数据库系统概念学习笔记

as关键字

as可以用作别名：

select avg(salary) as avg_salary
from instructor
where dept_name = 'Comp. Sci.';

distinct

select count(distinct ID)
from teachers
where semester = 'Spring' and year = 2010;

group by

"找出每个系在2010年春季学期讲授课程的教师人数"

select dept_name, count(distinct ID) as instr_count
from instructor natural join teachers
where semester = 'Spring' and year = 2010
group by dept_name;

some

"找出满足下面条件的所有教师的姓名，他们的工资至少比Biology系的某一个教师的工资要高"

select distinct T.name
from instructor as T, instructor as S
where T.salary > S.salary and S.dept_name = 'Biology' 

或者

select name
from instructor
where salary > some (select salary
                     from instructor
                     where dept_name = 'Biology');

some支持 =, < , > , <=, >=, <>

all

all的用法与some类似

exists not exists

空关系测试

unique

重复元组存在性测试

关系范式

简单的理解， 范式即使标准，衡量数据库设计的规范等级，一般地，考虑到BC范式即可。

可以参考知乎上的回答https://www.zhihu.com/question/24696366