someone who is not authorized can change or add to the data.
Data definition language describes the database in which there are possibly many programs that have been writing in different program languages. The description is in the form of names and characteristics of data items, data aggregates, records, areas, and sets included in the database, and the relationships that exist and that have to be maintained between occurrences of elements within the database.
A data item is represented by a value in a database, which is an occurrence of the smallest unit of the named data.
A Data aggregate is the occurrence of named collections of data items that are inside the record. There are two kinds, the first is a vector that is a one-dimensional sequence of data items. These all have identical characteristics. The other is a repeating group and is a collection of data that occurs several times within a record occurrence.
Record is an occurrence of a named collection of zero, one, or more items. Keys are a way to uniquely identify a record.
Set is an occurrence of a named collection of records. For every record in the schema there must be one occurrence.
Area is a named collection of records but it does not have to be linked between each record.
Database contains all the records within the schema; however each database must contain a separate schema.
Programs are the instructions that occur within a computer language.ur article here!