我是用的oracle数据库，级联删除时错

Exception occurred during processing request: could not execute statement; SQL [n/a]; constraint [SYSTEM.FK_O681PHLIAUJPVDBLO1VO2F6SF]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement

<set name="employee"  cascade="all" inverse="true" >

<key foreign-key="did"/>

<one-to-many class="com.ssh.entry.Employee" />

</set>