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MySQL 基础之 单表、多表联查

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标签:员工信息   com   区别   一个   HERE   老男孩   lin   ice   运维   

使用和不使用not null 的区别:

不使用: 查询时用‘name is null’ 作为条件

mysql>create table t8(
    -> id int auto_increment primary key,
    -> name varchar(32),
    -> email varchar(32)
    -> )charset=utf8;

mysql>insert into t8(email) values ('allen');

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

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

使用:查询时用‘name=’‘ ’作为查询条件

mysql> create table t9(
    -> id int auto_increment primary key,
    -> name varchar(32) not null default '',
    -> email varchar(32) not null default ''
    -> )charset=utf8;

mysql> insert into t9 (email) values ('allen');

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

mysql> select * from t9 where name='';
+----+------+-------+
| id | name | email |
+----+------+-------+
|  1 |      | allen |
+----+------+-------+
1 row in set (0.01 sec)

单表操作:

单表查询的语法:

select 字段1,字段2 from 表名  
                        where 条件
                        group by field
                        having 筛选
                        order by field
                        limit 限制条数

分组: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 聚合函数,字段名 from 表名 group by 分组的字段;

group by 是分组的关键词, 必须和聚合函数 一起出现

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

例子:

创建表:

create table emp(
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
);

插入内容:

insert into emp(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)
;
1、select count(id), gender from emp group by gender;

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:

+-----------+--------+
| count(id) | gender |
+-----------+--------+
|        10 | male   |
|         8 | female |
+-----------+--------+
2 rows in set (0.01 sec)
2、select depart_id,name, max(age) from emp group by depart_id;

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,name, max(age) from emp group by depart_id;
+-----------+--------+----------+
| depart_id | name   | max(age) |
+-----------+--------+----------+
|         1 | egon   |       81 |
|         2 | 歪歪   |       48 |
|         3 | 张野   |       28 |
+-----------+--------+----------+
3 rows in set (0.01 sec)
3、min : 求最小的;
4、sum : 求和; select depart_id, sum(age) from emp group by depart_id;
mysql> select depart_id, sum(age) from emp group by depart_id;
+-----------+----------+
| depart_id | sum(age) |
+-----------+----------+
|         1 |      362 |
|         2 |      150 |
|         3 |      100 |
+-----------+----------+
3 rows in set (0.01 sec)
5、count : 计数; select depart_id,count(depart_id) from emp group by depart_id;
mysql> select depart_id,count(depart_id) from emp group by depart_id;
+-----------+------------------+
| depart_id | count(depart_id) |
+-----------+------------------+
|         1 |                8 |
|         2 |                5 |
|         3 |                5 |
+-----------+------------------+
3 rows in set (0.00 sec)
6、avg : 平均数; select depart_id, avg(age) from emp group by depart_id;
mysql> select depart_id, avg(age) from emp group by depart_id;
+-----------+----------+
| depart_id | avg(age) |
+-----------+----------+
|         1 |  45.2500 |
|         2 |  30.0000 |
|         3 |  20.0000 |
+-----------+----------+
3 rows in set (0.00 sec)

having:

having用于对group by之后的数据进行进一步的筛选

mysql> select depart_id, avg(age) from emp group by depart_id having avg(age)>35;
+-----------+----------+
| depart_id | avg(age) |
+-----------+----------+
|         1 |  45.2500 |
+-----------+----------+
1 row in set (0.01 sec)

order by: order by 字段名 asc(升序)/desc(降序)

对多个字段进行排序:

age asc, depart_id desc; 表示先对age进行降序,再把age相等的行按部门号进行升序排列

mysql> select * from emp order by age asc, depart_id desc;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name       | gender | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 18 | 程咬铁     | female |  18 | 2014-05-12 | operation                               | NULL         |   17000.00 |    403 |         3 |
| 17 | 程咬铜     | male   |  18 | 2015-04-11 | operation                               | NULL         |   18000.00 |    403 |         3 |
| 16 | 程咬银     | female |  18 | 2013-03-11 | operation                               | NULL         |   19000.00 |    403 |         3 |
| 15 | 程咬金     | male   |  18 | 1997-03-12 | operation                               | NULL         |   20000.00 |    403 |         3 |
| 12 | 星星       | female |  18 | 2016-05-13 | sale                                    | NULL         |    3000.29 |    402 |         2 |
| 11 | 丁丁       | female |  18 | 2011-03-12 | sale                                    | NULL         |    1000.37 |    402 |         2 |
|  1 | egon       | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 |
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    401 |         1 |
| 14 | 张野       | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
| 13 | 格格       | female |  28 | 2017-01-27 | sale                                    | NULL         |    4000.33 |    402 |         2 |
|  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 |
|  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  2 | alex       | male   |  78 | 2015-03-02 | teacher                                 | NULL         | 1000000.31 |    401 |         1 |
|  3 | wupeiqi    | male   |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
18 rows in set (0.01 sec)

