分组查询，多表联查

null 和not null

使用null的时候：

　　where 查询条件为 ：where 列表名 is null;

例：

    create table t8(
                
                id int auto_increment primary key,
                name varchar(32),
                email varchar(32)
            )charset=utf8;
            
            insert into t8 (email) values ('xxxx');
            
            mysql> insert into t8 (email) values ('xxxx');
            Query OK, 1 row affected (0.05 sec)

            mysql> select * from t8;
            +----+------+-------+
            | id | name | email |
            +----+------+-------+
            |  1 | NULL | xxxx  |
            +----+------+-------+
            1 row in set (0.00 sec)

            mysql> select * from t8 where name='';
            Empty set (0.00 sec)

            mysql> select * from t8 where name is null;
            +----+------+-------+
            | id | name | email |
            +----+------+-------+
            |  1 | NULL | xxxx  |
            +----+------+-------+
            1 row in set (0.01 sec)        

使用not null的时候：

where 查询条件为 ：where 列表名 = ‘’;

例：

        create table t9(
                
                id int auto_increment primary key,
                name varchar(32) not null default '',
                email varchar(32) not null default ''
            )charset=utf8;
            
            insert into t9 (email) values ('xxxx');
            
            mysql> insert into t9 (email) values ('xxxx');
            Query OK, 1 row affected (0.03 sec)

            mysql> select * from t9;
            +----+------+-------+
            | id | name | email |
            +----+------+-------+
            |  1 |      | xxxx  |
            +----+------+-------+
            1 row in set (0.00 sec)

            mysql> select * from t9 where name='';
            +----+------+-------+
            | id | name | email |
            +----+------+-------+
            |  1 |      | xxxx  |
            +----+------+-------+
            1 row in set (0.00 sec)            

创建表：
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
gender enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, #一个部门一个屋子
depart_id int
)charset utf8;


insert into employee(name,gender,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

单表查询

分组：

　　group by

　　 将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等

　　 用法：　　

　　　　select 聚合函数, 选取的字段 from  employee group by 分组的字段;
　　　　group by : 是分组的关键词
　　　　group by 必须和 聚合函数(count) 出现
　　　　where 条件语句和groupby分组语句的先后顺序：
   　　　where > group by > having(*********)

例子：

　　1.以性别为例， 进行分组， 统计一下男生和女生的人数是多少个：

　　mysql> select gender,count(id) as total from  employee group by gender;
　　+--------+-------+
　　| gender | total |
　　+--------+-------+
　　| male   |    10 |
　　| female |     8 |
　　+--------+-------+
　　2 rows in set (0.00 sec)

　　　2.对部门进行分组， 求出每个部门年龄最大的那个人？

　　mysql> select depart_id,max(age) from employee group by depart_id;
　　+-----------+----------+
　　| depart_id | max(age) |
　　+-----------+----------+
　　|         1 |       81 |
　　|         2 |       48 |
　　|         3 |       28 |
　　+-----------+----------+
　　3 rows in set (0.01 sec)
      3.min : 求最小的

　　4. sum ： 求和
　　5. count ： 计数 数量

　　count 和 sum的区别：
　　mysql> select depart_id,count(age) from employee group by depart_id;
　　+-----------+------------+
　　| depart_id | count(age) |
　　+-----------+------------+
　　|         1 |          8 |
　　|         2 |          5 |
　　|         3 |          5 |
　　+-----------+------------+
　　3 rows in set (0.00 sec)

　　mysql> select depart_id,sum(age) from employee group by depart_id;
　　+-----------+----------+
　　| depart_id | sum(age) |
　　+-----------+----------+
　　|         1 |      362 |
　　|         2 |      150 |
　　|         3 |      100 |
　　+-----------+----------+
　　3 rows in set (0.03 sec)

