ALLBASE/SQL Reference Manual (36216-90216)

48 Chapter1

Introduction

ALLBASE/SQL Data Access

The DBEnvironment determines both what data can be accessed in a transaction and

what data can be recovered. Following a failure, a transaction can be recovered, or all data

can be recovered, as follows:

• A transaction is one or more SQL statements that together perform a unit of work on

one or more databases in a DBEnvironment. Work done within a transaction can be

made permanent (committed) or undone (rolled back).

• After a system or hardware failure,

all

data within a DBEnvironment is recovered to a

consistent state. Changes performed by any transactions incomplete at failure time are

rolled back. Changes performed by transactions completed before failure time are made

permanent.

You can have more than one DBEnvironment on your system. When you connect to a

DBEnvironment, ALLBASE/SQL establishes a DBE session for you. The query processor

can process statements only when you are in a DBE session. You can access any

DBEnvironment in either of the two following modes:

• Single-user mode—only one user or program can use a DBEnvironment.

• Multiuser mode—more than one user and/or program can use a DBEnvironment at

the same time.