-- 查询练习
-- 查询所有字段
-- select * from 表名;
select *
from students ;
-- 查询指定字段
-- select 列1,列2,... from 表名;
select
name,gender
from students;
-- 使用 as 给字段起别名
-- select 字段 as 名字.... from 表名;
select
name
as
"姓名",gender
as
"性别"
from students;
-- select 表名se.字段 .... from 表名;
select students.name,students.gender
from students;
-- 可以通过 as 给表起别名
-- select 别名.字段 .... from 表名 as 别名;
select s.name,s.gender
from students
as s;
失败的select students.name, students.age
from students
as s;
-- 消除重复行(查性别)
-- distinct 字段
select distinct gender
from students;
-- 条件查询
-- 比较运算符
-- select .... from 表名 where .....
-- >
-- 查询大于18岁的信息
select *
from students
where age >
18;
-- <
-- 查询小于18岁的信息
select *
from students
where age <
18;
-- >=
-- <=
-- 查询小于或者等于18岁的信息
select *
from students
where age <=
18;
-- =
-- 查询年龄为18岁的所有学生的名字
select *
from students
where age =
18;
(注意)
select *
from students
where age =
18;
-- != 或者 <>
select *
from students
where age !=
18;
-- 逻辑运算符
-- and
-- 18和28之间的所以学生信息
select *
from students
where age >
18
and age <
28;
失败select *
from students
where age>
18
and <
28;
-- 18岁以上的女性
select *
from students
where age >
18
and gender =
2;
-- or
-- 18以上或者身高高过180(包含)以上
select *
from students
where age >
18
or height >=
180;
-- not
-- 不在 18岁以上的女性 这个范围内的信息
-- select * from students where not age>18 and gender=2;
select *
from students
where
not (age >
18
and gender =
2);
-- 模糊查询(where name like 要查询的数据)
-- like
-- % 替换任意个
-- _ 替换1个
-- 查询姓名中 以 "小" 开始的名字
select *
from students
where
name
like
"小%";
-- 查询姓名中 有 "小" 所有的名字
select *
from students
where
name
like
"%小%";
-- 查询有2个字的名字
select *
from students
where
name
like
"__";
-- 查询有3个字的名字
select *
from students
where
name
like
"___";
-- 查询至少有2个字的名字
select *
from students
where
name
like
"__%";
-- 范围查询
-- in (1, 3, 8)表示在一个非连续的范围内
-- 查询 年龄为18、34的姓名
select *
from students
where age =
18
or age =
34;
select *
from students
where age
in (
18,
34);
-- not in 不非连续的范围之内
-- 年龄不是 18、34岁之间的信息
select *
from students
where age
not
in (
18,
34);
(注意)
select
name
from students
where
not age
in (
18,
34);
-- between ... and ...表示在一个连续的范围内
-- 查询 年龄在18到34之间的的信息
select *
from students
where age between
18
and
34;
-- not between ... and ...表示不在一个连续的范围内
-- 查询 年龄不在在18到34之间的的信息
select *
from students
where age
not between
18
and
34;
失败的select *
from students
where age
not (between
18
and
34);
-- 空判断
-- 判空is null
-- 查询身高为空的信息
select *
from students
where height
is
null;
-- 判非空is not null
select *
from students
where height
is
not
null;
失败select *
from students
where height
not
is
null;
-- 排序
-- order by 字段
-- asc 从小到大排列，即升序
-- desc 从大到小排序，即降序
-- 查询年龄在18到34岁之间的男性，按照年龄从小到大到排序
select *
from students
where (age between
18
and
34)
and gender =
1
order by age
asc;
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序
select *
from students
where (age between
18
and
34)
and gender =
2
order by height
desc;
-- order by 多个字段
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
select *
from students
where (age between
18
and
34)
and gender =
2
order by height
desc,age
asc;
-- 查询年龄在18到34岁之间的女性，身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,
-- 如果年龄也相同那么按照id从大到小排序
select *
from students
where (age between
18
and
34)
and gender =
2
order by height
desc,age
asc,id
desc;
-- 按照年龄从小到大、身高从高到矮的排序
select *
from students
order by age
asc,height
desc;
-- 聚合函数
-- 总数
-- count
-- 查询男性有多少人，女性有多少人
select
count(*)
from students
where gender =
1;
-- 最大值
-- max
-- 查询最大的年龄
select
max(age)
from students;
-- 查询女性的最高 身高
select
max(height)
from students
where gender =
2;
-- 最小值
-- min
select
min(height)
from students
where gender =
2;
-- 求和
-- sum
-- 计算所有人的年龄总和
select
sum(age)
from students ;
-- 平均值
-- avg
-- 计算平均年龄
select
avg(age)
from students;
-- 计算平均年龄 sum(age)/count(*)
select
sum(age)/
count(*)
from students;
-- 四舍五入 round(123.23 , 1) 保留1位小数
-- 计算所有人的平均年龄，保留2位小数
select
round(
avg(age),
2)
from students;
-- 计算男性的平均身高 保留2位小数
select
round(
avg(height),
2)
from students
where gender =
1;
-- 分组
-- group by
-- 按照性别分组,查询所有的性别
select gender
from students
group by gender;
select
name,gender
from students
group by gender;错误
-- select name,gender from students group by gender;
-- 失败select * from students group by gender;
-- 计算每种性别中的人数
select gender,
count(*)
from students
group by gender;
-- group_concat(...)
-- 查询同种性别中的姓名
select gender,group_concat(
name)
from students
group by gender;
-- 查询每组性别的平均年龄
select gender,
avg(age)
from students
group by gender;
-- having
-- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
select gender,
avg(age)
from students
group by gender
having
avg(age) >
30;
-- 查询每种性别中的人数多于2个的信息
select gender,
count(*)
from students
group by gender
having
count(*) >
2;
-- with rollup 汇总的作用(了解)
select gender,
count(*)
from students
group by gender
with
rollup;
-- 分页
-- limit start, count
-- 限制查询出来的数据个数
-- 查询前5个数据
select *
from students
limit
5;
-- 每页显示2个，第1个页面
select *
from students
limit
0,
2;
-- 每页显示2个，第2个页面
select *
from students
limit
2,
2;
-- 每页显示2个，第3个页面
select *
from students
limit
4,
2;
-- 每页显示2个，第4个页面
select *
from students
limit
6,
2;
-- 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
select *
from students
order by age
asc
limit
10,
2;
错误1
select *
from students
limit
10,
2
order by age
asc;
-- 工作错误的写法
错误2
select *
from students
limit
2*(
6-
1),
2;
-- limit 放在最后面(注意)
-- 连接查询
-- inner join ... on
-- select ... from 表A inner join 表B;
select *
from students
inner join classes;
-- 查询 有能够对应班级的学生以及班级信息
select *
from students
inner join classes
on students.cls_id = classes.id;
-- 按照要求显示姓名、班级
select students.name,classes.name
from students
inner join classes
on students.cls_id = classes.id;
-- 给数据表起名字
select s.name,c.name
from students
as s
inner join classes
as c
on s.cls_id = c.id;
-- 查询 有能够对应班级的学生以及班级信息，显示学生的所有信息 students.*，只显示班级名称 classes.name.
select s.*,c.name
from students
as s
inner join classes
as c
on s.cls_id = c.id;
-- 在以上的查询中，将班级姓名显示在第1列
select c.name,s.*
from students
as s
inner join classes
as c
on s.cls_id = c.id;
-- 查询 有能够对应班级的学生以及班级信息, 按照班级进行排序
-- select c.xxx s.xxx from students as s inner join clssses as c on .... order by ....;
select c.name,s.*
from students
as s
inner join classes
as c
on s.cls_id = c.id
order by c.name;
-- 当时同一个班级的时候，按照学生的id进行从小到大排序
select c.name,s.*
from students
as s
inner join classes
as c
on s.cls_id = c.id
order by c.name,s.id
asc;
-- left join
-- 查询每位学生对应的班级信息
select *
from students
inner join classes
on students.cls_id = classes.id;
select *
from students
left join classes
on students.cls_id = classes.id;
-- select * from students right join classes on students.cls_id = classes.id;
-- 查询没有对应班级信息的学生
-- select ... from xxx as s left join xxx as c on..... where .....
-- select ... from xxx as s left join xxx as c on..... having .....
select *
from students
left join classes
on students.cls_id = classes.id
where classes.name
is
null;
(注意)不建议使用
select *
from students
left join classes
on students.cls_id=classes.id
having classes.id
is
null;
-- right join on
-- 将数据表名字互换位置，用left join完成
-- 子查询
-- 标量子查询: 子查询返回的结果是一个数据(一行一列)
-- 列子查询: 返回的结果是一列(一列多行)
-- 行子查询: 返回的结果是一行(一行多列)
-- 查询出高于平均身高的信息(height)
-- 1 查出平均身高
select
avg(height)
from students;
-- 2 查出高于平均身高的信息
select *
from students
where height > (
select
avg(height)
from students);
-- 查询学生的班级号能够对应的 学生名字
-- select name from students where cls_id in (select id from classes);
-- 1 查出所有的班级id
select id
from classes;
-- 2 查出能够对应上班级号的学生信息
select *
from students
where cls_id
in (
select id
from classes);