http://blog.sina.com.cn/s/blog_7e5541250100ygb1.html
这里我们建几个SQL表，我们用个图来说明关系
这里tid的值和教师表中的id相匹配，pid和院系表中的id相匹配,也就是说这样建表就把三张表的关系给建立起来了。学生表在最低层，院系表在最高层。
而这里我们就要讲一个外键的概念：外键是作为与主表的主键相关联的键值，外键可以有多个，同时有外键的表称作对应主键所在表的从表，那主键所在的表就作主表。图中，tid是教师表中id的外键，也就学生表是教师表的从表，同理，教师表也就是院系表的从表。
外键的主要作用是保持数据一致性，完整性，主要目的是控制存储在外键表中的数据。
当然在这时我们还不需要去建立真正的外键联系关系
现在我们要查询学生表，教师表，院系表的所有信息：
select * from student,teacher,part //查询这三个表中的所有信息
我们现在要查询某学生的教师是的名字：
select teaname from student,teacher where (student.id =\'\') and (teacher.id = student.tid)
//这里有两个约束条件，一个是约束学生的id号，一个是约束学生表和教师表的数据要相对应，也就是，这样查询的结果，就是该学生tid所对应教师表中id号的teaname字段的数据，也就是这位学生的教师名字了
现在有一个问题，我想问这个学生所在院系是什么？这个问题就应该迎刃而解了吧。
当然这只是简单的多表查询，其实在真正多表查询时，是用的以下方法：
这里讲一个join方法
该语句是实现多对对查询或一对一查询的方法，当我们在用简单多表查询的时候，它的匹配是无逻辑的，也就是没有具体的约束条件，它不会按照逻辑关系来约束。有可能这个学生会和一个无关第教师联系起来，或者和一个无关的院系联系起来。
我们用一个简单的join方法查询：
select teacher.teaname from student join teacher on(student.tid = teacher.id)where (student.id=\'1\')
//查询id号为1的学生的教师的名字
这就是join语句的语法，它是个嵌套语句，也就是把一个表嵌套在另一个表中，一般都是自下而上的嵌套，如上例，就是学生表去加入教师表的集合。也就是从表加入主表。
这种join有两个参数，我们上例用的是缺省参数--innor,当我们直接用join时，就默认是这个参数，还有一个left，这两个参数在用时有很大的区别。
select * from student left join teacher on(student.tid = teacher.id)
这上面这个例子中，用left和不用left参数是有很大区别的。用如果在学生表中，有几个新报道的学生还未分配教师，那么，用left参数查询的时候也会显示出来，尽管他的tid为空，和教师表中的数据不匹配，但也会显示出来。 若不用left ，那么就不会显示这几个新学生。
通过这个例子应该能更好的理解left的用处了吧，在处理中，我们要根据需求来进行设计查询语句。
当然，我们先前讲的外键这个时候我们也需要拿出来用了。下图所示：
我们现在建好了这个表，我们有如下设计需求，当我在删掉这个类别的时候，我们需要把该类别下的产品也删掉。我们可以用最原始的方法，也就是先查询所有该类别的产品，然后把类别下的产品全删完后才删除该类别，但这样，不显得很麻烦吗？
这里就该用上外键的功能了，首先我们建立好外键，也就是把cid设为类别表中id的外键
ALTER TABLE product ADD CONSTRAINT fk_product_cid
FOREIGN KEY (cid)
REFERENCES category(id);
//第一行是在product(产品表)中添加一个外键，第二行是此外键对应的字段是cid，第三行是此外键对应的键值为category(类别表)中的id.
