关系的数据操纵语言按照表达式查询方式可分为两大类，关系代数和 ______ 。
参考答案：关系演算
Not until yesterday ( ) your payment.
A．have we received
B．had we received
C．did we receive
D．we have received
ST1-600型闸调器与ST2-250型闸调器的工作原理完全相同
