Ms sql unique constraint null

Privacy policy. 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.

Perhaps the rows that do have a value in that column should move to another table with the original table having a one-to-zero-or-one relationship with it. Then, drop the column in the original table. An efficient unique index can now be put on the second table. Howard Howard 1, 11 11 silver badges 11 11 bronze badges.

Mike Taylor Mike Taylor 2, 2 2 gold badges 17 17 silver badges 32 32 bronze badges. Yonatan Tuchinsky Yonatan Tuchinsky 3 3 silver badges 9 9 bronze badges. Very nice alternative if you have access to the designer — Francisco. Although, as I have just discovered, once you have data in your table, you can no longer use the designer.

It seems to ignore the filter and any attempted table updates are met with the message "Duplicate key not allowed" — MortimerCat. Quassnoi Quassnoi k 88 88 gold badges silver badges bronze badges. Lieven Keersmaekers Lieven Keersmaekers Marc Gravell Marc Gravell k gold badges silver badges bronze badges. Trent Hibbard Trent Hibbard 21 1 1 bronze badge. Seriously disagree with your first statement. In Australia every employee has a thing called a Tax File Number, which is, of course unique.

By law you are not required to supply it to your employee. This means that the column may be null, but should otherwise be unique. In this case an additional table might be regarded as over-engineering. Baris Akar Baris Akar 4, 1 1 gold badge 23 23 silver badges 54 54 bronze badges. Paul Paul 3, 1 1 gold badge 14 14 silver badges 22 22 bronze badges. Pang 8, gold badges 82 82 silver badges bronze badges.

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Podcast Helping communities build their own LTE networks. In other words, if the inserted data meets the constraint rule, it will be inserted successfully. If the inserted data violates the defined constraint, the insert operation will be aborted. Constraints in SQL Server can be defined at the column level, where it is specified as part of the column definition and will be applied to that column only, or declared independently at the table level.

In this case, the constraint rules will be applied to more than one column in the specified table. Adding the constraint after creating the table, the existing data will be checked for the constraint rule before creating that constraint. There are six main constraints that are commonly used in SQL Server that we will describe deeply with examples within this article and the next one. These constraints are:. Let us start discussing each one of these SQL Server constraints with a brief description and practical demo.

View all page feedback. 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.