Schema DDL and Hardware

A schema DDL entry does not include References to a physical device or media space. Thus, a schema written using a DDL is a description of a database that is not affected by the devices or media used to store the data. The database may, therefore, be stored on any combination of storage media that is supported in a particular DBMS. Because of their sequential nature, some devices, such as magnetic tape, may not take full advantage of the facilities included in a DDL. Such devices are not precluded, however, and may be perfectly adequate for some of the data.

Chances are the DDL you are using will follow the guidelines created by the Conference on Data Systems Languages (CODASYL) and their subcommittee, Data Base Task Group (DBTG). These guidelines have influenced the development of database systems, particularly those for the larger computer systems.

Because of space limitations, the format specifications for a DML and a schema DDL are not presented. The syntax rules for a data description language are similar to those for COBOL and are too technically extensive to include in this chapter. For example, a DDL has a character set, words (programmer supplied), reserved words, key words, names, literal and nonnumeric literal formatting, and many other qualification rules.

Schema/Subschema Data Conversion

Since data description in the subschema is host language oriented, the syntax used in the subschema to describe the characteristics of data items may differ from that in the schema or storage schema. This means that data types that turn out to have the same representation in a given implementation may be described differently in the schema and storage schema than in the subschema. Also, there maybe data types defined in the subschema that have characteristics and representations different from those of any schema type, and vice versa. However, any data item description is eligible for inclusion in a subschema for a particular host language subschema data description entry if one of the following conditions is satisfied in the implementation involved: 

l The data item has the same representation both in the database and in the UWA in that implementation, 

l A conversion procedure has been provided by the implementor, or the database administrator. 

. A conversion procedure has been provided by

The implementor is responsible for defining the correspondence between the schema data types and specifications and the sub schema data types and specifications, in terms of the representation of these respective data types in the implementation. An example of a correspondence that might be established by an implementor would be correspondence between coded arithmetic data in the schema and COMPUTATIONAL data in the COBOL subschema.

The implementor might provide special conversion procedures in addition to those in the DBMS for implementing the conversion rules. An example of a case where the implementor might provide a special conversion procedure would be in the interface between the DBMS and database procedures written in particular host languages. If the DBMS supplies a standard parameter list to database procedures, the representation of some of the parameter values might not match that of any data type in a particular host language. In this case, the implementor might wish to provide a standard conversion procedure to allow the host language to correctly access such values.

Developers of host language database facilities may provide rules defining the intended correspondence between data types allowed in their host language subschema DDL and the data types in the schema DDL. Such rules may be specified directly, naming characteristics of subschema data types so that they can be matched with the characteristics of schema data types. Different host languages may define their rules for intended data type correspondence in terms of the closest schema equivalents; for example, FORTRAN referring to schema TYPE specifications and COBOL referring to schema PICTURE specifications. In this case, the conversion rules specified as part of the schema DDL may be used in determining appropriate conversions involving data types not explicitly mentioned in the host language's defined rules. For example, the COBOL database facility might specify the intended correspondence between its subschema PICTURE specifications and schema PICTURE specifications. With the correspondence between schema and subschema PICTURES established, subschema PICTURE specifications may be interpreted as if they were schema PICTURE specifications. The schema DDL defined conversion rules (which define conversions between schema PICTURES and other schema data types) can then be used to determine appropriate conversions between subschema PICTURES and any schema data type.