删除所有年龄小于21岁的数据: delete from students where age删除表中的所有数据: delete from students; 在表的最后追加列 address: alter table students add address char(60); ..
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在某些情况中，您还需要删除表以及表中的所有数据，修改数据库的设计，然后重新加载数据。
In some cases, you may have to unload the tables, delete all the data, change the database design, then reload the data.
刷新表：在某些数据仓库中，整个表每年删除一次，然后装载一个替代的表，这个新表包含了除不再需要的数据以外的所有数据。
Refresh table: In some data warehouses, the entire table is dropped once per year and a replacement table is loaded with all the data except for the data that is no longer needed.
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