DQL数据查询语句使用示例

-- 查询所有的学生  select 字段 from 表
select * from student
-- 查询指定字段
select studentno,studentname from student
-- 给结果起一个名字 ， As，可以给字段起别名，也可以给表起别名
select studentno as 学号,studentname as 学生姓名 from student
select studentno 学号,studentname 学生姓名 from student
-- 函数 concat（a，b）
select CONCAT('姓名：',studentname) as 新名字 from student

-- 查询一下有哪些同学参加了考试
select * from result -- 查询全部的考试成绩
select studentno from result -- 查询有哪些学生参加了考试
select distinct studentno from result -- 去重

select version() -- 查询系统版本号（函数）
select 100*3-2 -- 用来计算（表达式）
select @@auto_increment_increment -- 查询自增的步长（变量）
-- 学生考试成绩+1分查看
select studentno,studentresult+1 as '提分后' from result

select studentno,studentresult from result
-- 查询考试成绩在95~100之间的
-- and &&
select studentno,studentresult from result 
where studentresult>=95 and studentresult<=100
-- 模糊查询（区间）
select studentno,studentresult from result 
where studentresult between 95 and 100
-- ! not
select studentno,studentresult from result 
where not studentno=1000

-- ===========模糊查询========================
-- 查询姓张的同学
-- like结合%（代表0到任意个字符） _（一个字符）
select studentno,studentname from student
where studentname like '张%'
-- 查询姓张的同学，名字只有两个字
select studentno,studentname from student
where studentname like '张_'
-- ======in（具体的一个或者多个值）========
-- 查询学号1000,1001的学生
select studentno,studentname from student
where studentno in(1000,1001)
-- 查询在北京的学生
select studentno,studentname from student
where address in('北京朝阳')
-- ========null not null====================
-- 查询地址为空的同学 null或者''
select studentno,studentname from student
where address='' or address is null
-- 查询出生日期不为空的学生
select studentno,studentname from student
where borndate is not null

 -- 查询参加考试的同学（学号，姓名，科目编号，分数）
select * from student
select * from result

select s.studentno,studentname,subjectno,studentresult 
from student as s
inner join  result as r
where s.studentno=r.studentno
-- right join 
select s.studentno,studentname,subjectno,studentresult 
from student as s
right join result as r
on s.studentno = r.studentno
-- left join 
select s.studentno,studentname,subjectno,studentresult 
from student as s
left join result as r
on s.studentno = r.studentno

-- ===============联表查询==================
 -- 查询参加考试的同学（学号，姓名，科目编号，分数）
select * from student
select * from result

-- join（连接的表） on（判断的条件） 连接查询
-- where 等值查询
select s.studentno,studentname,subjectno,studentresult 
from student as s
inner join  result as r
where s.studentno=r.studentno
select s.studentno,studentname,subjectno,studentresult 
from student as s
inner join  result as r
on s.studentno=r.studentno
-- right join 
select s.studentno,studentname,subjectno,studentresult 
from student as s
right join result as r
on r.studentno = s.studentno
-- left join 
select s.studentno,studentname,subjectno,studentresult 
from student as s
left join result as r
on s.studentno = r.studentno
-- 查询缺考的同学(只要让成绩为null，就能找到）
select s.studentno,studentname,subjectno,studentresult 
from student as s
left join result as r
on s.studentno = r.studentno where studentresult is null
-- 查询了参加考试的同学信息：学号，学生姓名，科目名称，分数
-- 表 student result subject
-- 右查询
-- 交叉点：学生表 studentno=成绩表 studentno  
--         成绩表 subjectno=科目表 subjectno
select s.studentno,studentname,sub.subjectname,studentresult
from student as s
right join result as r
on r.studentno=s.studentno
inner join `subject` as sub
on r.subjectno=sub.subjectno
-- 我要查询哪些数据 select ...
-- 从哪几个表中查询 from 表 xxx join 连接的表 on 较差条件
-- 假设存在一种多张表查询，先查询两张表开始，然后再慢慢增加
-- from a left join b
-- from a right join b

-- 查询父子信息:把一张表看为两个一模一样的表
select a.cateGoryname as '父栏目',b.categoryname as '子栏目'
from category as a,category as b
where a.categoryid = b.pid

-- 排序：升序 ASC  降序DESC
-- 通过哪个字段排序，怎么排
select s.studentno,studentname,subjectname,studentresult
from student as s
inner join result as r
on s.studentno = r.studentno
inner join `subject` as sub
on sub.subjectno=r.subjectno
where subjectname = '高等数学-1'
order by studentresult DESC

-- 为什么要分页
-- 缓解数据库的压力，给人更好的体验  ---瀑布流
-- 分页，每页显示五条数据
-- 语法：limit 起始值，页面的大小
-- 网页应用：当前页，总页数，页面的大小
-- limit 0,5 1~5条数据
-- limit 1,5 2~6条数据
select * from student limit 0,5
-- 第一页 limit 0,5   （1-1）*5
-- 第二页 limit 5,5   （2-1）*5
-- 第二页 limit 10,5  （3-1）*5
-- 第N页  limit       （n-1）*5
-- pageSize：页面的大小
-- （n-1）*pageSize：起始值
-- n：当前页
-- 数据总数/页面大小=总页数

-- 查询高等数学-1的所有考试结果（学号，科目编号，成绩），降序排列
-- 方式1
select studentno,subjectno,studentresult
from result as r
inner join `subject` as sub
on r.subjectno=sub.subjectno
where subjectname='高等数学-1'
order by studentresult DESC
-- 方式2：使用子查询
select studentno,subjectno,studentresult
from result
where subjectno=(
	select subjectno from `subject` 
	where  subjectname='高等数学-1'
)order by studentresult DESC
-- 查询所有高等数学-1的学生的学号
select subjectno from `subject` where subjectname='高等数学-1'
-- 查询分数不小于80分的学生的学号和姓名
select studentno,studentname
from student 
where studentno=(
select distinct studentno from result where studentresult>=80
)
-- 查询课程为高等数学-2并且分数不小于80分的同学的学号和姓名
-- 联表查询
select s.studentno,studentname
from student as s
inner join result as r
on s.studentno=r.studentno
inner join `subject` as sub
on r.subjectno=sub.subjectno
where subjectname='高等数学-2' and studentresult>=80
-- 子查询
select DISTINCT s.studentno,studentname
from student as s
inner join result as r
on s.studentno=r.studentno
where studentresult>=80 and subjectno=(
	select subjectno from `subject` where subjectname='高等数学-2'
)
-- 子查询：（由里及外）
select studentno,studentname
from student where studentno in(
	select studentno from result where studentresult>=80 and subjectno=(
		select subjectno from `subject` where subjectname='高等数学-2'
	)
)