Can you put a unique constraint on a nullable column?
As you know, when you create a UNIQUE constraint on a nullable column, SQL Server allows only one NULL value, thereby maintaining the UNIQUEness. However, there are situations when we need more than one NULL value in the column but still have to maintain uniqueness, ignoring all those NULL values.
Can a unique value be NULL?
Unique fields in SQL Server are created using unique constraints or unique indexes, furthermore, each unique constraint uses a unique index. Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a single row with null value.
How do you create a unique key on a column which is having multiple NULL values?
As per the ANSI, UNIQUE constraint allows multiple NULLs. But in the SQL Server, it allows only one NULL value. With the UNIQUE constraint, you cannot insert multiple NULLs. But you can create UNIQUE NONCLUSTERED INDEX with the NOT NULL filter and can insert multiple NULLs.
Can unique index have multiple NULL values?
Can a unique field accept a null value?
Unique fields in SQL Server are created using unique constraints or unique indexes, furthermore, each unique constraint uses a unique index. Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a single row with null value.
When do null values violate a unique constraint?
Quoting the manual about UNIQUE constraints: In general, a unique constraint is violated when there is more than one row in the table where the values of all of the columns included in the constraint are equal. However, two null values are not considered equal in this comparison.
Are there any databases that do not allow null values?
Some databases do not allow multiple null values, for example the SQL Server documentation states that “multiple null values are considered duplicates”. On databases that do not allow nullable UNIQUE constraints you could try this (from GuidoG’s answer to another question): Drop the email column from the table.
Which is an example of a unique constraint?
A constraint that doesn’t allow duplicate values with the twist that Null values are not considered duplicates of other Null values. A practical example This table/design goal is a good example of a situation where you might need a null agnostic unique constraint. Customers Table