Primary Key:
- There can only be one primary key constraint in a table
- In some DBMS it cannot be
NULL
– e.g. MySQL addsNOT NULL
- Primary Key is a unique key identifier of the record
Unique Key:
- Can be more than one unique key in one table
- Unique key can have
NULL
values - It can be a candidate key
- Unique key can be
NULL
; multiple rows can haveNULL
values and therefore may not be considered “unique”
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