Introduction to Data Management

Accessing Databases With NonStop SQL
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Shorthand views, however, are used strictly for convenience. They are derived from
any number of tables or other views, and use the security of the underlying tables or
views. You can read the data in a shorthand view, but you cannot update or delete it.
Files You can regard a relational database as consisting of two structural levels, as shown in
Figure 3-4. The top level includes the tables and views that you access directly
through SQL statements. This is the aspect of the database that you can see and
operate upon. Underlying this structure is a set of physical files on disks. These files
are managed by the GUARDIAN 90 operating system and are implicitly accessed
through the DP2 disk process. The data that you insert into your tables or protection
views is actually stored in these files. Tables and views are associated with their
corresponding physical files through entries in the data dictionary.
The structure of NonStop SQL tables determines the structure of their underlying
files. Thus, these tables can be supported by key-sequenced, relative, and entry-
sequenced files. Data transfers between these files is done in terms of logical records
and key fields within those records.
Access Coordination Through object locking, NonStop SQL safely coordinates different accesses to the
same data. When tables and views are audited, NonStop SQL makes most of the
locking decisions for you. For example, NonStop SQL determines whether to lock one
row at a time or to lock an entire table. NonStop SQL also determines whether many
users can lock and read the same data, or whether only the current lock holder can
access the data exclusively.