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Difference between sequence and identity column in SQL Server

In SQL Server, both sequences and identity columns are used to generate unique numeric values. However, there are some key differences between the two:

  1. Association with Tables: Identity columns are directly associated with a specific table, whereas sequences are standalone objects that are not directly tied to any table. This means that a sequence can be shared across multiple tables or even multiple databases, providing more flexibility in generating unique identifiers.
  2. Generation Control: Identity columns are automatically generated by the database engine during the insert operation. The values are assigned in a sequential manner based on the defined increment and seed values. On the other hand, sequences require explicit retrieval of the next value using the NEXT VALUE FOR syntax. This gives you more control over when and where you want to use the generated values.
  3. Reuse of Values: By default, identity columns in SQL Server do not reuse values, meaning once a value is used, it is not used again even if a row is deleted. In contrast, sequences can be configured to cycle back to the minimum value when the maximum value is reached. This allows reusing values within the specified range.
  4. Scalability: In terms of scalability, sequences provide more flexibility than identity columns. Since sequences are not bound to a specific table, they can be easily shared across multiple tables or even multiple databases. This can be beneficial in scenarios where you need to generate unique identifiers across different entities or when you have distributed systems.

Overall, the choice between using a sequence or an identity column depends on the specific requirements of your application. Identity columns are simpler to use and automatically generate values during insert operations, while sequences provide more control, flexibility, and reusability of values.

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