关于我的数据库——MySQL——第四篇

（叠甲：如有侵权请联系，内容都是自己学习的总结，一定不全面，仅当互相交流（轻点骂）我也只是站在巨人肩膀上的一个小卡拉米，已老实，求放过）。

函数

函数名称描述
current_date()当前日期
current_time()当前时间
current_timestamp()当前时间戳
date(datetime)返回datetime 参数的日期部分
date_add(date,interval d_value_type)在date中减去日期或时间 interval 后的数值单位可以是：year minute second day
date_sub(date, interval d_value_type)在date中减去日期或时间 interval 后的数值单位可以是：year minute second day
datediff(datel,date2)两天日期的差，单位是天
now()当前日期时间
获取年月日
mysql> select current_date();
+----------------+
| current_date() |
+----------------+
| 2024-10-26     |
+----------------+
1 row in set (0.00 sec)
获取时分秒
mysql> select current_time();
+----------------+
| current_time() |
+----------------+
| 09:37:40       |
+----------------+
1 row in set (0.00 sec)
获取时间戳
mysql> select current_timestamp();
+---------------------+
| current_timestamp() |
+---------------------+
| 2024-10-26 09:38:03 |
+---------------------+
1 row in set (0.00 sec)
在日期的基础上加日期
mysql> select date_add('2024-10-26',interval 59 day);
+----------------------------------------+
| date_add('2024-10-26',interval 59 day) |
+----------------------------------------+
| 2024-12-24                             |
+----------------------------------------+
1 row in set (0.00 sec)
在日期的基础上减日期
mysql> select date_sub('2024-10-26',interval 59 day)
);
+----------------------------------------+
| date_sub('2024-10-26',interval 59 day) |
+----------------------------------------+
| 2024-08-28                             |
+----------------------------------------+
1 row in set (0.00 sec)
例子：创建一张生日表
mysql> create table birthday( id int primary key auto_increment, birthday date);
Query OK, 0 rows affected (0.02 sec)//添加当前日期
mysql> insert into birthday values( 1, current_date(());
Query OK, 1 row affected (0.01 sec)查询结果
mysql> select *from birthday;
+----+------------+
| id | birthday   |
+----+------------+
|  1 | 2024-10-26 |
+----+------------+
1 row in set (0.00 sec)
案例2：创建一个留言表
//建表
mysql> create table msg( id int primary key auto_increment, content varchar(30)not null, sendtime datetime);
Query OK, 0 rows affected (0.02 sec)//插入数据
mysql> insert into msg(content,sendtime)values('hello1',now());
Query OK, 1 row affected (0.00 sec)mysql> insert into msg(content,sendtime)values('hello2',now());
Query OK, 1 row affected (0.00 sec)//查询数据
mysql> select *from msg;
+----+---------+---------------------+
| id | content | sendtime            |
+----+---------+---------------------+
|  1 | hello1  | 2024-10-26 09:52:32 |
|  2 | hello2  | 2024-10-26 09:52:40 |
+----+---------+---------------------+
2 rows in set (0.00 sec)//显示所有留言信息，发布日期只显示日期，不用显示时间
mysql> select content, date(sendtime)from msg;
+---------+----------------+
| content | date(sendtime) |
+---------+----------------+
| hello1  | 2024-10-26     |
| hello2  | 2024-10-26     |
+---------+----------------+
2 rows in set (0.00 sec)//显示所有信息，发布信息包含具体时间
mysql> select content, sendtime from msg;
+---------+---------------------+
| content | sendtime            |
+---------+---------------------+
| hello1  | 2024-10-26 09:52:32 |
| hello2  | 2024-10-26 09:52:40 |
+---------+---------------------+
2 rows in set (0.00 sec)//查询2分钟内发布的贴子--插入的信息均超过2分钟
mysql> select * from msg where date_add(sendtime,interval 2 minute)>now();
Empty set (0.00 sec)//插入新数据
mysql> insert into msg(content,sendtime)values('hello3',now());
Query OK, 1 row affected (0.01 sec)//再次查询
mysql> select * from msg where date_add(sendtime,interval 2 minute)>now();
+----+---------+---------------------+
| id | content | sendtime            |
+----+---------+---------------------+
|  3 | hello3  | 2024-10-26 09:55:19 |
+----+---------+---------------------+
1 row in set (0.00 sec)

