name symbol keyboard form che union union 并 intersect 交 difference - minus 差 //前三類是要條件的，就像矩陣運(yùn)算一樣 條件：大概意思是 狗只能跟狗比，不能跟貓比 product * times 笛卡兒積 2。native relational operations
project r[] r[] 取其中幾列 select r where c r where c 就像sql中的select * from c 一樣 join join 合并：所有“共有列”一樣的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(表格跟見上篇文章） 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]中國最大的web開發(fā)資源網(wǎng)站及技術(shù)社區(qū)，
