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applications.
Data and index definition for the tables Composer and Piece in SQL
Taking a look at the SQL statements in the music database, we instantly see possibibilities of automatic link generation.
The table Composer is referenced by the table Piece; obviously, this reference can be used in generating hypertext link.
Assuming we translate each table entry into a web page, one can easily imagine how for each piece of music in the
database a link can be added to the field ComposerID in table Piece. The SQL statement defines this field as a foreign
key which references the table Composer.
Unfortunately, SQL statements for table creation are not normally available in legacy systems. However, the information
is not lost: the RDBMS automatically manages the corresponding structure. In the following section we take a look at
the way a RDBMS is storing meta-data about applications.
4 The System Catalog
RDBMS store meta information about applications in system catalog tables. The structure of all objects derived from the
data definition language (DDL: SQL statements like CREATE TABLE, CREATE INDEX) in the user databases are
described in the catalog tables. Sophisticated systems store other information as well, e. g. on permissions, statements
embedded in application programs (DML). In [Pönighaus 1995] an analysis on the usage of SQL statements based on
catalog data was performed; here we concentrate on DDL. The meta information contained in the catalog tables is
accessible via standard SQL statements. [Fig. 1b] shows an ER-model of a simplified system catalog (our meta base).
System catalogs vary somewhat between database systems. In the following we describe relevant parts of our system
catalog here; most widely used database systems provide the information needed, although the corresponding tables and
fields would be named differently.
Database:
this table contains the names of the databases and their creators. In a typical RDBMS, a user can own more than
one database.
Tables:
this table contains the names of the tables which form the application database. Entries in this table consist of
table name, database name and creator name.
Columns:
this table describes the columns of all application tables. An additional Remarks field can be used for the web
page generation if any text has been supplied by the table designers.
Relationships:
this table contains information on tables that are referencing other tables (or itself). It states the referencing table
(Tbname), the table being referenced (Reftbname), and the name of the relationship.
Foreignkeys:
relationships to other tables are described here; for each relationship contained in the catalog table Relationships,
the columns involved in the relationship are stated here.