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Unleashing the Power of SQL Server Sequences: Examples and Differences with Identity Columns

Introduction to SQL Server Sequences

In the world of database management systems, sequences play a crucial role in generating numeric values for various purposes. From assigning unique IDs to rows and columns to generating invoice numbers and transaction IDs, sequences can be applied in numerous ways to enhance the functionality and performance of the database.

In this article, we will explore the definition and importance of sequences in SQL Server, the syntax and usage of the CREATE SEQUENCE statement, various examples of using sequences in SQL, and the differences between sequence and identity columns. Additionally, we will discuss the application requirements, sharing numbers, resetting numbers, and when sequences should be used.

Definition and Importance of Sequences

In SQL Server, a sequence is a user-defined object that generates a series of numeric values according to a specified increment and starting value. These values can be either positive or negative integers, decimal, or floating-point numbers.

Sequences are essential because they allow developers to generate a numeric code unique to each row in a table. Typically, a sequence is assigned to a column that will use these numbers as a primary key.

For instance, suppose a table has an ID column which stores a unique integer for each record (row) inserted. In that case, a sequence can generate these IDs automatically without the need for manual input by the user.

Sequences can also be useful in generating invoice numbers and transaction IDs, where sequential number assignment is essential.

SQL Server CREATE SEQUENCE Statement

The syntax for creating a sequence in SQL Server is as follows:

CREATE SEQUENCE sequence_name
AS integer_type
START WITH start_value
INCREMENT BY increment_value
MINVALUE/NO MINVALUE
MAXVALUE/NO MAXVALUE
CYCLE/NO CYCLE
CACHE/NO CACHE

Let us explore each of these elements in detail.

sequence_name

This parameter identifies the name of the sequence to be created. It is essential to keep sequence names unique to avoid conflicts in later use.

AS integer_type

The AS keyword is used to define the data type of the sequence being created. The integer_type can be any valid integer data types according to the SQL Server data type hierarchy.

Most commonly, sequences are created with either the INT or BIGINT data types, as these types provide sufficient space for storing large sequences.

START WITH start_value

This parameter specifies the starting value for the sequence. The start_value can either be an explicit value or a reference to another column.

If no value is specified, the default is 1.

INCREMENT BY increment_value

This parameter specifies the increment value for the sequence. The increment_value can be any positive or negative integer, decimal, or floating-point number.

By default, the increment value is set to 1. MINVALUE/NO MINVALUE

The MINVALUE parameter sets the lower boundary value for the sequence and ensures that the sequence generates values no lower than this value, while NO MINVALUE specifies that no lower-bound value is defined.

MAXVALUE/NO MAXVALUE

The MAXVALUE parameter sets the upper boundary value for the sequence and ensures that the sequence generates no value higher than this value, while NO MAXVALUE specifies that no upper-bound value is defined. CYCLE/NO CYCLE

If the sequence has reached its maximum or minimum value, the CYCLE parameter specifies whether the sequence will start over or stop generating values.

If No CYCLE is used, there will be an error message if the sequence reaches the minimum or maximum value. CACHE/NO CACHE

The CACHE parameter specifies how many sequence values will be stored in memory for faster access.

This value can increase the sequence’s performance since it reduces the number of disk reads required to generate new sequence values. However, the CACHE parameter may also lead to more significant gaps in the sequence if the server fails and does not recover correctly.

SQL Server Sequence Examples

Let us explore some examples of creating sequences in SQL Server.

Simple sequence example

CREATE SEQUENCE EmployeeID
    START WITH 1000  
    INCREMENT BY 1;

In this example, we create a sequence named EmployeeID that starts with the value of 1000 and increments by 1. This sequence can now be applied as a primary key to a table, ensuring that each employee has a unique ID number.

Single table example

Suppose we have a table named Orders that contains an OrderNumber column that must have a unique integer assigned to it for each new order. 

CREATE SEQUENCE OrderIDs
    START WITH 1000  
    INCREMENT BY 1;  
    
ALTER TABLE Orders
    ADD CONSTRAINT
    PK_Orders_OrderNumber PRIMARY KEY CLUSTERED (OrderNumber);

In this example, we create a sequence named OrderIDs, which generates a unique number for each new order. The sequence is then applied to the OrderNumber column using a primary key constraint.

