What is a unique constraint? How is it different from a primary key?
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What is a unique constraint? How is it different from a primary key?
A unique constraint is a constraint on a table that says that a column or set of columns needs to have unique values.
It’s different from a primary key in that a table can only have one primary key, but a table can have zero, one, or many unique constraints.
Unique constraints can also allow NULL values, but primary keys cannot.