Understanding @@IDENTITY and SCOPE_IDENTITY() in SQL Server

What Are @@IDENTITY and SCOPE_IDENTITY()?

When working with SQL Server, you’ll often encounter scenarios where you need to retrieve the last generated identity value after inserting data into a table. This is especially common when dealing with tables that have an identity column—a column that automatically generates unique values. SQL Server provides two functions for this purpose: @@IDENTITY and SCOPE_IDENTITY(). While they may seem similar, they behave differently in certain situations. Let’s break down what they are, how they work, and when to use each one.

Key Differences Between @@IDENTITY and SCOPE_IDENTITY()

Practical Example

Let’s look at an example to understand how these functions work in practice.

Scenario:

• You have a table called MyTable with an identity column ID.
• You want to insert rows into the table and retrieve the last generated identity value.

Table Definition:

CREATE TABLE MyTable (
ID INT IDENTITY(1,1) PRIMARY KEY,
Column1 VARCHAR(50),
Column2 VARCHAR(50)
);
	

Example Queries:

-- Insert a row and retrieve the last identity value using @@IDENTITY
INSERT INTO MyTable (Column1, Column2) VALUES ('Value1', 'Value2');
SELECT @@IDENTITY AS LastIdentityValue;

-- Insert another row and retrieve the last identity value using SCOPE_IDENTITY()
INSERT INTO MyTable (Column1, Column2) VALUES ('Value3', 'Value4');
SELECT SCOPE_IDENTITY() AS LastIdentityValueInScope;
	

Explanation:

1. First Query:
   • The INSERT statement adds a row to MyTable.
   • @@IDENTITY returns the last identity value generated in the current session. If a trigger inserts data into another table with an identity column, @@IDENTITY will return the identity value from that table, not necessarily from MyTable.
2. Second Query:
   • The INSERT statement adds another row to MyTable.
   • SCOPE_IDENTITY() returns the last identity value generated within the current scope. It will always return the identity value from MyTable, regardless of any triggers or other operations.

When to Use @@IDENTITY vs. SCOPE_IDENTITY()

Use @@IDENTITY:

• Rarely recommended.
• Only use it if you explicitly need the last identity value generated in the entire session, including those from triggers or other tables.

Use SCOPE_IDENTITY():

• Recommended for most scenarios.
• Use it when you want the last identity value generated within the current scope (e.g., the current stored procedure or batch).
• It ensures you get the correct identity value without interference from triggers or other processes.

Common Pitfalls to Avoid

1. Using @@IDENTITY with Triggers:
   • If your table has triggers that insert data into another table with an identity column, @@IDENTITY will return the identity value from the triggered table, not the original table. This can lead to incorrect results.
2. Assuming SCOPE_IDENTITY() Works Across Sessions:
   • SCOPE_IDENTITY() only works within the current scope. If you’re working across multiple sessions or connections, you’ll need to handle identity values differently.
3. Ignoring Multiple Rows:
   • If you insert multiple rows in a single statement, both @@IDENTITY and SCOPE_IDENTITY() will return the identity value of the last inserted row. If you need all identity values, consider using the OUTPUT clause instead.

Best Practices

1. Prefer SCOPE_IDENTITY():
   • It’s safer and more predictable in most scenarios.
2. Avoid @@IDENTITY Unless Necessary:
   • Its behavior can be unpredictable, especially when triggers are involved.
3. Test Thoroughly:
   • Always test your queries to ensure you’re retrieving the correct identity value.
4. Use OUTPUT Clause for Multiple Rows:
   • If you’re inserting multiple rows and need all identity values, use the OUTPUT clause to capture them.

Final Thoughts

Understanding the difference between @@IDENTITY and SCOPE_IDENTITY() is crucial for working with identity columns in SQL Server. While both functions serve a similar purpose, SCOPE_IDENTITY() is generally the better choice due to its precision and reliability. On the other hand, @@IDENTITY should be used sparingly and only when you specifically need the last identity value generated in the entire session.

By following best practices and testing your queries thoroughly, you can avoid common pitfalls and ensure your database operations run smoothly.