[sql] Add primary key to existing table

The PRIMARY KEY constraint uniquely identifies each record in a database table. Primary keys must contain UNIQUE values and column cannot contain NULL Values.

  -- DROP current primary key 
  ALTER TABLE tblPersons DROP CONSTRAINT <constraint_name>
  Example:
  ALTER TABLE tblPersons 
  DROP CONSTRAINT P_Id;


  -- ALTER TABLE tblpersion
  ALTER TABLE tblpersion add primary key (P_Id,LastName)

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