<pre><code>select distinct constraint_type from user_constraints;

C
-
C
P
R
U
</code></pre>

<p>Seems P means primary key and R means foreign key, correct?  What are U and C? </p>

```
```
Code Description                Acts On Level
---------------------------------------------
C    Check on a table           Column
O    Read Only on a view        Object
P    Primary Key                Object
R    Referential (Foreign Key)  Column
U    Unique Key                 Column
V    Check Option on a view     Object
```
```