参照完整性
参考答案：
参照完整性指的是关系中的外键必须与参照关系的键相符，要么在参照关系中有值，或者本身取空值。
补全对话。
Clark: What events do you ____1____ at your school?
David: We have an English Speech Contest each year.
Clark: ____2____ is it?
David: It's March 5th.
Clark: Do you have ___3_____ Art Festival?
David: Yes. It's March 17th. ___4___ we have a basketball game on March 10th.
Clark: When is your School Trip?
David: It's April 6th.
Clark: Is Linda's birthday ___5___in April?
David: Yes. It's April 18th.
荷载按作用范围可分为（）和（）。