select * from emp order by depart_id asc, age desc;

mysql> select * from emp order by depart_id asc, age desc;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name       | gender | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
|  3 | wupeiqi    | male   |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  2 | alex       | male   |  78 | 2015-03-02 | teacher                                 | NULL         | 1000000.31 |    401 |         1 |
|  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 |
|  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
|  1 | egon       | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 |
| 13 | 格格       | female |  28 | 2017-01-27 | sale                                    | NULL         |    4000.33 |    402 |         2 |
| 11 | 丁丁       | female |  18 | 2011-03-12 | sale                                    | NULL         |    1000.37 |    402 |         2 |
| 12 | 星星       | female |  18 | 2016-05-13 | sale                                    | NULL         |    3000.29 |    402 |         2 |
| 14 | 张野       | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
| 15 | 程咬金     | male   |  18 | 1997-03-12 | operation                               | NULL         |   20000.00 |    403 |         3 |
| 16 | 程咬银     | female |  18 | 2013-03-11 | operation                               | NULL         |   19000.00 |    403 |         3 |
| 17 | 程咬铜     | male   |  18 | 2015-04-11 | operation                               | NULL         |   18000.00 |    403 |         3 |
| 18 | 程咬铁     | female |  18 | 2014-05-12 | operation                               | NULL         |   17000.00 |    403 |         3 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
18 rows in set (0.00 sec)

limit 分页: limit offset, size

offset 表示 行数据索引; size 表示取多少条数据

从第offset行开始,取size行数据。

mysql> select * from emp limit 0,10;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name       | gender | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
|  1 | egon       | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  2 | alex       | male   |  78 | 2015-03-02 | teacher                                 | NULL         | 1000000.31 |    401 |         1 |
|  3 | wupeiqi    | male   |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
10 rows in set (0.00 sec)

从第6行开始取10行:

mysql> select * from emp limit 6,10;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name      | gender | age | hire_date  | post      | post_comment | salary   | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
|  7 | jinxin    | male   |  18 | 1900-03-01 | teacher   | NULL         | 30000.00 |    401 |         1 |
|  8 | 成龙      | male   |  48 | 2010-11-11 | teacher   | NULL         | 10000.00 |    401 |         1 |
|  9 | 歪歪      | female |  48 | 2015-03-11 | sale      | NULL         |  3000.13 |    402 |         2 |
| 10 | 丫丫      | female |  38 | 2010-11-01 | sale      | NULL         |  2000.35 |    402 |         2 |
| 11 | 丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |
| 12 | 星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |
| 13 | 格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |
| 14 | 张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |
| 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
| 16 | 程咬银    | female |  18 | 2013-03-11 | operation | NULL         | 19000.00 |    403 |         3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
10 rows in set (0.00 sec)

多表操作

外键: 占用空间少,方便修改数据

一对多:

语法: constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)

mysql> create table dep(
    -> id int auto_increment primary key,
    -> name varchar(32) not null default ''
    -> )charset=utf8;

mysql> insert into dep (name) values ('研发部'),('运维部'),('前台部'),('小卖部');

mysql> 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 dep(id)
    -> )charset utf8;

mysql> insert into userinfo (name, depart_id) values ('allen a',1);
mysql> insert into userinfo (name, depart_id) values ('allen b',2);
mysql> insert into userinfo (name, depart_id) values ('allen c',3);
mysql> insert into userinfo (name, depart_id) values ('allen d',4);
mysql> insert into userinfo (name, depart_id) values ('allen e',1);
mysql> insert into userinfo (name, depart_id) values ('allen f',2);
mysql> insert into userinfo (name, depart_id) values ('allen g',3);
以上7行符合外键要求,所以能插入不报错,但下边一行插入时会报错
mysql> insert into userinfo (name, depart_id) values ('allen h',5);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'mysql> insert into userinfo (name, depart_id) values ('allen h',5)' at line 1

