本篇主要介绍使用SQL查询数据库的操作，包括条件查询、排序、聚合函数、分组、分页、连接查询、自关联、子查询等命令操作。

　　首先我们先创建一个数据库、数据表、插入字段：

--------这部分在上篇以及介绍了-----------------

-- 创建数据库

create database blogs_test charset=utf8;

-- 使用数据库

use blogs_test;

-- 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

);

---------------插入记录----------------------

-- 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',38,160.00,2,1,0),

(0,'凤姐',28,150.00,4,2,1),

(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,'刘亦菲',25,166.00,2,2,0),

(0,'金星',33,162.00,3,3,1),

(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表中插入数据

insert into classes values (0, "python_01期"), (0, "python_02期");

---得到的students表结构如下：

+----+-----------+------+--------+--------+--------+-----------+

| id | name      | age  | height | gender | cls_id | is_delete |

+----+-----------+------+--------+--------+--------+-----------+

|  1 | 小明      |   18 | 180.00 | 女     |      1 |           |

|  2 | 小月月    |   18 | 180.00 | 女     |      2 |          |

|  3 | 彭于晏    |   29 | 185.00 | 男     |      1 |           |

|  4 | 刘德华    |   59 | 175.00 | 男     |      2 |          |

|  5 | 黄蓉      |   38 | 160.00 | 女     |      1 |           |

|  6 | 凤姐      |   28 | 150.00 | 保密   |      2 |          |

|  7 | 王祖贤    |   18 | 172.00 | 女     |      1 |          |

|  8 | 周杰伦    |   36 |   NULL | 男     |      1 |           |

|  9 | 程坤      |   27 | 181.00 | 男     |      2 |           |

| 10 | 刘亦菲    |   25 | 166.00 | 女     |      2 |           |

| 11 | 金星      |   33 | 162.00 | 中性   |      3 |          |

| 12 | 静香      |   12 | 180.00 | 女     |      4 |           |

| 13 | 郭靖      |   12 | 170.00 | 男     |      4 |           |

| 14 | 周杰      |   34 | 176.00 | 女     |      5 |           |

+----+-----------+------+--------+--------+--------+-----------+

14 rows in set (0.00 sec)

--得到的classes表结构如下：

+----+--------------+

| id | name         |

+----+--------------+

|  1 | python_01期  |

|  2 | python_02期  |

+----+--------------+

2 rows in set (0.00 sec)

创建数据库、数据表

-- 语法结构
select 字段,(聚合函数,表达式)
from 表名
where 条件

...............

order by 字段 desc
limit (第N页-1)*每个的个数

一、条件查询

    -- 消除重复行

    -- distinct 字段

    select district gender from students;

-- 条件查询

    -- 1、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 <>

    -- 2、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;

        -- 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岁以上或者身高查过180(包含)以上

        select * from students where age>18 and 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;

    --3、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.*";

        -- 查询已 周开始、伦结尾的姓名

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

    --4、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;

        -- 失败的select * from students where age not (between 18 and 34);

    --5、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;

条件查询

　　条件查询 -- 主要用 where +条件 进行查询；

二、排序

-- --------------- 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, 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;

排序

　　排序 -- order by + 以什么排序；

三、聚合函数

-- 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 round(avg(height), 2) from students where gender=1;

聚合函数

　　聚合函数 -- count（求个数） 、max（最大值） 、min（最小值） 、sum（求和）、avg（求平均值）、round（取小数点后几位、四舍五入的方式）。聚合函数通常配合下面的分组进行使用。

四、分组

-- 分组

    -- group by

    -- 按照性别分组, 查询所有的性别

    --以下两种方式会报错：由于name和*的记录数和分组不同步

    select name from students group by gender;

    select * from students group by gender;

    -- 正确

    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;

分组

　　分组 -- group by + 以什么分组；group_concat（）--以分组查看组内某些信息；having -- 筛选某种组显示；

五、分页

-- 分页

    -- 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

    -- 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 age desc limit 0,2;

