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 .  A  conversion  procedure  has  been  provided  by the database administrator. 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 3-25