Be Unique Or Be Null

We all knew that unique index can prevent duplicated values in the table. Does a column with unique constraint allow NULL values? The answer is positive. But only one. This is what this post for.

There are chances that we might need the value in a column to be unique, or be NULL. Take an example, you have a user table for your online social application. The user can be validated with their identities (PID) or be linked with another external ID like Live ID (now calls Microsoft Account) in the real world. Otherwise he can choose to stay as not validated. To avoid duplicated registration, you might need the PID to be unique. At the same time, user can also choose to keep their privacy, and leave the PID value to be empty.

For the scenario above, you will find some difficulties to define the unique constraint. There will be impossible to save two unverified user, because their PIDs are all NULL.

To resolve this problem, you can use filtered index which was first introduced in SQL Server 2008. You can define the unique index as

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