分页

　　分页 -- limit start count 从第几条数据开始显示，显示几个记录；

六、连接查询

-- 链接查询

    -- 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 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

    -- 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

连接查询

　　通常根据数据表B的某些条件显示数据表A的数据；inner/left/right  join ... on ..；

七、自关联

INSERT INTO `areas` VALUES ('', '北京市', NULL);

INSERT INTO `areas` VALUES ('', '北京市', '');

INSERT INTO `areas` VALUES ('', '东城区', '');

INSERT INTO `areas` VALUES ('', '西城区', '');

INSERT INTO `areas` VALUES ('', '朝阳区', '');

INSERT INTO `areas` VALUES ('', '丰台区', '');

INSERT INTO `areas` VALUES ('', '石景山区', '');

INSERT INTO `areas` VALUES ('', '海淀区', '');

INSERT INTO `areas` VALUES ('', '门头沟区', '');

INSERT INTO `areas` VALUES ('', '房山区', '');

INSERT INTO `areas` VALUES ('', '通州区', '');

INSERT INTO `areas` VALUES ('', '顺义区', '');

INSERT INTO `areas` VALUES ('', '昌平区', '');

INSERT INTO `areas` VALUES ('', '大兴区', '');

INSERT INTO `areas` VALUES ('', '怀柔区', '');

INSERT INTO `areas` VALUES ('', '平谷区', '');

INSERT INTO `areas` VALUES ('', '密云县', '');

INSERT INTO `areas` VALUES ('', '延庆县', '');

INSERT INTO `areas` VALUES ('', '天津市', NULL);

INSERT INTO `areas` VALUES ('', '天津市', '');

INSERT INTO `areas` VALUES ('', '和平区', '');

INSERT INTO `areas` VALUES ('', '河东区', '');

INSERT INTO `areas` VALUES ('', '河西区', '');

INSERT INTO `areas` VALUES ('', '南开区', '');

INSERT INTO `areas` VALUES ('', '河北区', '');

INSERT INTO `areas` VALUES ('', '红桥区', '');

INSERT INTO `areas` VALUES ('', '滨海新区', '');

INSERT INTO `areas` VALUES ('', '东丽区', '');

INSERT INTO `areas` VALUES ('', '西青区', '');

INSERT INTO `areas` VALUES ('', '津南区', '');

INSERT INTO `areas` VALUES ('', '北辰区', '');

INSERT INTO `areas` VALUES ('', '武清区', '');

INSERT INTO `areas` VALUES ('', '宝坻区', '');

INSERT INTO `areas` VALUES ('', '宁河县', '');

INSERT INTO `areas` VALUES ('', '静海县', '');

INSERT INTO `areas` VALUES ('', '蓟县', '');

INSERT INTO `areas` VALUES ('', '河北省', NULL);

INSERT INTO `areas` VALUES ('', '石家庄市', '');

INSERT INTO `areas` VALUES ('', '长安区', '');

INSERT INTO `areas` VALUES ('', '桥东区', '');

INSERT INTO `areas` VALUES ('', '桥西区', '');

INSERT INTO `areas` VALUES ('', '新华区', '');

INSERT INTO `areas` VALUES ('', '井陉矿区', '');

INSERT INTO `areas` VALUES ('', '裕华区', '');

INSERT INTO `areas` VALUES ('', '井陉县', '');

INSERT INTO `areas` VALUES ('', '正定县', '');

INSERT INTO `areas` VALUES ('', '栾城县', '');

INSERT INTO `areas` VALUES ('', '行唐县', '');

INSERT INTO `areas` VALUES ('', '灵寿县', '');

INSERT INTO `areas` VALUES ('', '高邑县', '');

INSERT INTO `areas` VALUES ('', '深泽县', '');

INSERT INTO `areas` VALUES ('', '赞皇县', '');

INSERT INTO `areas` VALUES ('', '无极县', '');

INSERT INTO `areas` VALUES ('', '平山县', '');

INSERT INTO `areas` VALUES ('', '元氏县', '');