那么这就建立好了外键的关系，但是这样还是不行，我们还需要了解一下外键的级联操作的相关信息：
CASCADE（连锁关系，当主表变更，从表的相应信息也会变更）
RESTRICT(禁止主表变更)
SET NULL(子表相应字段设置为空)
。。。。。
一般我们用的最多的是连锁关系，而上例所讲的设计我们就需要用到连锁关系 ：
ALTER TABLE product ADD CONSTRAINT fk_product_cid
FOREIGN KEY (cid)
REFERENCES category(id);
ON UPDATE CASCADE;
ON DELETE CASCADE;
//这就实现了更新的连锁和删除的连锁。也就是当主表的信息变更，从表的相关信息也会变更。当主表的信息删除，从表的相关联信息也会删除
问题及描述：
--1.学生表
Student(S#,Sname,Sage,Ssex) --S# 学生编号,Sname 学生姓名,Sage 出生年月,Ssex 学生性别
--2.课程表
Course(C#,Cname,T#) --C# --课程编号,Cname 课程名称,T# 教师编号
--3.教师表
Teacher(T#,Tname) --T# 教师编号,Tname 教师姓名
--4.成绩表
SC(S#,C#,score) --S# 学生编号,C# 课程编号,score 分数
*/
--创建测试数据
create table Student(S# varchar(10),Sname nvarchar(10),Sage datetime,Ssex nvarchar(10))
insert into Student values(\'01\' , N\'赵雷\' , \'1990-01-01\' , N\'男\')
insert into Student values(\'02\' , N\'钱电\' , \'1990-12-21\' , N\'男\')
insert into Student values(\'03\' , N\'孙风\' , \'1990-05-20\' , N\'男\')
insert into Student values(\'04\' , N\'李云\' , \'1990-08-06\' , N\'男\')
insert into Student values(\'05\' , N\'周梅\' , \'1991-12-01\' , N\'女\')
insert into Student values(\'06\' , N\'吴兰\' , \'1992-03-01\' , N\'女\')
insert into Student values(\'07\' , N\'郑竹\' , \'1989-07-01\' , N\'女\')
insert into Student values(\'08\' , N\'王菊\' , \'1990-01-20\' , N\'女\')
create table Course(C# varchar(10),Cname nvarchar(10),T# varchar(10))
insert into Course values(\'01\' , N\'语文\' , \'02\')
insert into Course values(\'02\' , N\'数学\' , \'01\')
insert into Course values(\'03\' , N\'英语\' , \'03\')
create table Teacher(T# varchar(10),Tname nvarchar(10))
insert into Teacher values(\'01\' , N\'张三\')
insert into Teacher values(\'02\' , N\'李四\')
insert into Teacher values(\'03\' , N\'王五\')
create table SC(S# varchar(10),C# varchar(10),score decimal(18,1))
insert into SC values(\'01\' , \'01\' , 80)
insert into SC values(\'01\' , \'02\' , 90)
insert into SC values(\'01\' , \'03\' , 99)
insert into SC values(\'02\' , \'01\' , 70)
insert into SC values(\'02\' , \'02\' , 60)
insert into SC values(\'02\' , \'03\' , 80)
insert into SC values(\'03\' , \'01\' , 80)
insert into SC values(\'03\' , \'02\' , 80)
insert into SC values(\'03\' , \'03\' , 80)
insert into SC values(\'04\' , \'01\' , 50)
insert into SC values(\'04\' , \'02\' , 30)
insert into SC values(\'04\' , \'03\' , 20)
insert into SC values(\'05\' , \'01\' , 76)
insert into SC values(\'05\' , \'02\' , 87)
insert into SC values(\'06\' , \'01\' , 31)
insert into SC values(\'06\' , \'03\' , 34)
insert into SC values(\'07\' , \'02\' , 89)
insert into SC values(\'07\' , \'03\' , 98)
go
--1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数
--1.1、查询同时存在"01"课程和"02"课程的情况
select a.* , b.score [课程\'01\'的分数],c.score [课程\'02\'的分数] from Student a , SC b , SC c
where a.S# = b.S# and a.S# = c.S# and b.C# = \'01\' and c.C# = \'02\' and b.score > c.score
--1.2、查询同时存在"01"课程和"02"课程的情况和存在"01"课程但可能不存在"02"课程的情况(不存在时显示为null)(以下存在相同内容时不再解释)
select a.* , b.score [课程"01"的分数],c.score [课程"02"的分数] from Student a
left join SC b on a.S# = b.S# and b.C# = \'01\'
left join SC c on a.S# = c.S# and c.C# = \'02\'
where b.score > isnull(c.score,0)
--2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
--2.1、查询同时存在"01"课程和"02"课程的情况
select a.* , b.score [课程\'01\'的分数],c.score [课程\'02\'的分数] from Student a , SC b , SC c
