SQL理论课-Class 2

语法：
select 需要的属性
from 表1，表2

where 表1.属性 = 表2.属性（这里面的属性需要在两个表里面都一样）（这里的 表1.和 表2.不能省略。）

select dept_name 
from course,section
where course.course_id = section.course_id and dept_name = 'Comp.Sci'

select
from 表1 join 表2 on 表1.属性 = 表2.属性
还可以写成这样。效率比较高

Select *

From emp join dept on emp.dpetno = dept.deptno

Where loc = 'new York'

from 表1 natural join 表2 on 条件

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

语法：
select 属性1 from 表1 where 条件1
集合运算符
select 属性2 from 表2 where 条件2

where 属性 is null

OR: (unknown or true) = true,
 (unknown or false) = unknown
(unknown or unknown) = unknown

– AND: (true and unknown) = unknown, 
(false and unknown) = false,
(unknown and unknown) = unknown

– NOT: (not unknown) = unknown
– “P is unknown” evaluates to true if predicate P evaluates to unknown

查询先修课的先修课的名字（自连接）
use university
select distinct P.prereq_id
from prereq as C,prereq as P
where C.prereq_id = P.course_id
查询两个工资相等的老师
select distinct A.name,B.name
from instructor as A,instructor as B
where A.salary = B.salary and A.name <> B.name