SQL | SEQUENCES
Sequence is a set of integers 1, 2, 3, … that are generated and supported by some database systems to produce unique values on demand.
- A sequence is a user defined schema bound object that generates a sequence of numeric values.
- Sequences are frequently used in many databases because many applications require each row in a table to contain a unique value and sequences provides an easy way to generate them.
- The sequence of numeric values is generated in an ascending or descending order at defined intervals and can be configured to restart when exceeds max_value.
CREATE SEQUENCE sequence_name START WITH initial_value INCREMENT BY increment_value MINVALUE minimum value MAXVALUE maximum value CYCLE|NOCYCLE ; sequence_name: Name of the sequence. initial_value: starting value from where the sequence starts. Initial_value should be greater than or equal to minimum value and less than equal to maximum value. increment_value: Value by which sequence will increment itself. Increment_value can be positive or negative. minimum_value: Minimum value of the sequence. maximum_value: Maximum value of the sequence. cycle: When sequence reaches its set_limit it starts from beginning. nocycle: An exception will be thrown if sequence exceeds its max_value.
Following is the sequence query creating sequence in ascending order.
- Example 1:
CREATE SEQUENCE sequence_1 start with 1 increment by 1 minvalue 0 maxvalue 100 cycle;
Above query will create a sequence named sequence_1.Sequence will start from 1 and will be incremented by 1 having maximum value 100. Sequence will repeat itself from start value after exceeding 100.
- Example 2:
Following is the sequence query creating sequence in descending order.
CREATE SEQUENCE sequence_2 start with 100 increment by -1 minvalue 1 maxvalue 100 cycle;
Above query will create a sequence named sequence_2.Sequence will start from 100 and should be less than or equal to maximum value and will be incremented by -1 having minimum value 1.
- Example to use sequence : create a table named students with columns as id and name.
CREATE TABLE students ( ID number(10), NAME char(20) );
Now insert values into table
INSERT into students VALUES(sequence_1.nextval,'Ramesh'); INSERT into students VALUES(sequence_1.nextval,'Suresh');
where sequence_1.nextval will insert id’s in id column in a sequence as defined in sequence_1.
______________________ | ID | NAME | ------------------------ | 1 | Ramesh | | 2 | Suresh | ----------------------
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