where a.S# = b.S# and a.S# = c.S# and b.C# = \'01\' and c.C# = \'02\' and b.score < c.score
--2.2、查询同时存在"01"课程和"02"课程的情况和不存在"01"课程但存在"02"课程的情况
select a.* , b.score [课程"01"的分数],c.score [课程"02"的分数] from Student a
left join SC b on a.S# = b.S# and b.C# = \'01\'
left join SC c on a.S# = c.S# and c.C# = \'02\'
where isnull(b.score,0) < c.score
--3、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
select a.S# , a.Sname , cast(avg(b.score) as decimal(18,2)) avg_score
from Student a , sc b
where a.S# = b.S#
group by a.S# , a.Sname
having cast(avg(b.score) as decimal(18,2)) >= 60
order by a.S#
--4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
--4.1、查询在sc表存在成绩的学生信息的SQL语句。
select a.S# , a.Sname , cast(avg(b.score) as decimal(18,2)) avg_score
from Student a , sc b
where a.S# = b.S#
group by a.S# , a.Sname
having cast(avg(b.score) as decimal(18,2)) < 60
order by a.S#
--4.2、查询在sc表中不存在成绩的学生信息的SQL语句。
select a.S# , a.Sname , isnull(cast(avg(b.score) as decimal(18,2)),0) avg_score
from Student a left join sc b
on a.S# = b.S#
group by a.S# , a.Sname
having isnull(cast(avg(b.score) as decimal(18,2)),0) < 60
order by a.S#
--5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
--5.1、查询所有有成绩的SQL。
select a.S# [学生编号], a.Sname [学生姓名], count(b.C#) 选课总数, sum(score) [所有课程的总成绩]
from Student a , SC b
where a.S# = b.S#
group by a.S#,a.Sname
order by a.S#
--5.2、查询所有(包括有成绩和无成绩)的SQL。
select a.S# [学生编号], a.Sname [学生姓名], count(b.C#) 选课总数, sum(score) [所有课程的总成绩]
from Student a left join SC b
on a.S# = b.S#
group by a.S#,a.Sname
order by a.S#
--6、查询"李"姓老师的数量
--方法1
select count(Tname) ["李"姓老师的数量] from Teacher where Tname like N\'李%\'
--方法2
select count(Tname) ["李"姓老师的数量] from Teacher where left(Tname,1) = N\'李\'
--7、查询学过"张三"老师授课的同学的信息
select distinct Student.* from Student , SC , Course , Teacher
where Student.S# = SC.S# and SC.C# = Course.C# and Course.T# = Teacher.T# and Teacher.Tname = N\'张三\'
order by Student.S#
--8、查询没学过"张三"老师授课的同学的信息
select m.* from Student m where S# not in (select distinct SC.S# from SC , Course , Teacher where SC.C# =Course.C# and Course.T# = Teacher.T# and Teacher.Tname = N\'张三\') order by m.S#
--9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
--方法1
select Student.* from Student , SC where Student.S# = SC.S# and SC.C# = \'01\' and exists (Select 1 from SC SC_2 where SC_2.S# = SC.S# and SC_2.C# = \'02\') order by Student.S#
--方法2
select Student.* from Student , SC where Student.S# = SC.S# and SC.C# = \'02\' and exists (Select 1 from SC SC_2 where SC_2.S# = SC.S# and SC_2.C# = \'01\') order by Student.S#
--方法3
select m.* from Student m where S# in
(
select S# from
(
select distinct S# from SC where C# = \'01\'
union all
select distinct S# from SC where C# = \'02\'
) t group by S# having count(1) = 2
)
order by m.S#
--10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
--方法1
select Student.* from Student , SC where Student.S# = SC.S# and SC.C# = \'01\' and not exists (Select 1 fromSC SC_2 where SC_2.S# = SC.S# and SC_2.C# = \'02\') order by Student.S#
--方法2
select Student.* from Student , SC where Student.S# = SC.S# and SC.C# = \'01\' and Student.S# not in (SelectSC_2.S# from SC SC_2 where SC_2.S# = SC.S# and SC_2.C# = \'02\') order by Student.S#
--11、查询没有学全所有课程的同学的信息
--11.1、
select Student.*
from Student , SC
where Student.S# = SC.S#
