This prevents salaries from being entered beyond the regular salary range. This allows for multiple conditions to be checked in one location. Constraints that include implicit or explicit data type conversion may cause certain operations to fail.
Avoid data type conversion in constraint definitions. If a table that has just been created does not have any rows, any CHECK constraint on this table is considered valid. This situation can produce unexpected results, as in the following example.
Therefore, executing DELETE statements on tables with certain types of check constraints may produce unexpected results. For example, consider the following statements executed on table CheckTbl. When schema changes are made using the Table Designer or the Database Diagram Designer, it attempts to drop and recreate the table.
You cannot drop published objects, therefore the schema change will fail. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. For SQL Server and earlier, you can do it without a view. I just added a unique constraint like you're asking for to one of my tables. You simply have to put something in the computed column that will be guaranteed unique across the whole table when the actual desired unique column is NULL.
In this case, PartyID is an identity column and being numeric will never match any SamAccountName , so it worked for me. You can try your own method—be sure you understand the domain of your data so that there is no possibility of intersection with real data.
That could be as simple as prepending a differentiator character like this:. Note that the presence of an index including the computed column implicitly causes each expression result to be saved to disk with the other data in the table, which DOES take additional disk space.
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. One might be to use a negative number, to put the "surrogate NULLs" only in the negative range, and the "real values" only in the positive range. Alternately, the following pattern could be used. Some thought will show that this constraint cannot be duplicated for any row in the table, and still allows multiple NULLs.
For people who are using Microsoft SQL Server Manager and want to create a Unique but Nullable index you can create your unique index as you normally would then in your Index Properties for your new index, select "Filter" from the left hand panel, then enter your filter which is your where clause.
It should read something like this:. I found this on MSDN. With a non-clustered non-unique index on the column to enable the lookup. This often suggests you're creating a separate sub-entity within the same table as a different entity.
It probably makes more sense to have this entity in a second table. In the provided example, I would put LibraryCardId in a separate LibraryCards table with a unique not-null foreign key to the People table:. This way you don't need to bother with a column being both unique and nullable.
If a person doesn't have a library card, they just won't have a record in the library cards table. Also, if there are additional attributes about the library card perhaps Expiration Date or something , you now have a logical place to put those fields.
There is a ticket on Microsoft Connect for this since As suggested there and here the best options as of today are to use a filtered index as stated in another answer or a computed column, e.
Creating an index can be costly on larger tables. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. How do I create a unique constraint that also allows nulls? Ask Question. Asked 12 years, 9 months ago. Active 9 months ago. Viewed k times. Improve this question. Jeroen 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.
The statement has been terminated. But to achieve this, we need to sacrifice the space utilised by the additional column i. Please see the below sample script which works as second solution. The above two solutions would work on any version of SQL, when it comes to SQL Server we have another solution by using filtered index which can replace the second solution above, by simply creating the below UNIQUE filtered index.
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