You use LEVEL with the SELECT CONNECT BY statement to organize rows from a database table into a tree structure. database, there might be gaps in the sequence.Įach time you reference the NEXTVAL value of a sequence, the sequence is incremented immediately and permanently, whether you commit or roll back the transaction. applications, and some NEXTVAL values might not be stored in the Because NEXTVAL values might be referenced by different users and INSERT INTO employees_temp2 VALUES (employees_seq.CURRVAL, 'value 1') ![]() If you need to store the same value somewhere else, you use CURRVAL. INSERT INTO employees_temp VALUES (employees_seq.NEXTVAL, 'value 1') You usually use NEXTVAL to create unique numbers when inserting data. SELECT employees_seq.NEXTVAL INTO next_value FROM dual The NEXTVAL value is the same no matter what table you select from. The following example shows how to generate a new sequence number and refer to that same number in more than one statement: CREATE TABLE employees_temp AS SELECT employee_id, first_name FROM employees ĬREATE TABLE employees_temp2 AS SELECT employee_id, first_name FROM employees You can use CURRVAL and NEXTVAL only in a SELECT list, the VALUES clause, and the SET clause. To get the current or next value in a sequence, use dot notation: sequence_name.CURRVALĪfter creating a sequence, you can use it to generate unique sequence numbers for transaction processing. ![]() NEXTVAL increments the sequence and returns the next value. A reference to NEXTVAL stores the current sequence number in CURRVAL. CURRVAL returns the current value in a specified sequence.īefore you can reference CURRVAL in a session, you must use NEXTVAL to generate a number. When you create a sequence, you can specify its initial value and an increment. In PL/SQL, pseudocolumns are only allowed in SQL queries, not in INSERT / UPDATE / DELETE statements, or in other PL/SQL statements such as assignments or conditional tests.Ī sequence is a schema object that generates sequential numbers. PL/SQL recognizes the SQL pseudocolumns: CURRVAL, LEVEL, NEXTVAL, ROWID, and ROWNUM. SET TRANSACTION sets transaction properties such as read-write access and isolation level. Used with ROLLBACK, SAVEPOINT undoes part of a transaction. ![]() SAVEPOINT marks the current point in the processing of a transaction. ROLLBACK ends the current transaction and undoes any changes made since the transaction began. COMMIT makes permanent any database changes made during the current transaction. You use the COMMIT, ROLLBACK, SAVEPOINT, and SET TRANSACTION commands to control transactions. If your program fails in the middle of a transaction, Oracle detects the error and rolls back the transaction, restoring the database to its former state. It is important not to allow one operation to succeed while the other fails.Īt the end of a transaction that makes database changes, Oracle makes all the changes permanent or undoes them all. For example, two UPDATE statements might credit one bank account and debit another. A transaction is a series of SQL data manipulation statements that does a logical unit of work. Oracle is transaction oriented that is, Oracle uses transactions to ensure data integrity.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |