| No | Word | Definition |
| 1 | Primary Key | An attribute or a set of attribute that enable data in a table to be uniquely identified. It is specified in table schema as a primary key. |
| 2 | Candidate Key | An attribute or a set of attribute that enable data in a table to be uniquely identified. There could be more than Candidate Key in a table. However, one only can be specified as a primary key. |
| 3 | Foreign Key | A set of attributes in table T1 that reference another set of corresponding primary key attributes in table T2. The foreign key attributes in T1 must have the same domain as the corresponding primary key attributes in T2. |