As per BOL (https://msdn.microsoft.com/en-us/library/ff878058(v=sql.110).aspx) : A sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The sequence of numeric values is generated in an ascending or descending order at a defined interval and may cycle (repeat) as requested. Sequences, unlike identity columns, are not associated with tables. An application refers to a sequence object to receive its next value. The relationship between sequences and tables is controlled by the application. User applications can reference a sequence object and coordinate the values keys across multiple rows and tables.
This feature is really a good to have and this feature is same as Oracle’s sequence objects. A sequence object generates sequence of unique numeric values as per specifications mentioned.
A sequence can be defined as any integer data type. If the data type is not specified, a sequence defaults to bigint. A sequence value is not effected by SQL server restart. Many people think sequence is similar to identity. But big difference is sequence object is independent of a table while identity columns are attached to a table.
Create Sequence
CREATE SEQUENCE TEMPSEQ AS INT
START WITH 1 — START WITH VALUE 1
INCREMENT BY 1– INCREMENT WITH VALUE 1
MINVALUE 0 — MINIMUM VALUE TO START IS 0
MAXVALUE 5 — MAXIMUM IT CAN GO TO 5
NO CYCLE — DO NOT GO ABOVE 5
CACHE 2 — INCREMENT 2 VALUES IN MEMORY RATHER THAN INCREMENTING FROM IO
Generate value from Sequence
SELECT NEXT VALUE FOR TEMPSEQ AS seq_no;
Error when Sequence reach to MAX value
Msg 11728, Level 16, State 1, Line 18
The sequence object ‘TEMPSEQ’ has reached its minimum or maximum value. Restart the sequence object to allow new values to be generated.
Alter Sequence to RESTART it from 1 or desired value
ALTER SEQUENCE TEMPSEQ RESTART WITH 1
Create Sequence with CYCLE mode, where value will be reset to MIN value when it reaches to MAX value
CREATE SEQUENCE TEMPSEQ_CYCLE AS INT
START WITH 1 — START WITH VALUE 1
INCREMENT BY 1– INCREMENT WITH VALUE 1
MINVALUE 0 — MINIMUM VALUE TO START IS 0
MAXVALUE 5 — MAXIMUM IT CAN GO TO 5
CYCLE — RESET to 0 once reached to MAX VALUE 5
CACHE 2 — INCREMENT 2 VALUES IN MEMORY RATHER THAN INCREMENTING FROM IO
Output of Sequence with CYCLE
SELECT NEXT VALUE FOR TEMPSEQ_CYCLE AS ID, NAME FROM SYS.OBJECTS
Alter existing Sequence to set it to CYCLE mode
ALTER SEQUENCE TEMPSEQ CYCLE — RESET to 0 once reached to MAX VALUE 5
Use Sequence in Table as Default constraint.
Scenario: We need to generate token for each customer coming to meet him with HR & finance personal.
- Create Sequence
CREATE SEQUENCE TOKENNO AS INT
START WITH 1 — START WITH VALUE 1
INCREMENT BY 1– INCREMENT WITH VALUE 1
MINVALUE 0 — MINIMUM VALUE TO START IS 0
MAXVALUE 5000 — MAXIMUM IT CAN GO TO 5000
NO CYCLE — RESET to 0 once reached to MAX VALUE 5000
CACHE 200 — INCREMENT 200 VALUES IN MEMORY RATHER THAN INCREMENTING FROM IO
- Create Table with Sequence
CREATE TABLE HR_EXECUTIVE_MEETING
(TOKENNO INT DEFAULT (NEXT VALUE FOR TOKENNO), DESKNO INT)
CREATE TABLE FINANCE_EXECUTIVE_MEETING
(TOKENNO INT DEFAULT (NEXT VALUE FOR TOKENNO), DESKNO INT)
- Insert into table having Sequence
INSERT INTO HR_EXECUTIVE_MEETING(TOKENNO,DESKNO) VALUES(DEFAULT,22)
INSERT INTO FINANCE_EXECUTIVE_MEETING(TOKENNO,DESKNO) VALUES(DEFAULT,7)
- Check the table values
SELECT * FROM HR_EXECUTIVE_MEETING
SELECT * FROM FINANCE_EXECUTIVE_MEETING
Reference: Rohit Garg (http://mssqlfun.com/)
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