Adding unique keys

It is sometimes possible to identify an entity by keys other than the primary
key. These are (combinations of) attributes that have
key characteristics (unique and not null); they are called unique keys.
Unique keys are preceded by a sequential number enclosed in brackets: <1> for the
first unique key, <2> for the second, etc.
To create an alternate key:
- Look for the Key tool
and click the arrow next to the key (a display
of keys numbered 1 to 5 will appear).
- Choose one of those numbered keys.
- Click the attribute(s) that will become the unique key.