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.
Different features of sequences:
1. Sequence is database object that generate produce integer values in sequential order.
2. It automatically generates primary key and unique key values
3. It may be ascending or descending order.
4. It can be used for multiple tables.
5. Sequence numbers are stored and generated independently of tables
6. It saves a time by reducing application code.
7. It is used to generate unique integers.
8. It is used to create an auto number field.
9. Useful when you need to create a unique number to act as a primary key
10. Oracle provides an object called as a Sequence that can generate numeric values. The value generated can have maximum of 38 digits
11. Provide intervals between numbers.
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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