　　6. avg ： 平均数

　　having

　　表示对group by 之后的数据， 进行再一次的二次筛选

　　mysql> select depart_id,avg(age) from employee group by depart_id ;
　　+-----------+----------+
　　| depart_id | avg(age) |
　　+-----------+----------+
　　|         1 |  45.2500 |
　　|         2 |  30.0000 |
　　|         3 |  20.0000 |
　　+-----------+----------+
　　3 rows in set (0.00 sec)

　　mysql> select depart_id,avg(age) from employee group by depart_id having avg(age) > 35;
　　+-----------+----------+
　　| depart_id | avg(age) |
　　+-----------+----------+
　　|         1 |  45.2500 |
　　+-----------+----------+
　　1 row in set (0.00 sec)
　　
　　mysql> select depart_id,avg(age) as pj from employee group by depart_id having pj > 35;
　　+-----------+---------+
　　| depart_id | pj      |
　　+-----------+---------+
　　|         1 | 45.2500 |
　　+-----------+---------+
　　1 row in set (0.00 sec)

　　where 条件语句和groupby分组语句的先后顺序：
   
　　where > group by > having(*********)

升序 降序

　　order by

   　order by 字段名 asc （升序） desc(降序)

   　如果对多个字段进行排序，
  　 比如：
      　　　age desc, id asc;
      　　　表示： 先对age进行降序， 如果age有相同的行， 则对id进行升序

　　例：
　　select * from employee order by age desc, id desc;

limit 

分页

limit offset， size

offset： 行数据索引
size: 取多少条数据

mysql> select * from employee limit 0,10;#第一行为0，从第一行开始，取10条信息

总结

使用顺序为：

select * from 表名 where 条件 group by 条件 having 条件 order by 条件 limit 条件;

where > group by > having > order by > limit

多表操作

外键

a. 减少占用的空间

b. 只需要修改department表中一次， 其余的表中的数据就会相应的修改

一对多：

使用方法：
      　　constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)
   
   　　　　　　create table department(
      　　　　　　　　id int auto_increment primary key,
      　　　　　　　　name varchar(32) not null default ''
   　　　　　　)charset utf8;
   
   　　　　　　insert into department (name) values ('研发部');
   　　　　　　insert into department (name) values ('运维部');
  　　　　　　 insert into department (name) values ('前台部');
  　　　　　　 insert into department (name) values ('小卖部');
   
  　　　　　　 create table userinfo (
     　　　　　　　　 id int auto_increment primary key,
     　　　　　　　　 name varchar(32) not null default '',
      　　　　　　　　depart_id int not null default 1,
      
      　　　　　　　　constraint fk_user_depart foreign key (depart_id) references department(id),
      　　　　　　　　#constraint fk_user_depart foreign key (depart_id) references department(id),
      　　　　　　　　#constraint fk_user_depart foreign key (depart_id) references department(id),
   　　　　　　)charset utf8;
   
   　　　　　　insert into userinfo (name, depart_id) values ('zekai', 1);
  　　　　　　 insert into userinfo (name, depart_id) values ('xxx', 2);
  　　　　　　 insert into userinfo (name, depart_id) values ('zekai1', 3);
　　　　　　   insert into userinfo (name, depart_id) values ('zekai2', 4);
　　　　　　   insert into userinfo (name, depart_id) values ('zekai3', 1);
 　　　　　　  insert into userinfo (name, depart_id) values ('zekai4', 2);
　　　　　　   insert into userinfo (name, depart_id) values ('zekai4', 5);
　　

