为了大家更容易理解我举出的SQL语句,本文假定已经建立了一个学生成绩管理数据库,全文均以学生成绩的管理为例来描述。
  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.交叉连接
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,如查询李山的同乡:
select * from students
where native in (select native from students where name=’ 李山’)
  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。

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