SQL Server MASK function

SQL Server introduced a feature called "Dynamic Data Masking" (DDM) that allows you to create masking rules to obfuscate sensitive data in database columns. This feature helps protect sensitive data while still allowing authorized users to access the data.

To create a masking function in SQL Server, you don't explicitly create a "MASK" function, but rather you define masking rules for specific columns. You can use predefined masking functions, such as `default()`, `email()`, `random()`, or create your custom masking functions using Transact-SQL. Here's an example of how to create a custom masking function in SQL Server:

Suppose you have a table called `Employees`, and you want to mask the `SocialSecurityNumber` column. You can create a custom masking function to mask the last four digits of the social security number while displaying the rest as "XXX-XX-1234."

1. Create a custom masking function:

```sql

CREATE FUNCTION dbo.CustomMaskingFunction (@inputString NVARCHAR(100))

RETURNS NVARCHAR(100)

WITH SCHEMABINDING

AS

BEGIN

    RETURN CONCAT('XXX-XX-', RIGHT(@inputString, 4));

END;

```

In this example, we created a function that takes an input string (the social security number) and returns the masked value. It masks all but the last four digits of the SSN.

2. Define a masking policy:

```sql

CREATE MASKING POLICY CustomMaskingPolicy

WITH (FUNCTION = 'dbo.CustomMaskingFunction');

```

This policy specifies that the custom masking function `dbo.CustomMaskingFunction` should be used to mask data in the columns where this masking policy is applied.

3. Apply the masking policy to a specific column in your table:

```sql

ALTER TABLE Employees

ALTER COLUMN SocialSecurityNumber ADD MASKING CustomMaskingPolicy;

```

Now, when you query the `Employees` table, the `SocialSecurityNumber` column will be masked according to the custom masking function you defined.

Remember that dynamic data masking is a security feature, and it's essential to have the necessary permissions to create and apply masking policies. Also, be careful when masking data, as the goal is to obfuscate sensitive information without compromising the usability of the data for authorized users.