多对多：
　　   create table boy (
    　　　　  id int auto_increment primary key,
   　　　　   bname varchar(32) not null default ''
  　　 )charset utf8;
   
  　　 insert into boy (bname) values ('zhangsan'),('lisi'),('zhaoliu');
   
  　　 create table girl (
   　　　　   id int auto_increment primary key,
    　　　　  gname varchar(32) not null default ''
 　　  )charset utf8;
 　　  insert into girl (gname) values ('cuihua'),('gangdan'),('jianguo');
   
  　　 create table boy2girl (
   　　　　   id int auto_increment primary key,
   　　　　   bid int not null default 1,
     　　　　 gid int not null default 1,
      
    　　　　  constraint fk_boy2girl_boy foreign key (bid) references boy(id),
     　　　　 constraint fk_boy2girl_girl foreign key (gid) references girl(id)
 　　  )charset utf8;
   
  　　 insert into boy2girl (bid, gid) values (1,1),(1,2),(2,3),(3,3),(2,2);
   
   
   　　select * from boy left join  boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid;
   
  　　 mysql> select * from boy left join  boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid;
   　　+----+----------+------+------+------+------+---------+
  　　 | id | bname    | id   | bid  | gid  | id   | gname   |
 　　  +----+----------+------+------+------+------+---------+
　　   |  1 | zhangsan |    1 |    1 |    1 |    1 | cuihua  |
　　   |  1 | zhangsan |    2 |    1 |    2 |    2 | gangdan |
　　   |  2 | lisi     |    5 |    2 |    2 |    2 | gangdan |
 　　  |  2 | lisi     |    3 |    2 |    3 |    3 | jianguo |
　　   |  3 | zhaoliu  |    4 |    3 |    3 |    3 | jianguo |
　　   +----+----------+------+------+------+------+---------+
　　   5 rows in set (0.00 sec)

　　   mysql> select bname, gname from boy left join  boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid;
　　   +----------+---------+
　　   | bname    | gname   |
　　   +----------+---------+
　　   | zhangsan | cuihua  |
　　   | zhangsan | gangdan |
　　   | lisi     | gangdan |
　　   | lisi     | jianguo |
　　   | zhaoliu  | jianguo |
　　   +----------+---------+
　　   5 rows in set (0.00 sec)
   
　　   mysql> select bname, gname from boy left join  boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid where bname='zhangsan';
　　   +----------+---------+
　　   | bname    | gname   |
　　   +----------+---------+
　　   | zhangsan | cuihua  |
　　   | zhangsan | gangdan |
　　   +----------+---------+
　　   2 rows in set (0.02 sec)


一对一：
   
   user :
      id   name  age  
      1    zekai  18   
      2    zhangsan 23  
      3    xxxx   19   
   
   由于salary是比较敏感的字段，因此我们需要将此字段单独拆出来， 变成一张独立的表
   
   private：
      
      id  salary   uid  (外键 + unique)
      1    5000     1
      2    6000     2
      3    3000     3
      
      
   create table user (
      id int auto_increment primary key,
      name varchar(32) not null default ''
   )charset=utf8;
   
   insert into user (name) values ('zhangsan'),('zekai'),('kkk');
   
   
   create table priv(
      id int auto_increment primary key,
      salary int not null default 0,
      uid int not null default 1,
      
      constraint fk_priv_user foreign key (uid) references user(id),
      unique(uid)
   )charset=utf8;
   
   insert into priv (salary, uid) values (2000, 1);
   insert into priv (salary, uid) values (2800, 2);
   insert into priv (salary, uid) values (3000, 3);

   insert into priv (salary, uid) values (6000, 1);
   ERROR 1062 (23000): Duplicate entry '1' for key 'uid'

多表联查：
   
   mysql> select * from department;
   +----+--------+
   | id | name   |
   +----+--------+
   |  1 | 研发部 |
   |  2 | 运维部 |
   |  3 | 前台部 |
   |  4 | 小卖部 |
   +----+--------+
   4 rows in set (0.07 sec)

   mysql> select * from userinfo;
   +----+--------+-----------+
   | id | name   | depart_id |
   +----+--------+-----------+
   |  1 | zekai  |         1 |
   |  2 | xxx    |         2 |
   |  3 | zekai1 |         3 |
   |  4 | zekai2 |         4 |
   |  5 | zekai3 |         1 |
   |  6 | zekai4 |         2 |
   +----+--------+-----------+
   6 rows in set (0.00 sec)

