ORA-31670：Username argument must be specified and non-null.
错误码
错误描述：Username argument must be specified and non-null.
错误原因：Called change user with a NULL username or a null string.
解决方法：Specify a valid username argument. If this error occurs from a Data Pump client (e.g. expdp or impdp), try the operation again. If the error occurs again, contact Oracle Customer Support and report the error.
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