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CodePudding user response:

This column is the primary key, the primary key constraint itself can guarantee uniqueness, so you add a unique, not was necessary!

You can put this table to add a field, the unique constraint on the second field!

CodePudding user response:

Look at the view

 select * from user_constraints where the lower (table_name)='customers' 

Constraint_type column values for P is a primary key

CodePudding user response:

Primary key contains the only constraints and not null constraint

CodePudding user response:

The primary key to ensure that the only, and is empty, the requirements of the more strict than the UNIQUE