核心！DB关系代数理论分析和用SQL语句实现

关系代数6种常见的基本操作：

select： σ

∧ and 、∨ or、┐not 

举个例子：

用 选择下表记录：

选择结果如下：

SQL语句实现方法：

select A,B,C,D from r where A=B and D>5;

 

project(投影):  ∏ 

注：Duplicate rows removed from result, since relations are sets集合

举个例子：

用投影下表：

投影结果如下（要去除重复）：

用SQL语句实现此功能：

select A,C from r;

 

union :  ∪

two tables must have the same arity 参数数量

The attribute domains must be compatible能共处的 [kəm'pætəbl]

 to find all courses taught in the Fall 2009 semester, or in the Spring 2010 semester, or in both

举个例子：

用实现下表合并：

结果如下：

用SQL语句实现此功能：

(select A,B from r)union (select A,B from s);

 

set difference : -

举个例子：

用操作下表：

结果如下：

(select A,B from r) except (select A,B from s);

 

Cartesian product:  x

Assume that attributes of two tables are disjoint不相交的

If attributes of r and s are not disjoint, then renaming must be used.

用操作下表：

结果如下：

用SQL语句实现如下：

select * from r,s;

 

rename : p

 

 Additional operators:

Set intersection

Natural join

Assignment

Outer Join

 

Extended relational algebra operatiors:

Generalized Projection

Aggregate Functions