algebra:Relation Algebra来源: 发布时间:星期一, 2009年2月23日 浏览:0次 评论:0
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分两大类:
1  SET-THEORETIC OPERATIONSNAME SYMBOL KEYBOARD FORM CHE UNION UNION 并 INTERSECT 交 DIFFERENCE - MINUS 差 //前 3类是要条件   就像矩阵运算 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 select *from C 样JOINJOIN合并:所有“共有列” 样record才保留DIVISION/ DIVIDBY 除以 // If the table R is  d 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] 0
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