INSERT INTO `areas` VALUES ('', '赵县', '');

INSERT INTO `areas` VALUES ('', '辛集市', '');

INSERT INTO `areas` VALUES ('', '藁城市', '');

INSERT INTO `areas` VALUES ('', '晋州市', '');

INSERT INTO `areas` VALUES ('', '新乐市', '');

INSERT INTO `areas` VALUES ('', '鹿泉市', '');

INSERT INTO `areas` VALUES ('', '唐山市', '');

INSERT INTO `areas` VALUES ('', '路南区', '');

INSERT INTO `areas` VALUES ('', '路北区', '');

INSERT INTO `areas` VALUES ('', '古冶区', '');

INSERT INTO `areas` VALUES ('', '开平区', '');

INSERT INTO `areas` VALUES ('', '丰南区', '');

INSERT INTO `areas` VALUES ('', '丰润区', '');

INSERT INTO `areas` VALUES ('', '滦县', '');

INSERT INTO `areas` VALUES ('', '滦南县', '');

INSERT INTO `areas` VALUES ('', '乐亭县', '');

INSERT INTO `areas` VALUES ('', '迁西县', '');

INSERT INTO `areas` VALUES ('', '玉田县', '');

INSERT INTO `areas` VALUES ('', '唐海县', '');

INSERT INTO `areas` VALUES ('', '遵化市', '');

INSERT INTO `areas` VALUES ('', '迁安市', '');

INSERT INTO `areas` VALUES ('', '秦皇岛市', '');

INSERT INTO `areas` VALUES ('', '海港区', '');

INSERT INTO `areas` VALUES ('', '山海关区', '');

INSERT INTO `areas` VALUES ('', '北戴河区', '');

INSERT INTO `areas` VALUES ('', '青龙满族自治县', '');

INSERT INTO `areas` VALUES ('', '昌黎县', '');

INSERT INTO `areas` VALUES ('', '抚宁县', '');

INSERT INTO `areas` VALUES ('', '卢龙县', '');

INSERT INTO `areas` VALUES ('', '邯郸市', '');

INSERT INTO `areas` VALUES ('', '邯山区', '');

INSERT INTO `areas` VALUES ('', '丛台区', '');

INSERT INTO `areas` VALUES ('', '复兴区', '');

INSERT INTO `areas` VALUES ('', '峰峰矿区', '');

INSERT INTO `areas` VALUES ('', '邯郸县', '');

INSERT INTO `areas` VALUES ('', '临漳县', '');

INSERT INTO `areas` VALUES ('', '成安县', '');

INSERT INTO `areas` VALUES ('', '大名县', '');

INSERT INTO `areas` VALUES ('', '涉县', '');

INSERT INTO `areas` VALUES ('', '磁县', '');

INSERT INTO `areas` VALUES ('', '肥乡县', '');

INSERT INTO `areas` VALUES ('', '永年县', '');

INSERT INTO `areas` VALUES ('', '邱县', '');

INSERT INTO `areas` VALUES ('', '鸡泽县', '');

INSERT INTO `areas` VALUES ('', '广平县', '');

INSERT INTO `areas` VALUES ('', '馆陶县', '');

INSERT INTO `areas` VALUES ('', '魏县', '');

INSERT INTO `areas` VALUES ('', '曲周县', '');

INSERT INTO `areas` VALUES ('', '武安市', '');

INSERT INTO `areas` VALUES ('', '邢台市', '');

INSERT INTO `areas` VALUES ('', '桥东区', '');

INSERT INTO `areas` VALUES ('', '桥西区', '');

INSERT INTO `areas` VALUES ('', '邢台县', '');

INSERT INTO `areas` VALUES ('', '临城县', '');

INSERT INTO `areas` VALUES ('', '内丘县', '');

INSERT INTO `areas` VALUES ('', '柏乡县', '');

INSERT INTO `areas` VALUES ('', '隆尧县', '');

INSERT INTO `areas` VALUES ('', '任县', '');

