ARTICLE
Relational Databases
The Relational Data Model
Application software models a part of the real world. This type of
modeling is based on the
relational data model . A data model describes reality using formal
structures. In the relational data model, the only formal structure used
is known as a relation or table. All the information about the modeled
area of reality is represented as values in tables.
A table is a two-dimensional matrix which consists of rows and columns
(fields).
The value range of a field (the set of permitted values for the field)
is also known as the domain of the field.
A field or combination of fields which uniquely identifies every row in
a table is known as the key. This is a minimal attribute: if one of the
fields is removed from the key, the remaining fields cannot uniquely
identify the table rows.
Each table must have at least one table key
(primary key integrity). However a table usually has multiple keys.
During the modeling process, only one key is defined as the
primary key .
In the relational model, the relationship between objects is expressed
in terms of a foreign key . A combination
of fields in a table is known as a foreign key, if it is also the
primary key in another table.
In an AS ABAP , the data model is managed in the
ABAP Dictionary , which results in a
close compliance with the ABAP type concept
.
The Relational Database
All the data relevant for all applications of an AS ABAP is
collected and managed centrally in a
relational database . All ABAP
application programs work with this data by default. The largely
standardized language SQL can be used to access
relational databases. Application Server ABAP supports database systems
from different vendors. SQL is embedded in the ABAP language either with
standardized Open SQL or
vendor-specific Native SQL .
Note
The primary table key (key fields for a database table of AS ABAP
that build the table key) can have a total size of 900 bytes. However
if a key is larger than 120 bytes, the restrictions apply:
Table contents cannot be transported by specifying the complete key
values. Instead you need to specify the generic key values with a
maximum length of 120 bytes.
The table cannot be used as the base table of a lock object.
Documentation extract taken from SAP system, � Copyright SAP AG. All rights reserved