Row-level security (RLS) in SQL Server allows you to control access to rows in a database table based on the characteristics of the user executing a query. It can be particularly useful for multi-tenant applications and scenarios where you want to ensure that users see only the data they are authorized to see.
To implement row-level security in SQL Server, you typically follow these steps:
1. Create a Security Policy
You first create a security policy that will apply the filtering logic to your tables. This policy will be associated with the user-defined function that defines the security predicate.
2. Create a Filter Function
Define a table-valued function that returns a table with a condition that determines which rows the current user can access. The function typically checks the user's context or roles.
3. Apply the Security Policy to the Table
You then associate the policy and the filter function with the relevant table.
Example Steps to Implement RLS
Below is an example illustrating how to implement RLS:
Step 1: Create a Sample Table
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
Name NVARCHAR(50),
Department NVARCHAR(50),
UserID NVARCHAR(50) -- This will be used for RLS
);
-- Insert sample data
INSERT INTO Employees (EmployeeID, Name, Department, UserID)
VALUES
(1, 'Alice', 'HR', 'user1'),
(2, 'Bob', 'IT', 'user2'),
(3, 'Charlie', 'HR', 'user1');
Step 2: Create the Security Predicate Function
This function will define the filtering logic.
CREATE FUNCTION dbo.fn_securitypredicate(@UserID AS NVARCHAR(50))
RETURNS TABLE
WITH SCHEMABINDING
AS
RETURN SELECT 1 AS result
WHERE @UserID = USER_NAME(); -- Compare the UserID with the current user
Step 3: Create the Security Policy
Now, create a security policy and associate it with the table and the filtering function.
CREATE SECURITY POLICY EmployeeSecurityPolicy
ADD FILTER PREDICATE dbo.fn_securitypredicate(UserID) ON dbo.Employees
WITH (STATE = ON);
Step 4: Testing the RLS
You can now test the RLS implementation by checking what users see based on their UserID.
1. Log in as User1:
EXECUTE AS USER = 'user1';
SELECT * FROM Employees;
REVERT;
Expected output:
EmployeeID | Name | Department | UserID
------------|---------|------------|--------
1 | Alice | HR | user1
3 | Charlie | HR | user1
2. Log in as User2:
EXECUTE AS USER = 'user2';
SELECT * FROM Employees;
REVERT;
Expected output:
EmployeeID | Name | Department | UserID
------------|------|------------|--------
2 | Bob | IT | user2
Important Considerations
1. Complexity: While RLS can enhance security, it can also increase complexity. Be sure to document your security policies and functions.
2. Performance: Filtering may have performance implications, especially on large datasets. Always test the performance impact.
3. Nested Queries: Be cautious with joins and aggregations, as RLS predicates apply to the rows returned.
4. Audit: It's essential to audit the security policies and functions to ensure they are functioning as intended.
With these steps, you can implement row-level security in SQL Server to fine-tune access control based on user identity.