1. Sections 17– database transactions and controlling User Access
Database Programming
Sections 17– database transactions and controlling User Access
10/3/06

2. Review DDL - defines a table and changes to the table
DML - manipulates the data in a table
DCL - controls who can use the table
Relate the data transaction terms learned in the lesson to the DML statements learned earlier. When data is being entered, updated or deleted, mistakes can be made. The data transaction processes COMMIT, SAVEPOINT and ROLLBACK enable users to make the task of data entry easier and more efficient. Control points prevent having to correct costly mistakes or having to re-enter data.
Oracle HTML DB does not allow ROLLBACKS and SAVEPOINTS, and implements an automatic COMMIT. Any time you send a transaction across Oracle HTML DB, an implicit COMMIT happens.
Because the COMMIT happens, there is no way for a ROLLBACK occur. The connection from HTTP is not a persistent connection to the database. Therefore, there is no concept of doing COMMIT or ROLLBACK of DML statements. Students can learn about COMMIT and ROLLBACK concepts in this class, but using Oracle HTML DB to demonstrate a ROLLBACK, COMMIT, or SAVEPOINT is not an option.
Marge Hohly

3. Transactions
Allow users to make changes to data then decide whether to save or not the work.
Database transactions bundle multiple steps into a logical unit.
Transactions consist of one of the following:
DML statements which constitute one consistent change to the data. The DML processes include INSERT, UPDATE, DELETE and MERGE
one DDL statement such as CREATE, ALTER, DROP, RENAME or TRUNCATE
one DCL statement such as GRANT or REVOKE
Explain the concept of a transaction using the bank analogy.
A bank database contains balances for various customer accounts, as well as total deposit balances for other branches. Suppose a customer wants to withdraw and transfer money from their account and deposit it into another customer's account at a different branch. There are several separate steps involved to accomplish this rather simple operation. Both bank branches want to be assured that either all steps in the transaction happen, or none of them happen and if the system crashes, the transaction is not left partially complete. Grouping the withdrawal and deposit steps into a transaction provides this guarantee. A transaction either happens completely or not at all.
Marge Hohly

4. Transactions COMMIT ROLLBACK SAVEPOINT ROLLBACK TO SAVEPOINT
When user wants to save group of changes and make pending changes permanent
ROLLBACK
Discards changes made to database, all pending changes are discarded.
SAVEPOINT
Creates a marker in a transaction, which divides the transaction into smaller pieces.
ROLLBACK TO SAVEPOINT
Allows user to roll back the current transaction to a specified savepoint. Discards only changes after the SAVEPOINT
Marge Hohly

5. Example UPDATE d_cds SET cd_number = 96
WHERE title = 'Graduation Songbook';
SAVEPOINT one
INSERT INTO d_cds(cd_number, title, producer, year)
VALUES(100, 'Go For It', 'The Music Man', 2004) );
SET cd_number = 101
ROLLBACK TO SAVEPOINT one
COMMIT;
Marge Hohly

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