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How Transactions Guard Your Database
The transaction is a sequence of SQL data manipulation statements which does a logical unit of work. The Oracle treats the sequence of SQL statements as a unit so that all the changes brought about by the statements are either committed (made permanent) or rolled back (undone) at similar time. If your program fails in the center of a transaction, then the database is automatically restored to its previous state.
The first SQL statements in your program begin a transaction. Whenever a transaction ends, the next SQL statement automatically begins the other transaction. And hence, every SQL statement is part of the transaction. The distributed transaction involves at least one SQL statement which updates the data at multiple nodes in a distributed database.
The COMMIT and ROLLBACK statements assures that all the database changes brought about by the SQL operations are either made permanent or undone at similar time. All the SQL statements executed as the last commit or rollback make up the present transaction. The SAVEPOINT statement names and marks the present point in the processing of a transaction.
CHECK Constraints in SQL A CHECK constraint is a table constraint defined using the key word CHECK, as already illustrated in several examples in this chapter. In particular,
Example of ADD CONSTRAINT in SQL Example: Alternative formulation for MAX_ENROLMENTS ALTER TABLE IS_ENROLLED_ON ADD CONSTRAINT MAX_ENROLMENTS CHECK ((SELECT COUNT (*)
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Example of Alternative formulation as a table constraint Example: Alternative formulation as a table constraint ALTER TABLE EXAM_MARK ADD CONSTRAINT Must_be_enrolled_to_
Using SET TRANSACTION You use the SET TRANSACTION statement to begin the read-only or read-write transaction, start an isolation level, or assign your present transaction to a
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