Set column as primary key if the table doesn’t have a primary key

This checks if primary key exists, if not it is created

IF NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
WHERE CONSTRAINT_TYPE = 'PRIMARY KEY' AND TABLE_NAME = 'Persons' 
AND TABLE_SCHEMA ='dbo')
BEGIN
   ALTER TABLE Persons ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id)
END
ELSE
BEGIN
   -- Key exists
END

fiddle: http://sqlfiddle.com/#!6/e165d/2