Question:  I need to understand how to add a unique constraint statement to make sure that all rows have distinct values.  How do you alter a unique constraint column value to make it into a primary key constraint?

Unique Constraints

Unique constraints are like alternative primary key constraints.

- A unique constraint defines a column, or series of columns, that must be unique in value.

- You can have a number of unique constraints defined and the columns can have NULL values in them, unlike a column that belongs to a primary key constraint.  

- If you need to add unique key constraints to a table after the fact, simply use the alter table command.

ALTER TABLE
   my_status
ADD CONSTRAINT
    uk_my_status
UNIQUE
   (status_id,
    person_id);

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