group by Student.S# , Student.Sname , Student.Sage , Student.Ssex having count(C#) < (select count(C#)from Course)
--11.2
select Student.*
from Student left join SC
on Student.S# = SC.S#
group by Student.S# , Student.Sname , Student.Sage , Student.Ssex having count(C#) < (select count(C#)from Course)
--12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息
select distinct Student.* from Student , SC where Student.S# = SC.S# and SC.C# in (select C# from SC whereS# = \'01\') and Student.S# <> \'01\'
--13、查询和"01"号的同学学习的课程完全相同的其他同学的信息
select Student.* from Student where S# in
(select distinct SC.S# from SC where S# <> \'01\' and SC.C# in (select distinct C# from SC where S# = \'01\')
group by SC.S# having count(1) = (select count(1) from SC where S#=\'01\'))
--14、查询没学过"张三"老师讲授的任一门课程的学生姓名
select student.* from student where student.S# not in
(select distinct sc.S# from sc , course , teacher where sc.C# = course.C# and course.T# = teacher.T# andteacher.tname = N\'张三\')
order by student.S#
--15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
select student.S# , student.sname , cast(avg(score) as decimal(18,2)) avg_score from student , sc
where student.S# = SC.S# and student.S# in (select S# from SC where score < 60 group by S# havingcount(1) >= 2)
group by student.S# , student.sname
--16、检索"01"课程分数小于60，按分数降序排列的学生信息
select student.* , sc.C# , sc.score from student , sc
where student.S# = SC.S# and sc.score < 60 and sc.C# = \'01\'
order by sc.score desc
--17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
--17.1 SQL 2000 静态
select a.S# 学生编号 , a.Sname 学生姓名 ,
max(case c.Cname when N\'语文\' then b.score else null end) [语文],
max(case c.Cname when N\'数学\' then b.score else null end) [数学],
max(case c.Cname when N\'英语\' then b.score else null end) [英语],
cast(avg(b.score) as decimal(18,2)) 平均分
from Student a
left join SC b on a.S# = b.S#
left join Course c on b.C# = c.C#
group by a.S# , a.Sname
order by 平均分 desc
--17.2 SQL 2000 动态
declare @sql nvarchar(4000)
set @sql = \'select a.S# \' + N\'学生编号\' + \' , a.Sname \' + N\'学生姓名\'
select @sql = @sql + \',max(case c.Cname when N\'\'\'+Cname+\'\'\' then b.score else null end) [\'+Cname+\']\'
from (select distinct Cname from Course) as t
set @sql = @sql + \' , cast(avg(b.score) as decimal(18,2)) \' + N\'平均分\' + \' from Student a left join SC b on a.S# = b.S# left join Course c on b.C# = c.C#
group by a.S# , a.Sname order by \' + N\'平均分\' + \' desc\'
exec(@sql)
--24、查询学生平均成绩及其名次
--24.1 查询学生的平均成绩并进行排名，sql 2000用子查询完成，分平均成绩重复时保留名次空缺和不保留名次空缺两种。
select t1.* , px = (select count(1) from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t2 where 平均成绩 > t1.平均成绩) + 1 from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t1
order by px
select t1.* , px = (select count(distinct 平均成绩) from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t2 where 平均成绩 >= t1.平均成绩) from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t1
order by px
--24.2 查询学生的平均成绩并进行排名，sql 2005用rank,DENSE_RANK完成，分平均成绩重复时保留名次空缺和不保留名次空缺两种。
select t.* , px = rank() over(order by [平均成绩] desc) from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t
order by px
select t.* , px = DENSE_RANK() over(order by [平均成绩] desc) from
(
select m.S# [学生编号] ,
m.Sname [学生姓名] ,
isnull(cast(avg(score) as decimal(18,2)),0) [平均成绩]
from Student m left join SC n on m.S# = n.S#
group by m.S# , m.Sname
