MySQL查询

-- 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.字段 .... from 表名;
select name as "姓名",gender as "性别" from students as s;

-- 可以通过 as 给表起别名
-- select 别名.字段 .... from 表名 as 别名;
select s.name as "姓名",s.gender as "性别" 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 gender from students;
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
(区别)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;

-- 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;
select count(*) from students where gender = 2;

-- 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(*)

-- 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3位小数
select round(avg(age),3) 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;
-- 失败select * from students group by gender;
select * from students group by gender;
-- 计算每种性别中的人数
select count(*),gender from students group by gender;

-- group_concat(...)
-- 查询同种性别中的姓名
select group_concat(name),gender from students group by gender;

-- 查询每组性别的平均年龄
select avg(age),gender from students group by gender;

-- having(注意having和group by)
(!)-- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30
select gender,group_concat(name) 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,group_concat(name) 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(limit 必须写到sql语句的最后)
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,id asc;

-- left join
-- 查询每位学生对应的班级信息
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 ; -- 172
-- 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; -- 1,2
-- 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);