Difference Between Unique & Primary Key
You can have more than one
UNIQUE constraint per table, but only one
Primary key.
Some DBMS might automatically create an index (data structure to speed search) in response to
PRIMARY KEY, but not
UNIQUE.
SQL does not allow nulls in
Primary key, but allows them in
Unique columns (which may have two or more nulls, but not repeated non-null values).
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