oracle里面的约束
数据库用户级别的约束表：user_constraints
数据库DBA级别的约束表：DBA_constraints
| type Code | Type Description | Acts On Level |
|---|---|---|
| C | Check on a table | Column |
| O | Read Only on a view | Object |
| P | Primary Key | Object |
| R | Referential AKA Foreign Key | Column |
| U | Unique Key | Column |
| V | Check Option on a view | Object |
