Creating Tables with Identity Columns in SQL Server

What is an Identity Column?

An identity column in SQL Server is a numeric column that automatically generates sequential numbers when new rows are inserted.

It is commonly used for primary keys to uniquely identify each record without manual input.

• Automatically increments numeric values for new rows.
• Defined using the IDENTITY property with seed and increment values.
• Simplifies row identification and key management.

Syntax for Creating Identity Columns

To create an identity column, include the IDENTITY(seed, increment) property in the column definition within the CREATE TABLE statement.

The seed specifies the starting value, and the increment defines how much the value increases for each new row.

• IDENTITY(1,1) starts numbering at 1 and increments by 1.
• You can customize seed and increment to fit your requirements.

Example: Creating a Table with an Identity Column

Here is an example of creating a simple table named Employees with an identity column EmployeeID as the primary key.

Inserting Data into Tables with Identity Columns

When inserting data, you do not specify a value for the identity column; SQL Server automatically generates it.

If you need to insert explicit values into the identity column, you must enable IDENTITY_INSERT for that table temporarily.

• Default inserts omit the identity column.
• Use SET IDENTITY_INSERT [table] ON to insert explicit values.
• Remember to set IDENTITY_INSERT OFF after.

Limitations and Considerations

Identity columns are not guaranteed to be gapless; deletions or transaction rollbacks can cause gaps in the sequence.

They are specific to a single table and cannot be shared across tables.

For more complex scenarios, consider using sequences.

• Gaps in identity values can occur.
• Identity values are unique per table.
• Sequences offer more flexibility if needed.