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]中国最大的web开发资源网站及技术社区，
