数据库的三大范式。
第一范式
A relation is in first normal form if and only if the domain of each attribute contains only atomic (indivisible) values, and the value of each attribute contains only a single value from that domain.
简而言之，就是每一个属性都满足不可分割的原子性。
举例：
| Customer ID | Telephone Number |
|---|---|
| 1 | 555-861-2025, 192-122-1111 |
| 2 | (555) 403-1659 Ext. 53; 182-929-2929 |
应该修改为：
| Customer ID | Telephone Number |
|---|---|
| 1 | 555-861-2025 |
| 1 | 192-122-1111 |
| 2 | (555) 403-1659 Ext. 53 |
| 2 | 182-929-2929 |
第二范式
A relation is in 2NF if it is in 1NF and every non-prime attribute of the relation is dependent on the whole of every candidate key.
简而言之，在满足第一范式的基础上，表中的字段必须完全依赖于全部主键(主要针对复合主键)。
举例：
| Manufacturer | Model | Model Full Name | Manufacturer Country |
|---|---|---|---|
| Forte | X-Prime | Forte X-Prime | Italy |
| Forte | Ultraclean | Forte Ultraclean | Italy |
| Hoch | Toothmaster | Hoch Toothmaster | Germany |
| Hoch | X-Prime | Hoch X-Prime | Germany |
其中，Model Full Name 依赖复合主键 Manufacturer & Model，而 Manufacturer Country 只依赖于 Manufacturer，存在非完全依赖。
应该修改为：
| Manufacturer | Manufacturer Country |
|---|---|
| Forte | Italy |
| Hoch | Germany |
| Manufacturer | Model | Model Full Name |
|---|---|---|
| Forte | X-Prime | Forte X-Prime |
| Forte | Ultraclean | Forte Ultraclean |
| Hoch | Toothmaster | Hoch Toothmaster |
| Hoch | X-Prime | Hoch X-Prime |
第三范式
A relation is in 3NF if (1) the entity is in 2NF, and (2) all the attributes are determined only by the candidate keys of that relation and not by any non-prime attributes.
简而言之，在满足第二范式的基础上，表中的字段必须完全依赖于主键，不存在传递依赖。
举例：
| Tournament | Year | Winner | Winner Date of Birth |
|---|---|---|---|
| Indiana Invitational | 1998 | Al Fredrickson | 21 July 1975 |
| Cleveland Open | 1999 | Bob Albertson | 28 September 1968 |
| Des Moines Masters | 1999 | Al Fredrickson | 21 July 1975 |
| Indiana Invitational | 1999 | Chip Masterson | 14 March 1977 |
其中，Winner 依赖复合主键 Tournament & Year，而 Winner Date of Birth 只依赖于 Winner，存在传递依赖: Winner Date of Birth -> Winner -> Tournament & Year。
应该修改为：
| Winner | Date of Birth |
|---|---|
| Chip Masterson | 14 March 1977 |
| Al Fredrickson | 21 July 1975 |
| Bob Albertson | 28 September 1968 |
| Tournament | Year | Winner |
|---|---|---|
| Indiana Invitational | 1998 | Al Fredrickson |
| Cleveland Open | 1999 | Bob Albertson |
| Des Moines Masters | 1999 | Al Fredrickson |
| Indiana Invitational | 1999 | Chip Masterson |