Multiple tables example

Suppose we have two tables Customers and Orders, both of which require unique integer values. In this case, we can use one sequence to generate values for both tables.

CREATE SEQUENCE UniqueIDs
START WITH 10000
INCREMENT BY 1;

CREATE TABLE Customers 
(
    CustomerID INT PRIMARY KEY CLUSTERED,
    Name VARCHAR(100) NOT NULL
);

CREATE TABLE Orders 
(
    OrderID INT PRIMARY KEY CLUSTERED,
    OrderNumber INT NOT NULL,
    FOREIGN KEY (OrderNumber) REFERENCES Customers(CustomerID)
);
ALTER TABLE Customers ADD CONSTRAINT PK_Customers_CustomerID PRIMARY KEY CLUSTERED (CustomerID);
ALTER TABLE Orders ADD CONSTRAINT PK_Orders_OrderID PRIMARY KEY CLUSTERED (OrderID);
INSERT INTO Customers (CustomerID, Name) VALUES (NEXT VALUE FOR UniqueIDs, N'John Doe');
INSERT INTO Orders (OrderID, OrderNumber) VALUES (NEXT VALUE FOR UniqueIDs, 1);

In this example, we create a sequence named UniqueIDs, which will generate unique values for both the Customers and Orders tables. We then apply the sequence to both tables, inserting the NEXT VALUE FOR function to access the sequence-generated value.

Sequence vs. Identity Columns

While sequences and identity columns serve similar purposes in generating unique values automatically, there are some essential differences between the two.

Identity columns are created on a per-column basis while sequences are created as objects. Identity columns can generate integers only, while sequences can generate different data types.

Identity columns are tightly linked to their respective tables, while sequences can be shared across different tables and schemas.

When to Use Sequences

Generally, sequences should be used in applications where the generation of unique numeric values is essential. Some applications that often require the use of sequences include:

  • Use in primary keys for the tables
  • Creating unique order numbers
  • Generating transaction IDs
  • Tracking inventory and serial numbers
  • Generating unique ID numbers for customers, employees, vendors, etc.

SQL Server Sequence Examples

Creating a simple sequence example

Suppose we have a table named Items and we want to add a column named item_counter. This column will hold the unique integer value for each item added to the table, and we want it to start with the value 1 and increment by 1.

To create the sequence, we use the following code:

CREATE SEQUENCE item_counter
    START WITH 1
    INCREMENT BY 1;

We can then apply this sequence to the Items table as follows:

ALTER TABLE Items
    ADD item_counter INT NOT NULL
    CONSTRAINT df_item_counter DEFAULT (NEXT VALUE FOR item_counter);

This code creates a non-null item_counter column in Items table with a default constraint of the NEXT VALUE FOR item_counter function. This means that every time a new item is added to the table, the item_counter will automatically be populated with the next value from the sequence.

Using a sequence object in a single table example

Suppose we have a Procurement schema with a purchase_orders table that needs a unique order_number generated for each new order. We can create a sequence for the order_number and use it in the table as follows:

CREATE SEQUENCE order_number
    START WITH 1
    INCREMENT BY 1;
    
CREATE TABLE purchase_orders (
    order_id INT PRIMARY KEY CLUSTERED,
    order_number INT NOT NULL,
    order_date DATE NOT NULL,
    total_amount MONEY NOT NULL
);

ALTER TABLE purchase_orders
    ADD CONSTRAINT order_number
    DEFAULT (NEXT VALUE FOR order_number) FOR order_number;

This code creates a sequence for the order_number, adds a purchase_orders table with an order_id primary key and an order_number field with a default constraint that uses the NEXT VALUE FOR order_number function to get the next value from the sequence.

Using a sequence object in multiple tables example

Suppose we have a Warehouse schema with goods_receipts and invoice_receipts tables. Both tables require a unique receipt_no value to be generated for each new item.

