SEQUNCES Lecture_Notes_Unit4_chapter11_sequence
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This document provides an overview of sequences in RDBMS, detailing their creation, characteristics, and usage within Oracle databases. It explains the create sequence command, the function of pseudo-columns like currval and nextval, and includes examples of creating and altering sequences. Additionally, it outlines the process of inserting sequence numbers into a database table.
SEQUNCES Lecture_Notes_Unit4_chapter11_sequence
- 1. RDBMS - Unit IV Chapter 11 SEQUENCES Prepared By Dr. S.Murugan, Associate Professor Department of Computer Science, Alagappa Government Arts College, Karaikudi. (Affiliated by Alagappa University) Mailid: muruganjit@gmail.com Reference Book: LEARN ORACLE 8i, JOSE A RAMALHO
- 2. Sequences ➢ A sequence is simply an automatic counter, which is enabled whenever it is accessed. ➢ ➢ This guarantees that there are no two rows with the same code. ➢ A default sequence has the following characteristics: • Always starts from the number 1. • In ascending order. • Increases by 1.
- 3. The CREATE SEQUENCE Command ➢ The SQL command used to create a sequence is the CREATE SEQUENCE command.
- 4. The CREATE SEQUENCE Command ➢ START WITH: Indicates the initial value of the sequence. ➢ INCREMENT BY: Indicates the value by which the sequence will be incremented each time it is accessed. ➢ MAXVALUE: Indicates the maximum value of the sequence. When omitted, the maximum value of the sequence can be 1.00e +27. ➢ NOMAXVALUE: Indicates that the sequence doesn’t have a predefined maximum value. ➢ CYCLE/NOCYCLE: CYCLE indicates that the sequence should return to the initial value when the maximum value is reached.
- 5. Pseudo-Columns ➢ Oracle has several pseudo-columns that behave as an extra column when a new table is created. ➢ The currval and nextval pseudo-columns are directly related to sequences. There are two additional pseudo- columns: level and rownum Currval: Returns the current value of the sequence. Nextval: Increases the value of the sequence and returns the next value. Rowid: It is used to individualize each row of a table. rowid offers a faster and more practical way to access a row in the entire database.
- 7. Example for ROWNUM ➢ The rownum pseudo-column receives a number that corresponds to the position of a row in the set of rows returned by a query.
- 8. Creating a Sequence – Example 1 ➢ The following example illustrates the creation of the Newseq sequence using the default values: ➢ SQL> create sequence newseq;
- 9. Checking a User’s List of Sequences To check the sequences that were created for a user and the values of their parameters, you use a table called USER_SEQUENCES.
- 10. Sequence – Example2 ➢ create a sequence with an initial value, an increment value, and the size of the cache specified by the current user:
- 11. Sequence – Example3 ➢ In this next example, the sequence is created with minimum and maximum values: SQL> create sequence newseq3 minvalue 1000 maxvalue 4000; Sequence created:
- 12. Deleting a Sequence ➢ The DROP SEQUENCE command removes the sequence from the schema in which it was created. In this example, Newseq2 is removed: SQL> drop sequence newseq2;
- 13. Changing a Sequence ➢ With the ALTER SEQUENCE command the user can change some of the sequence’s parameters. However, there are several restrictions. ➢ The initial value can not be altered ➢ The minimum value for the table cannot be greater than the current value. ➢ The increment value can be changed without any problems. In the next example, two successful changes were made to the Newseq2 sequence: SQL> alter sequence newseq2 minvalue 30 increment by 20; Sequence altered.
- 14. Using a Sequence ➢ SQL>create newseq3 minval 1000 maxval 1005. ➢ The sequence is enabled the first time it is used, and displays its initial value: SQL> select newseq3.nextval from dual; NEXTVAL ------- 1000 ➢ The second time it is used, it displays the value incremented by 1. SQL> run NEXTVAL ------- 1001
- 15. Using a Sequence ➢ When we access to currval, it does not change the value of the sequence: SQL> select newseq3.currval from dual; CURRVAL ------- 1001 SQL> run CURRVAL ------- 1001
- 16. Using a Sequence ➢ Inserting Sequence number to the database. 1. Create a table 2. create a sequence 3. Insert sequence into a table. 3. View the records from table. 1. Create table product(sno number(3), item_name char(20), bill_date date); 2. Create sequence serial minvalue 1 maxvalue 100; 3. Insert into product values (serial.nextval,’RICE’,’14- JUL-19’); 4. Repeat the step 3 as you want to insert. 5. Select * from product;