) t
order by px
--25、查询各科成绩前三名的记录
--25.1 分数重复时保留名次空缺
select m.* , n.C# , n.score from Student m, SC n where m.S# = n.S# and n.score in
(select top 3 score from sc where C# = n.C# order by score desc) order by n.C# , n.score desc
--25.2 分数重复时不保留名次空缺，合并名次
--sql 2000用子查询实现
select * from (select t.* , px = (select count(distinct score) from SC where C# = t.C# and score >= t.score) fromsc t) m where px between 1 and 3 order by m.c# , m.px
--sql 2005用DENSE_RANK实现
select * from (select t.* , px = DENSE_RANK() over(partition by c# order by score desc) from sc t) m where pxbetween 1 and 3 order by m.C# , m.px
--26、查询每门课程被选修的学生数
select c# , count(S#)[学生数] from sc group by C#
--27、查询出只有两门课程的全部学生的学号和姓名
select Student.S# , Student.Sname
from Student , SC
where Student.S# = SC.S#
group by Student.S# , Student.Sname
having count(SC.C#) = 2
order by Student.S#
--28、查询男生、女生人数
select count(Ssex) as 男生人数 from Student where Ssex = N\'男\'
select count(Ssex) as 女生人数 from Student where Ssex = N\'女\'
select sum(case when Ssex = N\'男\' then 1 else 0 end) [男生人数],sum(case when Ssex = N\'女\' then 1 else 0end) [女生人数] from student
select case when Ssex = N\'男\' then N\'男生人数\' else N\'女生人数\' end [男女情况] , count(1) [人数] from studentgroup by case when Ssex = N\'男\' then N\'男生人数\' else N\'女生人数\' end
--29、查询名字中含有"风"字的学生信息
select * from student where sname like N\'%风%\'
select * from student where charindex(N\'风\' , sname) > 0
--30、查询同名同性学生名单，并统计同名人数
select Sname [学生姓名], count(*) [人数] from Student group by Sname having count(*) > 1
--31、查询1990年出生的学生名单(注：Student表中Sage列的类型是datetime)
select * from Student where year(sage) = 1990
select * from Student where datediff(yy,sage,\'1990-01-01\') = 0
select * from Student where datepart(yy,sage) = 1990
select * from Student where convert(varchar(4),sage,120) = \'1990\'
--32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
select m.C# , m.Cname , cast(avg(n.score) as decimal(18,2)) avg_score
from Course m, SC n
where m.C# = n.C#
group by m.C# , m.Cname
order by avg_score desc, m.C# asc
--33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩
select a.S# , a.Sname , cast(avg(b.score) as decimal(18,2)) avg_score
from Student a , sc b
where a.S# = b.S#
group by a.S# , a.Sname
having cast(avg(b.score) as decimal(18,2)) >= 85
order by a.S#
--34、查询课程名称为"数学"，且分数低于60的学生姓名和分数
select sname , score
from Student , SC , Course
where SC.S# = Student.S# and SC.C# = Course.C# and Course.Cname = N\'数学\' and score < 60
--35、查询所有学生的课程及分数情况；
select Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course
where Student.S# = SC.S# and SC.C# = Course.C#
order by Student.S# , SC.C#
--36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数；
select Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course
where Student.S# = SC.S# and SC.C# = Course.C# and SC.score >= 70
order by Student.S# , SC.C#
--37、查询不及格的课程
select Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course
where Student.S# = SC.S# and SC.C# = Course.C# and SC.score < 60
order by Student.S# , SC.C#
--38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名；
select Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course
where Student.S# = SC.S# and SC.C# = Course.C# and SC.C# = \'01\' and SC.score >= 80
order by Student.S# , SC.C#
--39、求每门课程的学生人数
select Course.C# , Course.Cname , count(*) [学生人数]
