分两大类：
1。SET-THEORETIC OPERATIONS
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]
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