• 1. nullable columns: 空值的字段

• 1.Though most Rails developers do not like to keep constraints in the database, you should consider things like nullable columns.
  虽然大多数Rails开发人员都不喜欢在数据库中保留限制，但您应该考虑像空列这样的事情。
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• 2.However ElectricityDeal had too many of its own properties. Accordingly many electricity-specific nullable columns were added to the Deal table.
  但是ElectricityDeal有太多自己的属性，因此有很多电相关的可空字段加入了Deal表。
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• 3.The pros include compact tables (no unnecessary nullable columns), data partitions across three subclass tables and easy association with other tables using the top super class table.
  优点包括数据表比较紧凑(没有不需要的可空字段)，数据跨三个子类的表进行分区，容易使用超类的表与其他表进行关联。
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