from Course , SC
where Course.C# = SC.C#
group by Course.C# , Course.Cname
order by Course.C# , Course.Cname
--40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩
--40.1 当最高分只有一个时
select top 1 Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course , Teacher
where Student.S# = SC.S# and SC.C# = Course.C# and Course.T# = Teacher.T# and Teacher.Tname = N\'张三\'
order by SC.score desc
--40.2 当最高分出现多个时
select Student.* , Course.Cname , SC.C# , SC.score
from Student, SC , Course , Teacher
where Student.S# = SC.S# and SC.C# = Course.C# and Course.T# = Teacher.T# and Teacher.Tname = N\'张三\' and
SC.score = (select max(SC.score) from SC , Course , Teacher where SC.C# = Course.C# and Course.T# =Teacher.T# and Teacher.Tname = N\'张三\')
--41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
--方法1
select m.* from SC m ,(select C# , score from SC group by C# , score having count(1) > 1) n
where m.C#= n.C# and m.score = n.score order by m.C# , m.score , m.S#
--方法2
select m.* from SC m where exists (select 1 from (select C# , score from SC group by C# , score havingcount(1) > 1) n
where m.C#= n.C# and m.score = n.score) order by m.C# , m.score , m.S#
--42、查询每门功成绩最好的前两名
select t.* from sc t where score in (select top 2 score from sc where C# = T.C# order by score desc) order byt.C# , t.score desc
--43、统计每门课程的学生选修人数（超过5人的课程才统计）。要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列
select Course.C# , Course.Cname , count(*) [学生人数]
from Course , SC
where Course.C# = SC.C#
group by Course.C# , Course.Cname
having count(*) >= 5
order by [学生人数] desc , Course.C#
--44、检索至少选修两门课程的学生学号
select student.S# , student.Sname
from student , SC
where student.S# = SC.S#
group by student.S# , student.Sname
having count(1) >= 2
order by student.S#
--45、查询选修了全部课程的学生信息
--方法1 根据数量来完成
select student.* from student where S# in
(select S# from sc group by S# having count(1) = (select count(1) from course))
--方法2 使用双重否定来完成
select t.* from student t where t.S# not in
(
select distinct m.S# from
(
select S# , C# from student , course
) m where not exists (select 1 from sc n where n.S# = m.S# and n.C# = m.C#)
)
--方法3 使用双重否定来完成
select t.* from student t where not exists(select 1 from
(
select distinct m.S# from
(
select S# , C# from student , course
) m where not exists (select 1 from sc n where n.S# = m.S# and n.C# = m.C#)
) k where k.S# = t.S#
)
--46、查询各学生的年龄
--46.1 只按照年份来算
select * , datediff(yy , sage , getdate()) [年龄] from student
--46.2 按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一
select * , case when right(convert(varchar(10),getdate(),120),5) < right(convert(varchar(10),sage,120),5) thendatediff(yy , sage , getdate()) - 1 else datediff(yy , sage , getdate()) end [年龄] from student
--47、查询本周过生日的学生
select * from student where datediff(week,datename(yy,getdate()) +right(convert(varchar(10),sage,120),6),getdate()) = 0
--48、查询下周过生日的学生
select * from student where datediff(week,datename(yy,getdate()) +right(convert(varchar(10),sage,120),6),getdate()) = -1
--49、查询本月过生日的学生
select * from student where datediff(mm,datename(yy,getdate()) +right(convert(varchar(10),sage,120),6),getdate()) = 0
--50、查询下月过生日的学生
select * from student where datediff(mm,datename(yy,getdate()) +right(convert(varchar(10),sage,120),6),getdate()) = -1
drop table Student,Course,Teacher,SC