mariadb 3

Posted 2020-11-14 openstack彭

 

基本查询

--查询基本使用(条件，排序，聚合函数，分组，分页)


--创建学生表
create table students (
    id int unsigned not null auto_increment primary key,
    name varchar(20) default ‘‘,
    age tinyint unsigned default 0,
    high decimal(5,2),
    gender enum(‘男‘, ‘女‘, ‘中性‘, ‘保密‘) default ‘保密‘,
    cls_id int unsigned default 0,
    is_delete bit default 0
);


--创建班级表
create table classes(
    id int unsigned auto_increment primary key not null,
    name varchar(20) not null
);

--往students表里插入数据
insert into students values
(0,‘小明‘,18,180.00,1,1,0),
(0,‘小月月‘,19,180.00,1,2,0),
(0,‘彭于晏‘,28,185.00,1,1,0),
(0,‘刘德华‘,58,175.00,1,2,0),
(0,‘黄蓉‘,108,160.00,2,1,0),
(0,‘凤姐‘,44,150.00,4,2,1),
(0,‘王祖贤‘,52,170.00,2,1,1),
(0,‘周杰伦儿‘,34,null,1,1,0),
(0,‘程坤‘,44,181.00,1,2,0),
(0,‘和珅‘,55,166.00,1,2,0),
(0,‘刘亦菲‘,29,162.00,2,3,0),
(0,‘金星‘,45,180.00,3,4,0),
(0,‘静香‘,18,170.00,2,4,0),
(0,‘郭靖‘,22,167.00,1,5,0),
(0,‘周杰‘,33,178.00,1,1,0),
(0,‘钱小豪‘,56,178.00,1,1,0),
(0,‘谢霆锋‘,38,175.00,1,1,0),
(0,‘陈冠希‘,38,175.00,1,1,0);

--查询
-- 查询所有列
    --select * from 表名
    select * from students;
    
    --一定条件查询(where)
    select * from where id=5;
    
    
    -- 查询制定列
    select id,name from students;
    
    
    -- 使用as给字段起别名
    select id,name as ‘姓名‘, age, high, gender from students;
    
    -- 通过表名字段查询
    select students.name from students;
    
    -- 给表起别名查询
    select s.id,s.name,s.age from students as s;
    
    --消除重复行
    -- distinct
    select distinct age from students;


    
    --条件查询
        --比较运算符
        -- 查询年纪大于18岁的信息
        select * from students where age > 18;
        
        --18岁到28岁之间(and)
        select * from students where age >= 18 and age =< 28;
        select * from students where age between 18 and 28
        --在18岁以上或者身高180以上的人(or)
        select * from students where age > 18 or high > 180;

        

    -- 模糊查询
        -- like
        -- % 替代1个或者多个甚至是没有
        -- 查询姓名中有‘小’的所有名字
        select * from students where name like ‘%小%‘;
        
        -- 查询两个字人的名字
        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 年纪为18和34的人
        select * from students where age in (18, 34);
     
        
        --查询 年龄在17岁到34岁之间的信息
        select * from students where age between 17 and 34;
        
        
        --查询 年纪不在18到34岁的信息
        select * from students where age not between 17 and 34;
    
    -- 空判断
        -- 判断is null
        -- 查询身高为空的信息
        select * from students where high is null;
        
        -- 判断非空is not null
        select * from students where high 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
        
        -- 查询年纪在18到34岁之间的男性，按照年纪从小到大
        select * from students where gender=1 and age between 18 and 34 order by age;
        
        
        -- 查询年纪在18到34岁之间的女性，身高从高到矮
        select * from students where gender=2 and age between 18 and 34 order by high desc;
        
        -- order by 多字段
        -- 查询年纪在18到34岁的女性，身高从高到矮排序，如果身高相同的情况下按照年纪从小到大排序
            select * from students where age between 18 and 34 and gender=2 order by high desc;
        
        -- 查询年纪在18到34岁的男性，身高从高到矮排序，如果身高相同的情况下按照年纪从小到大排序，如果年龄也相等那么按照id从小到大排序；
            select * from students where age between 18 and 34 and gender=1 order by high desc, age, id desc;
        
        

        

        
--聚合函数
    -- 总数
    -- count 
    -- 查询男性有多少人
    select count(*) from students where gender=1;
    
    
    -- 最大值
    -- max
    -- 查询最大的年纪
    select max(age) from students;
    
    
    -- 查询女性的最高 身高
    select max(high) from students where gender=2;
    
    -- 最小值
    -- min
    select min(high) from students;
    
    -- 求和
    -- sum
    -- 计算所有人的年龄总和
    select sum(age) from students;
    
    -- 平均值
    -- avg
    -- 计算平均年纪
    -- 计算平均年纪 sum(age)/count(*)
    select sum(age)/count(*) from students;
    select avg(age),2 from students;
    -- 保留2位小数
    select round(avg(age),2) from students;
    
-- 分组
    -- group by
    -- 按照性别分组，查询所有的性别
    select gender from students group by gender;
    
    
    -- 计算每组性别的人数
    select gender, count(*) from students group by gender;

    
    -- 查询男性组中的姓名 group_concat
    select gender,group_concat(name) from students where gender=1 group by gender; 
    
    -- having
    -- 查询每个性别平均年纪超过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4个的组的信息
    select gender,group_concat(name) from students group by gender having count(*)>4;
-- 分页
    -- 显示5页
    select * from students limit 5;

    -- 分页显示，每页显示2条数据
    select * from students limit 0, 2;

    -- 按照身高从高到矮排序，查找出所有女性，并且分页显示，每页显示2条数据
    select * from students where gender=2 order by high desc limit 0,2;