NAME SYMBOL KEYBOARD FORM CHE UNION UNION 并 INTERSECT 交 DIFFERENCE - MINUS 差 //前三类是要条件的，就像矩阵运算一样 条件：大概意思是 狗只能跟狗比，不能跟猫比 PRODUCT * TIMES 笛卡儿积 2。 NATIVE RELATIONAL OPERATIONS
PROJECT R[] R[] 取其中几列 SELECT R WHERE C R WHERE C 就像SQL中的select * from C 一样 JOIN JOIN 合并：所有“共有列”一样的record才保留 DIVISION / DIVIDBY 除以 // If the table R is defined by R = T*S,then it is true that T = R DIVIDBY S.
EXAMPLE 2.96(表格跟见上篇文章） 0 := ORDERS --alias GET cids of customers who order all products that anybody orders.This is division again,and the divisor,the list of “all” products,must be projected from ORDERS rather than PRODUCTS. 0[cid,pid]/0[pid]
