计算任意两个表的简单自然连接过程。假设有两个表A、B，分别为m1行、n1列和m2行、n2列，求它们简单自然连接的结果C=A B(i=j)。(A simple natural join procedure for computing two arbitrary tables. Suppose there are two tables A and B, which are M1 rows, N1 columns and M2 rows, N2 columns, and the result of their simple natural connection is C=A B (i=j).)