3 SEQUENCE Automatically generates sequence number (For rows in tables) Is sharable among tables. Typically used to create a primary key value (Unique numbers and may have gaps) Stored and generated independently of tables. So the same sequence may be used in several tables.
4 CREATE SEQUENCE: CREATE SEQUENCEsequence [INCREMENT BY n] --negative number to decrement [START With n] [MAXVALUE n| NOMAXVALUE ] [MINVALUE n| NOMINVALUE ] [CYCLE | NOCYCLE ] --start over or not [CACHE n | NOCACHE ]; --n numbers stored in memory for efficiency; default is 20;
5 CREATE SEQUENCE: Example: CREATE SEQUENCE id_seq INCREMENT BY 1 START WITH 1000 MAXVALUE NOCACHE NOCYCLE ;
6 CONFIRMING SEQUENCE: Since it is an object, it can be query: SELECT sequence_name, min_value, max_value, increment_by, last_number --next number stored FROMuser_sequences; -- or use SEQ
7 CONFIRMING SEQUENCE: NEXTVAL: --next available sequence number CURRVAL: --present value NEXTVAL & CURRVAL can not be used in: SELECT list of a view SELECT with distinct clause SELECT with group by SELECT having SELECT order by SELECT union/intersect/minus Subquery in SELECT, INSERT, or UPDATE
8 USING A SEQUENCE: INSERT INTO student (id, name) VALUES(id_seq.NEXTVAL, '&name'); View the current value of the sequence id_seq: SELECTid_seq.CURRVAL FROMdual;
9 USING A SEQUENCE: SELECT id_seq.CURRVAL, id_seq.NEXTVAL FROM dual; --if currval and nextval is used in the same query, both return the same number.
10 USING A SEQUENCE: INSERT INTO student_course (Id, C_num, Dept_name) VALUES(Stud_Seq.NEXTVAL, Course_Seq.NEXTVAL, ’&Dept_name’);
11 Gaps in sequence values: it is possible to have gaps in the sequence: A rollback The system crashes A sequence is used in another table
12 Modifying a sequence: ALTER SEQUENCE id_seq INCREMENT BY 10 START WITH 1000 MAXVALUE NOCACHE NOCYCLE;
13 Rules for Modifying a Sequence: You must be the owner or have the ALTER privilege Only future sequence numbers are affected To restart a sequence, the sequence must be dropped and recreated.
14 Dropping a Sequence: DROP SEQUENCE id_seq;
15 Question? id_seq is 150; We would like to change the lastvalue to 350.