Rollforward reco very preserves logical and physical data in tegrity due to a hard crash.

Dynamic restructuring allows y ou to change data structure, table capacities, and securit y without unloading and loading the database.

Null data values allow use of ￿elds that are relev ant to some, but not all, records in a table.

HP SQL does not require y ou to de￿ne explicit relationships bet ween data sets. When y ou perform a query, it determines relationships b y matching values between ￿elds common to two or more data sets. If y ou consider a data set to be analogous to a table, a giv en ￿eld in the data set w ould occur as a column in the table. Data from an y number of tables that share a common column can be related, as needed, and y ou can de￿ne them to be an HP SQL database. The query optimizer determines the best data access strategy based on factors suc h as the presence of indexes and the relative sizes of accessed tables.

Interactive SQL (ISQL) is the in teractive interface to HP SQL. It pro vides the functionalit y of a data de￿nition language (DDL) and a data manipulation language (SQL). DDL allo ws the database administrator to con trol all aspects of database creation and modi￿cation. SQL allows the programmer or frequen t user to interactively query a database.

Security

Security is maintained by allowing speci￿cation of appropriate lev els of access privileges to individual users or groups of users.

HP SQL allows read (SELECT) access and write access privileges to be assigned at the table level. Read or write access restriction at a ￿ner gran ularity than the table level, such as at the column lev el, may be obtained by de￿ning a view of the table that omits the sensitive information. Modi￿cation (UPD ATE) authorit y may be granted at the column lev el without requiring that a view be speci￿ed. W rite access may be assigned to allow an y combination of the following capabilities:

Row modi￿cation (UPD ATE)

Row insertion (INSERT)

Row deletion (DELETE)

The database administrator (DBA) assigns access privileges b y grouping users with common access needs into authorization groups. The particular read and write authorities are then assigned by the DBA to each of these groups. Users with unique access requiremen ts may also be granted privileges directly .

Logical Transaction

A logical transaction is a series of database modi￿cations of whic h either all or none must be performed to lea ve the database in a consisten t state. The particular grouping of modi￿cations de￿ned b y the user to be a transaction v aries depending on the particular application. An example of a transaction is an accoun ting entry to pay a bill: the credit to the cash account and debit to the accoun ts payable account must both be performed to avoid leaving the accounts out of balance. This concept of a logical transaction is essen tial to ensuring that data in tegrity is maintained when multiple users are concurren tly accessing the database or in the ev ent of system failure.

Data Management 7-7