SQL Server 2022 Tutorial : What Is Sequence In SQL? Why Sequence?

A Sequence is a database object that generates a sequence of unique numbers. These numbers are used as primary keys for tables or for other purposes where a unique identifier is required. For example, a sequence can be used to generate unique customer IDs, order numbers, or invoice numbers. Sequences can also be used for other purposes requiring a unique number, such as generating unique session IDs for web applications.

Sequences provide a simple and efficient way to generate unique numbers in a SQL database.

Why Sequences?

Generating unique identifiers

Sequences can be used to generate unique primary keys for tables, which are used to identify each row in the table. This ensures each row has a unique identifier, even if the table has millions of rows.

Consistency

Sequences ensure that each generated value is unique and consistent.

This can be important in many applications, especially those that involve financial transactions or other sensitive data.

Flexibility

Sequences can be configured with different starting values, increment values, and maximum or minimum values. 

Sequence Syntax 

CREATE SEQUENCE customer_id_seq
START WITH 1
INCREMENT BY 1
MINVALUE 1
MAXVALUE 9999
CYCLE;
INSERT INTO customers (customer_id, customer_name, customer_email)
VALUES (NEXT VALUE FOR customer_id_seq, 'Raj Bhatt', '[email protected]');

SQL

A Sequence in SQL is created using the CREATE SEQUENCE statement, which specifies the Sequence's starting value, increment value, maximum value, and other attributes. Once a sequence is created, it can be used to generate unique numbers using the NEXTVAL or CURRVAL functions.

The NEXTVAL function returns the next available value in the Sequence, and the CURRVAL function returns the current value of the Sequence. The Sequence is automatically incremented each time a value is generated, so the next value returned by NEXTVAL will always be unique.

Sequences in SQL are commonly used to generate primary keys for tables. 

How To drop a sequence in SQL

DROP SEQUENCE customer_id_seq;

How To check all Sequence available in Database

SELECT *
FROM sys.sequences
Order by name desc

Summary

• A sequence is an object in a database that generates a sequence of unique numbers.
• Sequences are often used to generate primary key values for tables.
• Sequences can be created using the "CREATE SEQUENCE" statement in Oracle or the "SEQUENCE" object in Microsoft SQL Server.
• The starting value, increment, and other parameters of a sequence can be specified when creating it.
• To use a sequence to generate values in a table, you can reference the Sequence in an INSERT statement using the "NEXT VALUE FOR" syntax in SQL Server or the "SEQUENCE_NAME.NEXTVAL" syntax in Oracle.
• Sequences can be dropped using the "DROP SEQUENCE" statement in SQL.

Note
When using sequences, it's important to ensure that the values generated are unique and don't exceed any constraints set on the table column.

Conclusion

Thank you for reading, and I hope this blog post has helped provide you with a better understanding of Sequence in SQL.

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