PG中的6大约束，详情参考：https://www.postgresql.org/docs/current/static/ddl-constraints.html
5.3.1. Check Constraints 5.3.2. Not-Null Constraints 5.3.3. Unique Constraints 5.3.4. Primary Keys 5.3.5. Foreign Keys 5.3.6. Exclusion Constraints
PG中的6大约束，详情参考：https://www.postgresql.org/docs/current/static/ddl-constraints.html
5.3.1. Check Constraints 5.3.2. Not-Null Constraints 5.3.3. Unique Constraints 5.3.4. Primary Keys 5.3.5. Foreign Keys 5.3.6. Exclusion Constraints