   left join 。。。 on
      
      　　select * from userinfo left join department on depart_id = department.id
      
      
      　　mysql> select name  from userinfo left join department on depart_id = department.id;
      　　ERROR 1052 (23000): Column 'name' in field list is ambiguous
      
     　　 mysql> select userinfo.name as uname, department.name as dname  from userinfo left join department on depart_id = department.id;
     　　 +--------+--------+
   　　   | uname  | dname  |
   　　   +--------+--------+
  　　    | zekai  | 研发部 |
     　　 | zekai3 | 研发部 |
   　　   | xxx    | 运维部 |
   　　   | zekai4 | 运维部 |
   　　   | zekai1 | 前台部 |
   　　   | zekai2 | 小卖部 |
 　　     +--------+--------+
    　　  6 rows in set (0.00 sec)
      

      
     right join ... on
      
      　　mysql> insert into department (name) values ('财务部');
      　　Query OK, 1 row affected (0.04 sec)

      　　mysql>
      　　mysql> select * from department;                     );
      　　+----+--------+
     　　 | id | name   |
     　　 +----+--------+
     　　 |  1 | 研发部 |
   　　   |  2 | 运维部 |
     　　 |  3 | 前台部 |
  　　    |  4 | 小卖部 |
    　　  |  5 | 财务部 |
    　　  +----+--------+
   　　   5 rows in set (0.00 sec)

    　　  mysql> select * from userinfo;
   　　   +----+--------+-----------+
  　　    | id | name   | depart_id |
  　　    +----+--------+-----------+
   　　   |  1 | zekai  |         1 |
   　　   |  2 | xxx    |         2 |
   　　   |  3 | zekai1 |         3 |
   　　   |  4 | zekai2 |         4 |
   　　   |  5 | zekai3 |         1 |
   　　   |  6 | zekai4 |         2 |
    　　  +----+--------+-----------+
   　　   6 rows in set (0.00 sec)

   　　   mysql> select userinfo.name as uname, department.name as dname  from userinfo left join department on depart_id = department.id;
    　　  +--------+--------+
    　　  | uname  | dname  |
   　　   +--------+--------+
   　　   | zekai  | 研发部 |
   　　   | zekai3 | 研发部 |
 　　     | xxx    | 运维部 |
　　      | zekai4 | 运维部 |
  　　    | zekai1 | 前台部 |
   　　   | zekai2 | 小卖部 |
  　　    +--------+--------+
  　　    6 rows in set (0.00 sec)

  　　    mysql> select userinfo.name as uname, department.name as dname  from userinfo right join department on depart_id = department.id;
 　　     +--------+--------+
 　　     | uname  | dname  |
 　　     +--------+--------+
　　      | zekai  | 研发部 |
 　　     | zekai3 | 研发部 |
  　　    | xxx    | 运维部 |
   　　   | zekai4 | 运维部 |
   　　   | zekai1 | 前台部 |
  　　    | zekai2 | 小卖部 |
  　　    | NULL   | 财务部 |
　　      +--------+--------+
 　　     7 rows in set (0.00 sec)

   inner join
   
      mysql> select * from department inner join userinfo on department.id=userinfo.depart_id;
      +----+--------+----+--------+-----------+
      | id | name   | id | name   | depart_id |
      +----+--------+----+--------+-----------+
      |  1 | 研发部 |  1 | zekai  |         1 |
      |  1 | 研发部 |  5 | zekai3 |         1 |
      |  2 | 运维部 |  2 | xxx    |         2 |
      |  2 | 运维部 |  6 | zekai4 |         2 |
      |  3 | 前台部 |  3 | zekai1 |         3 |
      |  4 | 小卖部 |  4 | zekai2 |         4 |
      +----+--------+----+--------+-----------+
      6 rows in set (0.00 sec)