标签:students 18 数据库 day2 id Mysql where age select
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;
-- =
-- 查询年龄为18岁的所有学生的名字
select * from students where age = 18;
-- != 或者 <>
select * from students where age != 18; select * from students where age <> 18;
逻辑运算符
-- and
-- 18和28之间的所有学生信息
select * from students where age>18 and age<28;
-- 18岁以上的女性
select * from students where age>18 and gender="女";
-- 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="女");
-- 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 in (18, 34); select * from students where age=18 or age=34;
-- not in 不非连续的范围之内
-- 年龄不是 18、34岁的信息
select * from students where age not in (18, 34); select * from students where not (age 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; select * from students where age>=18 and age<=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;
-- 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;
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(默认是asc升序)
select * from students where (age between 18 and 34) and (gender='男'); select * from students where (age between 18 and 34) and (gender='男') order by age;
-- 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='女') 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大到小排序
select * from students where (age between 18 and 34) and (gender='女') order by height desc, age desc;
-- 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大到小排序,
-- 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='女') order by height desc, age desc, id desc;
聚合函数
-- 总数
-- count
-- 计算班级学生的总数
select * from students; select count(*) from students;
-- 查询男性有多少人,女性有多少人
select count(*) from students where gender='男'; select count(*) from students where gender='女';
-- 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='女';
-- 最小值
-- min
select min(age) from students;
-- 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;
分组(重点)
-- group by
-- 按照性别分组,查询所有的性别
select gender from students group by gender; select distinct gender from students; select name,gender from students group by name,gender; select name,gender from students group by gender;错误
-- 计算每种性别中的人数
select gender,count(*) from students group by gender;
-- 计算每个年龄中的人数
select age,count(*) from students group by age;
-- 查询 男、女性别中年龄的最大值
select gender,max(age) from students group by gender; select gender,max(age) from students group by gender having gender in ('男', '女'); select gender,max(age) from students where gender in ('男', '女') 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;
-- with rollup 汇总的作用
select gender,group_concat(name),max(age),max(height) from students group by gender with rollup;
limit 查询
-- 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 0, 5;
-- 分页查询:
select * from students limit (n-1)*m, m;
-- 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;
连接查询(重点)
-- 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 students.name as 学生名,classes.name as 班级名 from students inner join classes on students.cls_id=classes.id;
-- 内连接的另外一种写法
select * from students, classes where students.cls_id=classes.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 classes.name,students.* from students inner join classes on students.cls_id=classes.id;
-- 查询 有能够对应班级的学生以及班级信息, 按照班级编号进行排序
-- select c.xxx s.xxx from students as s inner join clssses as c on .... order by ....;
select classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.id;
-- 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 classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.id, students.id; select c.name,s.* from students as s inner join classes as c on s.cls_id=c.id order by c.id, s.id; select c.name,s.* from students s inner join classes c on s.cls_id=c.id order by c.id, s.id; select c.name,s.* from students s, classes c where s.cls_id=c.id order by c.id, s.id;
-- 自连接查询
-- 表和自己建立连接
-- 1、查询一共有多少个省
select * from areas where pid is not null;
-- 2、查询省的名称为“广东省”的所有城市
select * from areas as c inner join areas as p on c.pid=p.id where p.title='广东省';
-- 3、查询省的名称为“深圳市”的所有县
select * from areas as a inner join areas as c on a.pid=c.id where c.title='深圳市';
-- 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(age) from students; select avg(height) from students;
-- 2 查出高于平均身高的信息
select * from students where age > (select avg(age) from students); select * from students where height > (select avg(height) from students);
-- 查询学生的学号和班级编号对应的学生信息
-- 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(查询学生的班级号,在班级表中有, 的学生信息)
select * from students where cls_id in (select id from classes);
-- 例3. 查找年龄最大,身高最高的学生:
select max(age),max(height) from students; select * from students where (age,height) = (select max(age),max(height) from students);
标签:students,18,数据库,day2,id,Mysql,where,age,select 来源: https://www.cnblogs.com/tracydzf/p/12581563.html
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