集合交(Set-Intersection Operation) 定义: r ∩ s = 要点: r ,s 必须包含相同属性,即同元 r ,s 属性的域必须相容 注意,任何使用了集合交的关系代数表达式
one or more relations as inputs and give a new relation as a result Operations select project union set-Intersection
one or more relations as inputs and give a new relation as a result Operations select project union set-Intersection