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);
Normally, a unique constreaint is the same functinally as a unique
index.
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