mysql语句与使用

— 数据的准备
— 创建一个数据库
create database python_test charset=utf8;

— 使用一个数据库
use python_test;

— 显示使用的当前数据是哪个?
select database();

   — 创建一个数据表
   — students表
   create table students(
   id int unsigned primary key auto_increment not null,
   name varchar(20) default \’\’,
   age tinyint unsigned default 0,
   height 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
   );

   — classes表
   create table classes (
   id int unsigned auto_increment primary key not null,
   name varchar(30) not null
   );

— 查询
   — 查询所有字段
   — select * from 表名;
   select * from students;
   select * from classes;
   select id, name 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 名字…. 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 students.name, students.age from students;
   select s.name, s.age from students as s;
   — 失败的select students.name, students.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;
       select id,name,gender 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

       — =
       — 查询年龄为18岁的所有学生的名字
       select * from students where age=18;

— != 或者 <>

   — 逻辑运算符
       — and
       — 18到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=\”女\”;
       select * from students where age>18 and gender=2;

       — or
       — 18以上或者身高查过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);

— 年龄不是小于或者等于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 name from students where name=\”小\”;
       select name from students where name like \”小%\”;

— 查询姓名中有 \”小\” 所有的名字
select name from students where name like \”%小%\”;

— 查询有2个字的名字
select name from students where name like \”__\”;

— 查询有3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18、34的姓名
       select name,age from students where age=18 or age=34;
       select name,age from students where age=18 or age=34 or age=12;
       select name,age from students where age in (12, 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 name,age from students where age not in (12, 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 name, age 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;
       select * from students where not age 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;
       select * from students where height is NULL;
       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，按照年龄从小到到排序
   select * from students where (age between 18 and 34) and gender=1;
   select * from students where (age between 18 and 34) and gender=1 order by age;
   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,id 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 * from students where gender=1;
   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 age from students;
   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

   — 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(sum(age)/count(*), 2) from students;
   select round(sum(age)/count(*), 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;
   — select name, 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 name from students group by gender;
   select * from students group by gender;
   select 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;

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

   — group_concat(…)
   — 查询同种性别中的姓名
    select gender,group_concat(name) from students where gender=1 group by gender;
    select gender,group_concat(name, age, id) from students where gender=1 group by gender;
    select gender,group_concat(name, \”_\”, age, \” \”, id) from students where gender=1 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, group_concat(name),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, group_concat(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id6-10（包含）的书序
select * from students limit 5, 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; — —–> limit (第N页-1)*每个的个数, 每页的个数;

   — 每页显示2个，显示第6页的信息, 按照年龄从小到大排序
   — 失败select * from students limit 2*(6-1),2;
   — 失败select * from students limit 10,2 order by age asc;
   select * from students order by age asc limit 10,2;

select * from students where gender=2 order by height desc limit 0,2;

— 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.*, classes.name from students inner join classes on students.cls_id=classes.id;
   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，只显示班级名称
   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 student 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;

   — 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 having c.id is null;
   select * from students as s left join classes as c on s.cls_id=c.id where 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
— 省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/

— 查询所有省份
select * from areas where pid is null;

   — 查询出山东省有哪些市
   select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle=\”山东省\”;
   select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle=\”山东省\”;

   — 查询出青岛市有哪些县城
   select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle=\”青岛市\”;
   select * from areas where pid=(select aid from areas where atitle=\”青岛市\”)

— 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 = 188;
   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);