We can create a sequence for the receipt_no and use it in both tables as follows:

CREATE SEQUENCE receipt_no
    START WITH 1
    INCREMENT BY 1;

CREATE TABLE goods_receipts (
    receipt_id INT PRIMARY KEY CLUSTERED,
    receipt_no INT NOT NULL,
    receipt_date DATETIME NOT NULL,
    supplier VARCHAR(50) NOT NULL
);

ALTER TABLE goods_receipts
    ADD CONSTRAINT goods_receipts_receipt_no
    DEFAULT (NEXT VALUE FOR receipt_no) FOR receipt_no;

CREATE TABLE invoice_receipts (
    receipt_id INT PRIMARY KEY CLUSTERED,
    receipt_no INT NOT NULL,
    invoice_date DATETIME NOT NULL,
    customer VARCHAR(50) NOT NULL
);

ALTER TABLE invoice_receipts
    ADD CONSTRAINT invoice_receipts_receipt_no
    DEFAULT (NEXT VALUE FOR receipt_no) FOR receipt_no;

This code creates a sequence for the receipt_no, adds goods_receipts and invoice_receipts tables with a primary key of receipt_id and a field for receipt_no. Both tables have a default constraint that uses the NEXT VALUE FOR receipt_no function to get the next value from the sequence.

Sequences vs. Identity Columns

Differences between Sequences and Identity Columns

While sequences and identity columns serve similar purposes, there are some notable differences between the two. Minimum and maximum increment values: Sequences can increment values by more than one at a time, whereas identity columns can only increment by one at a time.

Resetting the increment value: Sequences can have their increment value reset to a new value manually, whereas identity columns’ increment values cannot be reset. Caching values: Sequences can cache their values in memory, whereas identity columns do not and may have to read values from disk every time they need to generate a new value.

Using in multiple tables: Sequences can be used in multiple tables, whereas each identity column is attached to one table.

Similarities between Sequences and Identity Columns

Both sequences and identity columns generate a sequence of numbers automatically and can be used to create unique, sequential integer values for tables in a SQL Server Database.

When to Use Sequences

Sequences are an important feature of SQL Server and have various use cases. In this section, we will discuss situations where sequences should be used.

Requiring a number before inserting values

In SQL Server, a sequence can be used to generate a unique value for a column before inserting data, making working with databases more efficient. By doing this, developers don’t have to worry about specifying a value for the column before inserting data, saving time and resources.

Sharing numbers across tables or columns

A sequence can be used to share a unique number across multiple tables or columns. This can be essential when storing related data in different tables, and developers need to reference data between them.

Sequences can provide this functionality by generating a sequence of unique numbers that can be shared across tables and columns.

Resetting the number

Sequences offer the functionality to reset numbers, which is useful when reusing a table for new data while preserving the previous data. This reduces the potential for errors in data entry and ensures that the new data is kept separate from previously entered data.

Assigning multiple numbers at once

A sequence can also assign multiple numbers at once. This is useful when dealing with bulk data inserts.

Rather than calling the NEXT VALUE FOR sequence_name function for each row inserted, multiple values can be obtained in one call. This saves on valuable SQL resources, allowing data to be inserted into the database at a quicker pace.

Conclusion

In summary, SQL Server Sequences are an essential feature for generating a series of unique numeric values used for various purposes, including primary key values for tables, generating invoice numbers and transaction IDs, assigning unique numbers for customers, employees, vendors, course IDs, and many more. Sequences are created using the CREATE SEQUENCE statement, which defines the data type for the sequence and its increment and starting values.

Sequences differ from identity columns in that they remember the last generated value and allow for resetting or assigning multiple values at once. Sequences are best used when unique numeric values are required before inserting data, sharing numbers across tables or columns, resetting the number, or assigning multiple numbers at once.

Overall, SQL Server Sequences offer a powerful and flexible way to ensure that the database data is unique and consistent while maintaining high performance and reliability. When used correctly, sequences can greatly enhance the functionality of any SQL Server database, and they are an essential tool for any database professional.

SQL Server Sequences are an essential feature in generating unique numeric values for various purposes. With the ability to create a series of numeric values and manipulate them to fit application requirements, it is important to understand the syntax and usage of the CREATE SEQUENCE statement.

Sequences can be used to assign unique IDs to rows and columns, generate invoice numbers and transaction IDs, and track inventory and serial numbers, among other use cases. Understanding the differences between sequences and identity columns can help database professionals to choose the best option for their specific database requirements.

Overall, SQL Server Sequences offer a powerful and flexible way to ensure that the database data is unique and consistent while maintaining high performance and reliability. The key takeaway is to explore and experiment with sequences to unlock their full potential and reap their benefits in creating a robust and efficient database management system.

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