1.metadata(data about data)
Metadata is data about the data, such as the
name of a database or table, the data type of a column, or access
privileges. Other terms that sometimes are used for this information
are data dictionary and system
catalog.
2. INFORMATION_SCHEMA数据库存储所有其他数据库的信息，这些表是views,而非base table,没有实际对应的文件。基本表是视图的来源，视图丰富了基本表的表达。只能对INFORMATION_SCHEMA数据库的表进行select操作。
3. select schema的操作和show 相应项的效果是相同，不过shema更容易统一处理吧。
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