How many nulls are allowed in unique key

Despite a few basic similarities, primary and unique keys have significant differences. You can only have one primary key per table, but multiple unique keys. And finally, the primary key column has a unique clustered index while a unique key column has a unique non-clustered index. CodingSight is open for new authors and partnership proposals. Send your offers and ideas to marketing devart. Search Menu. Primary Key A primary key is used to uniquely identify each record in the table. Unique Key A column with a unique key constraint can only contain unique values — this is the result of the unique key prohibiting duplicate values to be stored inside a column.

Similarities between Primary and Unique Keys Columns with primary or unique keys cannot have duplicate values. The error message will look like this: Since both primary key and unique columns do not accept duplicate values, they can be used for uniquely identifying a record in the table.

Explicit index partitioning is not required. However, if you only want the constraint to apply to a single column, it can be applied at the column level. Why CockroachDB. CockroachDB Free for everyone. Contact Us. Sign In. Am I remembering correctly? A Unique constraint would need a definitive value for the comparison of the column values.

In other words, when comparing a single column value against any other column value using the equality operator, it must evaluate to false to be valid.

Unknown isn't really false even though it is often treated as falsy. Two NULL values could be equal, or not It helps to think of a unique constraint as restricting values that can be determined to be distinct from one another. Most people would expect one result, given that there is a unique constraint.

Given that, I believe that regardless of whether or not it's implemented accurately with respect the definition of NULL, it's definitely a lot more practical in most situations than allowing multiple NULL values.

If one needs to have a table in which there can be multiple records where a value is "unknown", but no two records are allowed to have the same "known" value, then the unknown values should be assigned artificial unique identifiers before they are added to the table. There are a few rare cases in which a column which has a UNIQUE constraint and contains a single null value; for example, if a table contains a mapping between column values and localized text descriptions, a row for NULL would make it possible to define the description that should appear when that column in some other table is NULL.

The behavior of NULL allows for that usage case. Otherwise, I see no basis for a database with a UNIQUE constraint on any column to allow the existence of many identical records, but I see no way to prevent that while allowing multiple records whose key values are not distinguishable.

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Add a comment. Active Oldest Votes. Accept Solution Reject Solution. There is a difference between Unique key and unique index. Cannot insert duplicate key in object 'dbo. The statement has been terminated. Posted Jan am Member Richard Deeming Jan am. The index only allows multiple Null values because it's a filtered index which explicitly excludes those values.

Given the age of the question, it's possible that the OP may still have been using an earlier version. Posted 3-Nov pm Sandeep Mewara. If you mean on a primary key column, then no - you can't have null primary keys. If you mean a unique index on a nullable column, well Posted 3-Nov pm OriginalGriff. Sandeep Mewara 4-Nov am. Add your solution here. OK Paste as.