sql并-交-差的处理

并集:union

    eg:

        求学过002或者003课程同学的学号:

        select s# from sc where c#='002'
        union

        select s# from sc where c#='003'

        等同于

        select s# from sc where c#='002' or c#='003'

交集:intersect

    eg:求学过002和003课程同学的学号:

    select s# from sc where c#='002'
    intersect
    select s# from sc where c#='003'

    等同于

    select s# from sc where c#='002'and s# in(

    select s# from sc where c#='003')

差集:except

    eg:求没学过002号课程的学号

    select distinct s# from sc
    except

    select s# from sc where c#='002'

    等同于

    select distinct s# from sc sc1 
    where not exists(
    select * from sc

    where c#='002' and s#=sc1.s#)

先写下了吧，以后写作业用到可以回来看看呢；