How to Create an Identity Column in Oracle

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Summary: Learn how to efficiently create and manage identity columns in Oracle databases, including step-by-step instructions and key considerations for using this feature.
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Creating an identity column in Oracle databases is a straightforward task that can greatly simplify the process of generating unique identifiers for rows within a table. This functionality is especially crucial for handling primary keys and is used extensively in database management for ensuring data integrity and performance. Here’s a guide on how to implement identity columns in Oracle.

What is an Identity Column?

An identity column automatically generates sequential values every time a new row is inserted into the table. It essentially replaces the manual sequence creation and assignment process, making it easier to manage data entries and reduce the likelihood of errors.

Steps to Create an Identity Column in Oracle

Oracle introduced identity columns in Oracle 12c, providing a more direct approach compared to the earlier versions that required manually creating sequences and triggers. Here’s how you can create an identity column in a new table:

Creating a New Table with an Identity Column
To create a new table with an identity column, you can specify the GENERATED AS IDENTITY clause in your CREATE TABLE statement. Here’s an example:

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In this example, employee_id is an identity column. The GENERATED ALWAYS option tells Oracle to always generate a new value for the column whenever a new row is added. You can also use GENERATED BY DEFAULT AS IDENTITY if you want to allow the possibility of manually inserting a value into the identity column under specific circumstances.

Modifying an Existing Table to Add an Identity Column

If you need to add an identity column to an existing table, you first need to ensure that the table does not already contain data that might conflict with the identity mechanism. You can add an identity column using the ALTER TABLE command like so:

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This command adds an identity column named employee_id to the employees table. The GENERATED BY DEFAULT option allows for the manual insertion of values into the identity column.

Key Considerations

Uniqueness: Ensure that the identity column is used for data that requires unique values. It is commonly used for primary keys.

Performance: Using identity columns can improve the performance of inserts because the database handles value generation.

Backward Compatibility: If you are upgrading from an older version of Oracle or porting a database, consider the impact of changing to identity columns.

Conclusion

Identity columns in Oracle provide an efficient and error-free way to manage auto-incrementing fields within your database tables. By following the guidelines and commands outlined above, you can set up identity columns that will help maintain the integrity and uniqueness of your data across the database.