INSERT INTO `areas` VALUES ('', '南和县', '');

INSERT INTO `areas` VALUES ('', '宁晋县', '');

INSERT INTO `areas` VALUES ('', '巨鹿县', '');

INSERT INTO `areas` VALUES ('', '新河县', '');

INSERT INTO `areas` VALUES ('', '广宗县', '');

INSERT INTO `areas` VALUES ('', '平乡县', '');

INSERT INTO `areas` VALUES ('', '威县', '');

INSERT INTO `areas` VALUES ('', '清河县', '');

INSERT INTO `areas` VALUES ('', '临西县', '');

INSERT INTO `areas` VALUES ('', '南宫市', '');

INSERT INTO `areas` VALUES ('', '沙河市', '');

INSERT INTO `areas` VALUES ('', '保定市', '');

INSERT INTO `areas` VALUES ('', '新市区', '');

INSERT INTO `areas` VALUES ('', '北市区', '');

INSERT INTO `areas` VALUES ('', '南市区', '');

INSERT INTO `areas` VALUES ('', '满城县', '');

INSERT INTO `areas` VALUES ('', '清苑县', '');

INSERT INTO `areas` VALUES ('', '涞水县', '');

INSERT INTO `areas` VALUES ('', '阜平县', '');

INSERT INTO `areas` VALUES ('', '徐水县', '');

INSERT INTO `areas` VALUES ('', '定兴县', '');

INSERT INTO `areas` VALUES ('', '唐县', '');

INSERT INTO `areas` VALUES ('', '高阳县', '');

INSERT INTO `areas` VALUES ('', '容城县', '');

INSERT INTO `areas` VALUES ('', '涞源县', '');

INSERT INTO `areas` VALUES ('', '望都县', '');

INSERT INTO `areas` VALUES ('', '安新县', '');

INSERT INTO `areas` VALUES ('', '易县', '');

INSERT INTO `areas` VALUES ('', '曲阳县', '');

INSERT INTO `areas` VALUES ('', '蠡县', '');

INSERT INTO `areas` VALUES ('', '顺平县', '');

INSERT INTO `areas` VALUES ('', '博野县', '');

INSERT INTO `areas` VALUES ('', '雄县', '');

INSERT INTO `areas` VALUES ('', '涿州市', '');

INSERT INTO `areas` VALUES ('', '定州市', '');

INSERT INTO `areas` VALUES ('', '安国市', '');

INSERT INTO `areas` VALUES ('', '高碑店市', '');

INSERT INTO `areas` VALUES ('', '白沟新城县', '');

INSERT INTO `areas` VALUES ('', '张家口市', '');

INSERT INTO `areas` VALUES ('', '桥东区', '');

INSERT INTO `areas` VALUES ('', '桥西区', '');

INSERT INTO `areas` VALUES ('', '宣化区', '');

INSERT INTO `areas` VALUES ('', '下花园区', '');

INSERT INTO `areas` VALUES ('', '宣化县', '');

INSERT INTO `areas` VALUES ('', '张北县', '');

INSERT INTO `areas` VALUES ('', '康保县', '');

INSERT INTO `areas` VALUES ('', '沽源县', '');

INSERT INTO `areas` VALUES ('', '尚义县', '');

INSERT INTO `areas` VALUES ('', '蔚县', '');

INSERT INTO `areas` VALUES ('', '阳原县', '');

INSERT INTO `areas` VALUES ('', '怀安县', '');

INSERT INTO `areas` VALUES ('', '万全县', '');

INSERT INTO `areas` VALUES ('', '怀来县', '');

INSERT INTO `areas` VALUES ('', '涿鹿县', '');

INSERT INTO `areas` VALUES ('', '赤城县', '');

INSERT INTO `areas` VALUES ('', '崇礼县', '');

INSERT INTO `areas` VALUES ('', '承德市', '');

INSERT INTO `areas` VALUES ('', '双桥区', '');

INSERT INTO `areas` VALUES ('', '双滦区', '');

