In the R/3 System, long-life data is stored in relational database tables. In a relational database model, the real world is represented by tables. A table is a two-dimensional matrix, consisting of lines and columns (fields). The smallest possible combination of fields that can uniquely identify each line of the table is called the key. Each table must have at least one key, and each table has one key that is defined as its primary key. Relationships between tables are represented by foreign keys.
Standard SQL
SQL (Structured Query Language) is a largely standardized language for accessing relational databases. It can be divided into three areas:
· Data Manipulation Language (DML)
Statements for reading and changing data in database tables.
· Data Definition Language (DDL)
Statements for creating and administering database tables.
· Data Control Language (DCL)
Statements for authorization and consistency checks.
Each database has a programming interface that allows you to access the database tables using SQL statements. The SQL statements in these programming interfaces are not fully standardized. To access a specific database system, you must refer to the documentation of that system for a list of the SQL statements available and their correct syntax.
The Database Interface
To make the R/3 System independent of the database system with which you use it despite the differences in the SQL syntax between various databases, each work process on an application server has a database interface. The R/3 System communicates with the database by means of this interface. The database interface converts all of the database requests from the R/3 System into the correct Standard SQL statements for the database system. To do this, it uses a database-specific component that shields the differences between database systems from the rest of the database interface. You choose the appropriate layer when you install the R/3 System.
There are two ways of accessing the database from a program - with Open SQL or Native SQL.
Open SQL
Open SQL statements are a subset of Standard SQL that is fully integrated in ABAP. They allow you to access data irrespective of the database system that the R/3 installation is using. Open SQL consists of the Data Manipulation Language (DML) part of Standard SQL; in other words, it allows you to read (SELECT) and change (INSERT, UPDATE, DELETE) data.
Open SQL also goes beyond Standard SQL to provide statements that, in conjunction with other ABAP constructions, can simplify or speed up database access. It also allows you to buffer certain tables on the application server, saving excessive database access. In this case, the database interface is responsible for comparing the buffer with the database. Buffers are partly stored in the working memory of the current work process, and partly in the shared memory for all work processes on an application server. Where an R/3 System is distributed across more than one application server, the data in the various buffers is synchronized at set intervals by the buffer management. When buffering the database, you must remember that data in the buffer is not always up to date. For this reason, you should only use the buffer for data which does not often change. You specify whether a table can be buffered in its definition in the ABAP Dictionary.
Native SQL
Native SQL is only loosely integrated into ABAP, and allows access to all of the functions contained in the programming interface of the respective database system. Unlike Open SQL statements, Native SQL statements are not checked and converted, but instead are sent directly to the database system. When you use Native SQL, the function of the database-dependent layer is minimal. Programs that use Native SQL are specific to the database system for which they were written. When writing R/3 applications, you should avoid using Native SQL wherever possible. It is used, however, in some parts of the R/3 Basis System - for example, for creating or changing table definitions in the ABAP Dictionary.
The ABAP Dictionary
The ABAP Dictionary, part of the ABAP Workbench, allows you to create and administer database tables. Open SQL contains no statements from the DDL part of Standard SQL. Normal application programs should not create or change their own database tables.
The ABAP Dictionary uses the DDL part of Open SQL to create and change database tables. It also administers the ABAP Dictionary in the database. The ABAP Dictionary contains metadescriptions of all database tables in the R/3 System. Only database tables that you create using the ABAP Dictionary appear in the Dictionary. Open SQL statements can only access tables that exist in the ABAP Dictionary.
Authorization and Consistency Checks
The DCL part of Standard SQL is not used in R/3 programs. The work processes within the R/3 System are logged onto the database system as users with full rights. The authorizations of programs or users to read or change database tables is administered within the R/3 System using the R/3 authorization concept. Equally, transactions must ensure their own data consistency using the R/3 locking concept. For more information, refer to Authorization Concept and Programming Database Updates.
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