With this strategy, you can move up or down to any logical database schema state.
用这个策略，可以前后转移到逻辑数据库模式的任何状态。
Holding data attributes in XML could allow a very flexible schema in which attributes can be removed or added without changing the logical database schema.
将数据属性保存在XML中可以允许采用非常灵活的模式，能在不用更改逻辑数据库模式的情况下添加或删除属性。
Some database changes require changes in data as well as schema, and some of those data changes require logical changes.
有些数据库变化要求数据和模式一起变化，有些数据变化要求逻辑变化。
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