Database Reference
In-Depth Information
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The host language is that which supports the DSL in addition to
other non-database facilities such as manipulation of variables,
computations and Boolean logic. Host languages are typically
high level languages (HLL); examples include COBOL, C, C++,
Java, Pascal, RPG-400, etc.
Typically, the sublanguage consists of a
data definition language
(DDL), a
data
manipulation language
(DML), and a
data control language
(DCL). These components
are not necessarily distinct entities, but are typically part of a single coherent product.
The above-mentioned facilities allow users to define assorted logical views of data in
the database. In summary, the external schema is the user interpretation of the database,
but facilitated by the DSL.
2.1.2 Conceptual Level
The
conceptual level
is an abstract representation of the entire information content of
the database; it also referred to as the logical or community user view. It is defined by
means of the
conceptual schema
, which includes definition of each of the various types of
conceptual records.
The conceptual schema includes defining the structure of the database, security
constraints, operational constraints and integrity checks. It represents a closer picture of
how data will be actually stored and managed, and is the level that most technical user
will relate.
The conceptual schema must adhere to the data independence requirement. Also,
it must be comprehensive since it represents the realization of the entire database design.
2.1.3 Internal Level
Also called the
storage view
, the
internal level
is the low level representation of the
database. It is one level above the physical level, which deals with pages, cylinders and
tracks on the storage device.
The internal level is defined by the
internal schema
, which addresses issues such as
record types, indexes, field representation, physical storage sequence of records, data
access, etc., and written in the internal DDL.