INSERT INTO `areas` VALUES ('', '鹰手营子矿区', '');

INSERT INTO `areas` VALUES ('', '承德县', '');

INSERT INTO `areas` VALUES ('', '兴隆县', '');

INSERT INTO `areas` VALUES ('', '平泉县', '');

INSERT INTO `areas` VALUES ('', '滦平县', '');

INSERT INTO `areas` VALUES ('', '隆化县', '');

INSERT INTO `areas` VALUES ('', '丰宁满族自治县', '');

INSERT INTO `areas` VALUES ('', '宽城满族自治县', '');

INSERT INTO `areas` VALUES ('', '围场满族蒙古族自治县', '');

INSERT INTO `areas` VALUES ('', '沧州市', '');

INSERT INTO `areas` VALUES ('', '新华区', '');

INSERT INTO `areas` VALUES ('', '运河区', '');

INSERT INTO `areas` VALUES ('', '沧县', '');

INSERT INTO `areas` VALUES ('', '青县', '');

INSERT INTO `areas` VALUES ('', '东光县', '');

INSERT INTO `areas` VALUES ('', '海兴县', '');

INSERT INTO `areas` VALUES ('', '盐山县', '');

INSERT INTO `areas` VALUES ('', '肃宁县', '');

INSERT INTO `areas` VALUES ('', '南皮县', '');

INSERT INTO `areas` VALUES ('', '吴桥县', '');

INSERT INTO `areas` VALUES ('', '献县', '');

INSERT INTO `areas` VALUES ('', '孟村回族自治县', '');

INSERT INTO `areas` VALUES ('', '泊头市', '');

INSERT INTO `areas` VALUES ('', '任丘市', '');

INSERT INTO `areas` VALUES ('', '黄骅市', '');

INSERT INTO `areas` VALUES ('', '河间市', '');

INSERT INTO `areas` VALUES ('', '廊坊市', '');

INSERT INTO `areas` VALUES ('', '安次区', '');

INSERT INTO `areas` VALUES ('', '广阳区', '');

INSERT INTO `areas` VALUES ('', '固安县', '');

INSERT INTO `areas` VALUES ('', '永清县', '');

INSERT INTO `areas` VALUES ('', '香河县', '');

INSERT INTO `areas` VALUES ('', '大城县', '');

INSERT INTO `areas` VALUES ('', '文安县', '');

INSERT INTO `areas` VALUES ('', '大厂回族自治县', '');

INSERT INTO `areas` VALUES ('', '霸州市', '');

INSERT INTO `areas` VALUES ('', '三河市', '');

INSERT INTO `areas` VALUES ('', '衡水市', '');

INSERT INTO `areas` VALUES ('', '桃城区', '');

INSERT INTO `areas` VALUES ('', '枣强县', '');

INSERT INTO `areas` VALUES ('', '武邑县', '');

INSERT INTO `areas` VALUES ('', '武强县', '');

INSERT INTO `areas` VALUES ('', '饶阳县', '');

INSERT INTO `areas` VALUES ('', '安平县', '');

INSERT INTO `areas` VALUES ('', '故城县', '');

INSERT INTO `areas` VALUES ('', '景县', '');

INSERT INTO `areas` VALUES ('', '阜城县', '');

INSERT INTO `areas` VALUES ('', '冀州市', '');

INSERT INTO `areas` VALUES ('', '深州市', '');

某几个省市县的数据

　　由于每个省、市、以及县的数据均在一个表内，这时候我们需要用到自关联,由于省的aid == 市的pid，而市的aid == 县的pid，则：

 --- 创建一个表将上面的数据插入

    create table areas(

        aid int primary key,

        atitle varchar(20),

        pid int

    ); 

 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";

自查询

　　其实相当于将一个表做两次命名，再做连接查询；

八、子查询

-- 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);

子查询

　　主要通过将子查询的结果作为值传入，用做其他地方；

　　over~~~ 本篇关系某培训机构的的课件~~~~，只是方便记忆~~~~

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