students a, b, C, courses: x, y, Z,w (ay)2(a,w),(b,x)2by)(b,w),(c,w) R={(a2y),(a,w)2(b,x)2(b2y)(b,w)} RCAXB. ie.R is a subset ofAXB relationstudents a,b,c, courses:x,y,z,w (a,y),(a,w),(b,x),(b,y),(b,w)，(c,w) R={(a,y),(a,w),(b,x),(b,y),(b,w)} RA×B, i.e. R is a subset of A×B relation