Relation Algebra

王朝other·作者佚名  2008-05-31
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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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