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IIS/HP SQL allows views to be created. A view is a table derived by defining a filter over one or more tables to let users or programs view only certain data in the tables. Views improve security by allowing users to access only the data they have a need to know. Since the view is not actually a physical table, use of views does not result in redundant data. When data in a table is updated, all views that use the data are automatically updated.

An IIS/HP SQL query requires that the programmer specify only the data needed, minimizing the amount of preplanning and coding necessary. A non-procedural interface allows the query of IIS/HP SQL databases without specifying data access path information. You can operate on entire sets of data at one time, rather than on one record at a time. The major features of IIS/HP SQL are:

• Relational data model allows you to specify required data, without specifying the retrieval method.
  
  
• Language preprocessors allow the same statements that are used for an interactive query to be embedded in HP Pascal/iX and HP COBOL II/XL application programs.
  
  
• Concurrent access allows multiple users to simultaneously access data.
  
  
• Specification of levels of access privileges maintain security for users and groups of users.
  
  
• Data independence allows you to make changes to the database structure without requiring modification of applications.
  
  
• You can define views that allow a user or group of users to see parts of one or more tables as a single, virtual, table. These act as a filter to customize a table.
  
  
• Query optimizer reduces requirement for query planning details.
  
  
• B-tree indexes support fast data access.
  
  
• User controlled transactions ensure that data is always in a consistent state.
  
  
• Automatic locking ensures data integrity in a multiuser environment by preventing access to data while it is being updated.
  
  
• Automatic rollback recovery preserves logical data integrity due to a soft crash.
  
  
• You can invoke rollback capability in a program to allow erroneous data, usually generated in an on-line situation, to be removed before the transaction is completed.
  
  
• Rollforward recovery preserves logical and physical data integrity due to a hard crash.
  
  
• Dynamic restructuring allows you to change data structure, table capacities, and security without unloading and loading the database.
  
  
• Null data values allow use of fields that are relevant to some, but not all, records in a table.

IIS/HP SQL does not require you to define explicit relationships between data sets. When you perform a query, it determines relationships by matching values between fields common to two or more data sets. If you consider a data set to be analogous to a table, a given field in the data set would occur as a column in the table. Data from any number of tables that share a common column can be related, as needed, and you can define them to be an IIS/HP SQL database. The query optimizer determines the best data access strategy based on factors such as the presence of indexes and the relative sizes of accessed tables.

Interactive SQL (ISQL) is the interactive interface to IIS/HP SQL. It provides the functionality of a data definition language (DDL) and a data manipulation language (SQL). DDL allows the database administrator to control all aspects of database creation and modification. SQL allows the programmer or frequent user to interactively query a database.

Security is maintained by allowing specification of appropriate levels of access privileges to individual users or groups of users.

IIS/HP SQL allows read access and write access privileges to be assigned at the table level. Read or write access restriction at a finer granularity than the table level, such as at the column level, may be obtained by defining a view of the table that omits the sensitive information. Modification authority may be granted at the column level without requiring that a view be specified. Write access may be assigned to allow any combination of the following capabilities:

• Row modification
• Row insertion
• Row deletion

The database administrator (DBA) assigns access privileges by 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 requirements may also be granted privileges directly.

A logical transaction is a series of database modifications of which either all or none must be performed to leave the database in a consistent state. The particular grouping of modifications defined by the user to be a transaction varies depending on the particular application. An example of a transaction is an accounting entry to pay a bill: the credit to the cash account and debit to the accounts payable account must both be performed to avoid leaving the accounts out of balance. This concept of a logical transaction is essential to ensuring that data integrity is maintained when multiple users are concurrently accessing the database or in the event of system failure.

IIS/HP SQL preserves data integrity, when multiple users are accessing a database, through a comprehensive locking scheme based on the transaction concept. When a user begins a transaction, a lock is automatically granted for each page read or modified by the transaction. (A page is a unit of data storage that contains 4,096 characters.) This ensures that no one else may update the data on those pages while the user is reading or updating them. If data is only being read, then other users are not prevented from reading it simultaneously; they just can't update it. If data is being updated, however, data integrity is ensured by preventing the other users from reading or updating the data. When the user's transaction is completed, all acquired locks are automatically released.

Tables (data sets) may also be blocked explicitly. This feature is provided to allow the programmer greater flexibility in applications where it is advantageous to lock large portions of the database. Since explicit locks reduce concurrency, they are not recommended for general use.

IIS/HP SQL ensures that the logical and physical integrity of the database is protected in the event of a program abort, system failure, or destruction of the media on which the database resides.

Rollback recovery is an automatically activated recovery feature that ensures that the database is always in a logically consistent state. IIS/HP SQL logs write transactions to a log file on disc. In the event of a system failure or program abort, IIS/HP SQL uses this log file to automatically back out any partially completed transactions.

The rollback capability may also be invoked in an IIS/HP SQL program. This is a particularly valuable feature in an on-line application, as it allows a user who has entered incorrect information to nullify the transaction before its completion.

Rollforward recovery protects the physical and logical integrity of the database against media failure. In the event of a hardware or software failure, the transactions from the log file are reapplied to a backup copy of the data to bring it up to the current state.

A simultaneously updated copy of the log files used for rollback or rollforward recovery may be kept on another disc to provide additional protection in the case of disc failure.

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