MySQL 数据库使用详解 之 查询命令

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库是哪个?

select databases();

• 创建一个students数据表

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

);

• 创建classes表

create table classes (

id int unsigned auto_increment primary key not null,

name varchar(30) not null

);

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查询详解正式开始:

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基本查询:

• 查询所有字段

方法: select * from 表名;

select * from students;

select * from classes;

• 查询指定字段

方法: select 列1,列2,... from 表名;

select name, age from classes;

• 使用 as 给 字段 起别名

方法: select 字段 as 名字.... from 表名;

select name as 姓名, age as 年龄 from classes;

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

方法: 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;

• 消除重复行

方法: distinct 字段

select distinct gender from students;

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条件查询:

• 比较运算符

方法: 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岁之间所有学生的信息

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";

select * from students where age>18 and gender=2;

• or

18岁以上或者身高查过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;

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模糊查询:

• like

方法: % 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.*";

查询已 "周" 开始、"伦"结尾的姓名

select name from students where name rlike "^周.*伦$";

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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的名字

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;

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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;

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排序查询:

• 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, 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动大到小排序

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;

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聚合函数查询:

• 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;

• 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 ( 方法同max一样 )
• 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 round(avg(height), 2) from students where gender=1;

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分组查询:

• group by

按照性别分组, 查询所有的性别

select name from students group by gender;

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_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;

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分页查询:

• limit start, count

限制查询出来的数据格式

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

select * from students order by age asc limit 10,2;

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

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链接查询:

• inner join ... on ( 交集 )

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 又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 ( 并集--以左边的表参考 )

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 s.*, c.name from students as s left join classes as c on s.cls_id=c.id where c.name is null;

select s.*, c.name from students as s left join classes as c on s.cls_id=c.id having c.name is null;

 

• right join

将数据表名字互换位置，用left join完成

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自关联查询:

• 省级联动例子: 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 * from areas as city inner join areas as province on city.pid=province.aid where province.atitle="青岛市";

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子查询:

• 标量子查询

查询出高于平均身高的信息

select * from students where height > (select avg(height) from students);

 

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 (1, 2);

select * from students where cls_id in (select id from classes);