mysql查询命令详细

-- 数据库准备
    -- 创建一个数据库
    create database python_test charset=utf8
    -- 使用数据库
    use python_test
    -- 显示当前使用的数据库是哪个
    select database()
    -- 创建一个数据表students
    create table students(
        id int unsigned not null primary key auto_increment,
        name varchar(20) default '',
        age tinyint unsigned default 0,
        height decimal(5,2),
        gender enum('男','女','保密') default '保密',
        cls_id int unsigned default 0,
        is_delete bit default 0
    );
    -- 创建classes表
    create table classes(
        id int unsigned not null primary key auto_increment,
        name varchar(30) not null
    );
    -- 向students表中插入数据
    insert into students values
        (0,'小明',18,180.00,2,1,0),
        (0,'小月月',18,180.00,2,2,1),
        (0,'彭于晏',29,185.00,1,1,0),
        (0,'刘德华',59,175.00,1,2,1),
        (0,'黄蓉',38,160.00,2,3,0)，
        (0,'凤姐',28,150.00,3,2,1)，
        (0,'王祖贤',18,172.00,2,1,1)，
        (0,'周杰伦',36,NULL,1,1,0)，
        (0,'程坤',27,181.00,1,2,0)，
        (0,'周杰伦',36,NULL,1,1,0)，
        (0,'刘亦菲',25,166.00,2,2,0)，
        (0,'静香',12,180.00,2,4,0)，
        (0,'郭靖',12,170.00,1,4,0)，
        (0,'周杰',34,176.00,2,5,0);
    -- 向classes中插入数据
    insert into classes values
        (0,'python_01期'),
        (0,'python_02期'),
        (0,'python_03期');


-- 查询
    -- 查询所有字段
    -- select * from 表名；
    select * from students;
    select * from classes;

    -- 查询指定字段
    -- select 列1，列2... from 表名；
    select name,age from students;

    -- 使用as给字段起别名
    -- select 字段 as 别名... from 表名;
    select name as 姓名,age as 年龄 from students;

    -- select 表名.字段... from 表名；
    select students.name, students.age 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.age from students as s;

    -- 消除重复行
    -- 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;

        -- >=
        -- <=
        -- 查询小于或等于18岁的信息
        select * from students where age<=18;

        -- != 或者<>
        -- 查询年龄不等于18岁的消息
        select * from students where age!=18;

    -- 逻辑运算符
        -- and 
        -- 18到28之间的所有学生信息
        select * from students where age>18 and age<28;
        -- 失败 select * from students where age>18 and <28;

        -- or
        -- 18以上或者身高超过180（包含）
        select * from students where age>18 or height>=180;

        -- not
        -- 不是 18岁以上的女性
        select * from students where not (age>18 and gender=12);

        -- 年龄不是小于或等于18 并且是女性
        select * from students where (not age<=18) and gender=2;

    -- 模糊查询
        -- like
        -- % 替换1个或者多个
        -- _ 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’的所有名字
        select name from students where name like '%小%';

        -- 查询有2个字的名字
        select name from students where name like '__';

        -- 查询至少有2个字的名字
        select name from students where name like '__%';

        -- rlike 正则
        -- 查询以 周开始的姓名
        select name from students where name rlike '^周.*';

        -- 查询以周开始、伦结尾的姓名
        select name from students where name rlike '^周.*伦$';

    -- 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12，18,34的姓名
        select name from students where age in (12,18,34);

        -- not in 不非连续的范围内
        -- 年龄不是18，34之间的信息
        select name    from students where age not in (18,34);

        -- between ... and ... 表示在一个连续的范围内
        -- 查询年龄在18到34之间的信息
        select name 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18到34之间的数据
        select name from students where age not between 18 and 34;
        -- 失败 select name 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 name from students where height is null;

        -- 判非空 is not null

-- 排序
    -- order by 字段
    -- asc从小到大排序 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，按照年龄从小到大排序
    select name from students where (age between 18 and 34) and gender=1 order by age asc

    -- 查询年龄在18岁到34岁之间的女性，身高从高到矮排序
    select name 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 name 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 name 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;
    select count(*) as 男性人数 from students where gender=1;
    select count(*) as 女性人数 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
    -- main
    select main(height) 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;

    -- 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;

    -- 计算每种性别中的人数
    select gender,count(*) from students group by gender;

    -- 计算男性的人数
    select gender,count(*) from students where gender=1 group by gender;

    -- group_count(...)
    -- 查询同种性别中的人名
    select gender,group_count(name) from students group by gender;
    -- group_count内多种参数，用符号分隔。查询同性别的人名、年龄和id
    select gender,group_countn(name, '_' , age, '_' ,id) from students group by gender_count;

    -- having
    -- 查询平均年龄超过30岁的性别，以及姓名 having avg(age)>30
    select gender,group_count(name) from students group by gender having avg(age)>30;

    -- 查询每种性别中人数大于2的性别及姓名
    select gender,group_count(name) from students group by gender having count(*)>2;

-- 分页
    -- limit start, count

    -- 限制查询出来的数据个数
    select * from students where gender=1 limit 2;

    -- 查询前5个数据
    select * from students limit 0, 5;

    -- 查询id 6-10(包含)的数据
    select * from students limit 5, 5;

    -- 每页显示2个，显示第1页数据
    select * from students limit 0, 2;

    -- 每页显示2个，显示第2页数据
    select * from students limit 2, 2;

    -- 每页显示2个，显示第3页数据
    select * from students limit 4, 2;

    -- 每页显示2个，显示第4页数据
    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 limit 10, 2;
    -- 失败select * from students order by age limit (6-1)*2, 2

-- 连接查询
    -- 内连接
    -- inner join ... on
    -- 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，只显示班级名臣
    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;
    -- select c.name,s.* from classes as c inner join students as s on c.id=s.cls_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.name,s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

    -- 当是同一个班级的时候，按照学生的id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;

    -- 左连接
    -- left join
    -- 查询每位学生对应的班级信息
    select * from students as s left join classes as c on s.cls_id=c.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 as s left join classes as c on s.cls_id=c.id where c.id is null;
    select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;

    -- right join on 
    -- 将数据表名字互换位置，用left join完成

-- 自关联
    -- 导入sql语句
    -- sql语句文件所在目录，进入数据库(use 数据库名)。 source sql文件名 即可导入
    source areas.sql
    -- 省级联动 http://demo.lanrenzhijia.com/2014/city0605/

    -- 查询所有省份
    select * from areas where pid=0;

    -- 查询出江西省有哪些市
    select * from areas as province inner join areas as city on city.pid=province.id having province.atitle='江西省';
    -- 只显示省份和市的名称
    select province.atitle,city.atitle from areas as province inner join areas as city on city.pid=province.id having province.atitle='江西省';

    -- 查询出景德镇市有哪些县城
    select province.atitle,city.atitle from areas as province inner join areas as city on city.pid=province.id having province.atitle='景德镇市';

-- 子查询
    -- 标量子查询
    -- 查询出高于平均身高的信息

    -- 查询最高的男生信息
    select * from students where height=(select max(height) from students);

    -- 列级子查询
    -- 查询学生的班级号能够对应的学生信息
    select * from students where cls_id in (select id from classes);

查询的完整格式：

SELECT select_expr [,select_expr,...] [      
      FROM tb_name
      [WHERE 条件判断]
      [GROUP BY {col_name | postion} [ASC | DESC], ...] 
      [HAVING WHERE 条件判断]
      [ORDER BY {col_name|expr|postion} [ASC | DESC], ...]
      [ LIMIT {[offset,]rowcount | row_count OFFSET offset}]
]