1, follow the principle of
https://mp.weixin.qq.com/s/Yjh_fPgrjuhhOZyVtRQ-SA
2, ER model
E represents entry, entity, design entity defined as a class, specify what the object is described, an entity is converted to a table in the database
Relationship R represents a relationship, the relationship between the description of the corresponding rule two entities, including the type of relationship comprises one, one-to-many
3, tombstone
(1) For important data, do not want to delete by drop physics, once removed, data can not be retrieved
(2) deletion scheme: setting isDelete column is bit, showing a tombstone, the default value is 0
(3) for non-critical data can be physically deleted