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oracle中某表的字段设置为number，现欲改为Varchar2，但是库中有很多数据，怎样能保证数据不丢失，把类型改过来？
SQL> create table aaa (a number(9));
Table created.
Elapsed: 00:00:00.32
SQL> insert into aaa values(999);
1 row created.
Elapsed: 00:00:00.03
SQL> commit;
Commit complete.
Elapsed: 00:00:00.00
SQL> alter table aaa add b varchar2(5);
Table altered.
Elapsed: 00:00:00.17
SQL> update aaa set a=b;
1 row updated.
Elapsed: 00:00:00.02
SQL> commit;
Commit complete.
Elapsed: 00:00:00.03
SQL> alter table aaa drop column a;
Table altered.
Elapsed: 00:00:00.52
SQL> alter table aaa rename column b to a;
Table altered.
Elapsed: 00:00:00.04
SQL>
