Beginning SQL

The other statement that the ANSI SQL standard defines is the ROLLBACK statement.
ROLLBACK
If for any reason you need to undo the transaction as a whole, execute the ROLLBACK statement.
ROLLBACK tells the DBMS that all changes to data caused by all SQL statements since the last COMMIT
statement must be undone. It is very important to understand where the transaction begins. Many
interactive applications perform a COMMIT after each statement the user enters, which essentially
means that ROLLBACK is unavailable. On the other hand, many of these same DBMSs when used
programmatically start a transaction with the first SQL statement executed by the program and never
perform a COMMIT until explicitly told to do so. Thus, a ROLLBACK statement undoes every statement
since the program began or back to the first statement after the last COMMIT. Figure 11-2 shows a set
of three SQL statements occurring and then a ROLLBACK statement. The ROLLBACK causes the previous
three statements to be undone. After the ROLLBACK, the database is left in the same state as if the first
three SQL statements had never even been performed.
Rolled Back
Transaction
Undo All
Changes
Figure 11-2
Select
Update
Insert
Roll Back
Database
Transactions
COMMIT and ROLLBACK are executable SQL statements, which for this discussion are called transaction
statements or keywords. In the ANSI model, either of these transaction statements is valid after at least one
nontransaction statement such as SELECT, INSERT, DELETE, and so on.
In fact, many DBMSs perform a COMMIT after every statement entered directly into the user interface.
This is known as autocommit and can be turned on or off in some products. In other words, you receive
a COMMIT immediately after each SQL statement typed in, and a ROLLBACK is not valid in that environment.
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