Ms sql server unique constraint null

Applies to: SQL Server Creating a unique constraint automatically creates a corresponding unique index. See Primary key, foreign key, and unique key in Azure Synapse Analytics for information on unique constraints in Azure Synapse Analytics. In Object Explorer , right-click the table to which you want to add a unique constraint, and select Design. In the grid under General , select Type and choose Unique Key from the drop-down list box to the right of the property, and then select Close.

Please note that some database professionals will see this as a case of "surrogate NULLs", which definitely have problems mostly due to issues around trying to determine when something is a real value or a surrogate value for missing data ; there can also be issues with the number of non-NULL surrogate values multiplying like crazy.

However, I believe this case is different. The computed column I'm adding will never be used to determine anything. It has no meaning of itself, and encodes no information that isn't already found separately in other, properly defined columns.

It should never be selected or used. Have fun adding a new column to your base table you'll at minimum have to drop the index, and then drop the view or alter the view to not be schema bound. See the full long list of requirements for creating an indexed view in SQL Server also later versions , If your column is numeric, there may be the challenge of ensuring that the unique constraint using Coalesce does not result in collisions.

In that case, there are some options. Both clustered and nonclustered index can be unique. We can create a unique clustered index and multiple nonclustered indexes on the same table. From the above explanation it is clearly understood that unique constraint does not allow duplicate values and the same is true with NULL values as well.

As mentioned above if we insert multiple NULL values to maintain data integrity SQL Server will throw an error, so the work-around would be as follows:. By doing a small change we can achieve the uniqueness. No error raised after executing the above query, now check if the uniqueness is retained by inserting the existing value into TITLE column. In this article.

Create Unique Constraints. Modify Unique Constraints. Delete Unique Constraints. Describes how to disable a check constraint when a replication agent inserts or updates data in your table. Disable Check Constraints for Replication. Describes how to disable a check constraint when data is added to, updated in, or deleted from a table.

Describes how to change the constraint expression or the options that enable or disable the constraint for specific conditions. Modify Check Constraints.