java - In SQL, can I add a unique constraint to a field that's not required?

Question

I have a standard User entity, with a name, mobile phone number, and email address. Of course, I don't want any duplication in the email address, but I also don't want it to be required. I'm not sure if a unique constraint lets multiple records be blank. (Wouldn't that violate uniqueness?) Is this even possible? If so, how do I specify this constraint in JPA?

Answer 1

In a word - yes, this is possible. Uniqueness means the column can't have two equal values. nulls in SQL are not values - they are the lack thereof. Since they are not values, they don't affect the uniqueness, and a unique column can have as many nulls as you like.

In JPA, you can specify this with the unique attribute of the @Column annotation:

@Column(unique=true)
String email;