多对多:

创建男生表

mysql> create table boy(
    -> id int auto_increment primary key,
    -> bname varchar(32) not null default ''
    -> )charset=utf8;

insert into boy (bname) values ('xiaoming'),('xiaogang'),('xiaoqiang');

创建女生表

mysql> create table girl(
    -> id int auto_increment primary key,
    -> gname varchar(32) not null default ''
    -> )charset=utf8;

mysql> insert into girl (gname) values ('xiaohong'),('xiaoli'),('xiaojiao');

创建关联表

mysql> create table b2g(
    -> id int auto_increment primary key,
    -> bid int not null default 1,
    -> gid int not null default 0,
    ->
    -> constraint fk_b2g_boy foreign key (bid) references boy(id),
    -> constraint fk_b2g_girl foreign key (gid) references girl(id)
    -> )charset utf8;

mysql> insert into b2g (bid, gid) values (1,1),(1,2),(2,3),(3,3),(2,2);

用到 left jion :

mysql> select * from boy left join b2g on boy.id=b2g.bid left join girl on girl.id=b2g.gid;
+----+-----------+------+------+------+------+----------+
| id | bname     | id   | bid  | gid  | id   | gname    |
+----+-----------+------+------+------+------+----------+
|  1 | xiaoming  |    1 |    1 |    1 |    1 | xiaohong |
|  1 | xiaoming  |    2 |    1 |    2 |    2 | xiaoli   |
|  2 | xiaogang  |    5 |    2 |    2 |    2 | xiaoli   |
|  2 | xiaogang  |    3 |    2 |    3 |    3 | xiaojiao |
|  3 | xiaoqiang |    4 |    3 |    3 |    3 | xiaojiao |
+----+-----------+------+------+------+------+----------+
5 rows in set (0.01 sec)
mysql> select bname, gname from boy left join b2g on boy.id=b2g.bid left join girl on girl.id=b2g.gid;
+-----------+----------+
| bname     | gname    |
+-----------+----------+
| xiaoming  | xiaohong |
| xiaoming  | xiaoli   |
| xiaogang  | xiaoli   |
| xiaogang  | xiaojiao |
| xiaoqiang | xiaojiao |
+-----------+----------+
5 rows in set (0.00 sec)

一对一:

创建员工信息表

mysql> create table user(
    -> id int auto_increment primary key,
    -> name varchar(32) not null default ''
    -> )charset=utf8;

mysql> insert into user (name) values ('xiaoming'),('xiaogang'),('xiaoqiang');

mysql> select * from user;
+----+-----------+
| id | name      |
+----+-----------+
|  1 | xiaoming  |
|  2 | xiaogang  |
|  3 | xiaoqiang |
+----+-----------+
3 rows in set (0.00 sec)

创建员工工资表

mysql> 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;

mysql> insert into priv (salary, uid) values (2000, 1),(2500,2),(3000,3);

mysql> select * from priv;
+----+--------+-----+
| id | salary | uid |
+----+--------+-----+
|  1 |   2000 |   1 |
|  2 |   2500 |   2 |
|  3 |   3000 |   3 |
+----+--------+-----+
3 rows in set (0.00 sec)

多表联查:

left join ...on... :

mysql> select userinfo.name as uname, dep.name as dname from userinfo left join dep on depart_id=dep.id;
+---------+-----------+
| uname   | dname     |
+---------+-----------+
| allen a | 研发部    |
| allen e | 研发部    |
| allen b | 运维部    |
| allen f | 运维部    |
| allen c | 前台部    |
| allen g | 前台部    |
| allen d | 小卖部    |
+---------+-----------+
7 rows in set (0.01 sec)



mysql> select userinfo.name as uname, dep.name as dname from userinfo left join dep on depart_id=dep.id;
+---------+-----------+
| uname   | dname     |
+---------+-----------+
| allen a | 研发部    |
| allen e | 研发部    |
| allen b | 运维部    |
| allen f | 运维部    |
| allen c | 前台部    |
| allen g | 前台部    |
| allen d | 小卖部    |
+---------+-----------+
7 rows in set (0.01 sec)

right join ...on...

inner join

MySQL 基础之 单表、多表联查

标签:员工信息   com   区别   一个   HERE   老男孩   lin   ice   运维   

原文地址:https://www.cnblogs.com/allenchen168/p/11768705.html

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