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Difference between primary key and unique key in sql

Difference between primary key and unique key in sql.
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Primary key                            Unique key
1-can not be null                    1-can be null and multiple null
2-can not be same value        2-can not be same value
3-only be one column name  3-multiple column could be unique
suppose emp_id is primary key and city is the unique key
primary key                                   unique key
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emp_id        emp_nm        sal        city
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0                  AA            18000        (null)      
1                 BB             18000        NEW DELHI
3                CC             18000        NOIDA
2                DD             18000        (null)
4                EE             18000        KANPUR
5                FF             18000        (null)
6                GG             18000        (null)
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