由于关系代数运算的结果与其输入的类型一样，仍为关系，所以我们可以把多个关系代数运算组合成一个关系代数表达式。-As a result of relational algebra operations as its input type, still relationship, so we can combine multiple relational algebra into a relational algebra expression

数据库关系代数作业,包含4章数据库的ppt资料文件-Database relational algebra operations, including Chapter 4 ppt file information