null 和notnull:
null ：
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)
notnull
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)单表操作：
group by
分组指的是：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 集合函数，选取的字段 from employee group by 分组的字段。
group by ：时分组的关键词
group by： 必须 和 聚合函数(count)出现
先后顺序
where 条件语句和group by分组语句的先后顺序：
where >group by >having(**********)
列子：
1.以性别为列，进行分组，统计一下男生和女生的人数是多少：
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：
select count(id), gender from employee group by gender;
+-----------+--------+
| count(id) | gender |
+-----------+--------+
| 10 | male |
| 8 | female |
+-----------+--------+
2 rows in set (0.00 sec)
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
6.avg ： 平均值count 和 sum 的区别：
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)having
having 表示对 group by 之后的数据。进行再一次的二次筛选。
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 先后顺序
where 条件语句 和 group by 分组语句的先后顺序。
where > group by > having(****)
升序 和 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。
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;总结（顺序）：
使用顺序：
select *from 表名 where 条件 group by 条件 order by 条件 limit 条件；
where > group by > having > order by > limit
多表操作 (**************************)
外键
一对多
使用的原因：
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)
constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)
# 列：
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)