关系代数的5个基本操作是（）（）（）（）（）。
关系代数的理论基础是（），关系演算的理论基础是（），关系逻辑的理论基础是（）。
实体完整性规则是对（）的约束，参照完整性规则是对（）的约束。
Most Computer operating systems in use today, including all consumer-targeted systems, include a TCP/IP（）. ["plan","requirement","implementation","design"]
Today there are （）computers in our homes and offices than there are people who live and work in them. ["less","more","little","lotsof"]
关系中没有行序的原因是（）