字符串函数

charset(str)返回字符串字符集
concat(sting2 [,……])连接字符串
insert(sting,substing)返回substring在string中出现的位置，没有返回0
ucase(string2)转换成大写
lcase(string2)转换成小写
left(string2,length)从string2中的左边起去length个字符
length(string)sting的长度
replace(str,serch_str,replace_str)在str中用replace_str 替换search_str
strcmp(string1,string2)逐字比较两字符串大小
substring(str,position [,length])从str的potion开始,取出length个字符
ltrim(string)             |          rtrim(string)trim(string)去除前空格或后空格
获取emp表中的ename列的字符集
mysql> select charset(ename) from emp;
+----------------+
| charset(ename) |
+----------------+
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
| utf8           |
+----------------+
14 rows in set (0.00 sec)
要求显示student表中的信息，显示格式：‘XXX的XX是XXX分，XXXXXX分’
mysql> select concat(name, '的语文是',chinese,'分,
数学是',math,'分')as'分数'from exam_result;
+---------------------------------------------+
| 分数                                        |
+---------------------------------------------+
| 唐三藏的语文是134分,数学是107分             |
| 猪八戒的语文是110分,数学是114分             |
| 曹孟德的语文是112分,数学是108分             |
| 刘玄德的语文是176分,数学是81分              |
| 诸葛亮的语文是196分,数学是97分              |
| 荀彧的语文是192分,数学是95分                |
| 马超的语文是176分,数学是98分                |
| 孙武的语文是176分,数学是98分                |
+---------------------------------------------+
求学生表中学生姓名占用的字节数
mysql> select length(name),name from exam_result;
+--------------+-----------+
| length(name) | name      |
+--------------+-----------+
|            9 | 唐三藏    |
|            9 | 猪八戒    |
|            9 | 曹孟德    |
|            9 | 刘玄德    |
|            9 | 诸葛亮    |
|            6 | 荀彧      |
|            6 | 马超      |
|            6 | 孙武      |
+--------------+-----------+
将emp中所有名字有S的替换成‘上海’
mysql> select replace(ename,'S','上海'),ename from
emp;
+-----------------------------+--------+
| replace(ename,'S','上海')   | ename  |
+-----------------------------+--------+
| 上海MITH                    | SMITH  |
| ALLEN                       | ALLEN  |
| WARD                        | WARD   |
| JONE上海                    | JONES  |
| MARTIN                      | MARTIN |
| BLAKE                       | BLAKE  |
| CLARK                       | CLARK  |
| 上海COTT                    | SCOTT  |
| KING                        | KING   |
| TURNER                      | TURNER |
| ADAM上海                    | ADAMS  |
| JAME上海                    | JAMES  |
| FORD                        | FORD   |
| MILLER                      | MILLER |
+-----------------------------+--------+
14 rows in set (0.00 sec)
截取emp表中ename字段的第1个到第三个字符
mysql> select substring(ename,2,2),ename from emp;
+----------------------+--------+
| substring(ename,2,2) | ename  |
+----------------------+--------+
| MI                   | SMITH  |
| LL                   | ALLEN  |
| AR                   | WARD   |
| ON                   | JONES  |
| AR                   | MARTIN |
| LA                   | BLAKE  |
| LA                   | CLARK  |
| CO                   | SCOTT  |
| IN                   | KING   |
| UR                   | TURNER |
| DA                   | ADAMS  |
| AM                   | JAMES  |
| OR                   | FORD   |
| IL                   | MILLER |
+----------------------+--------+
14 rows in set (0.00 sec)
以首字母小写的方式显示所有员工的姓名
mysql> select concat(lcase(substring(ename,1,1)),susbstring(ename,2)) from emp;
+--------------------------------------------------------+
| concat(lcase(substring(ename,1,1)),substring(ename,2)) |
+--------------------------------------------------------+
| sMITH                                                  |
| aLLEN                                                  |
| wARD                                                   |
| jONES                                                  |
| mARTIN                                                 |
| bLAKE                                                  |
| cLARK                                                  |
| sCOTT                                                  |
| kING                                                   |
| tURNER                                                 |
| aDAMS                                                  |
| jAMES                                                  |
| fORD                                                   |
| mILLER                                                 |
+--------------------------------------------------------+
14 rows in set (0.00 sec)

数学函数

函数名称描述
abs(number)绝对值函数
bin(decimal_number)十进制转换二进制
hex(decimal_number)十进制转换十六进制
conv(number,from_base,to_base)进制转换
ciling(number)向上取整
floor(number)向下取整
format(number,decimal_places)格式化，保留小数位数
hex(decimaNumber)转换成十六进制
rand()返回随机浮点数，范围[0.0,1.0]
mod(number,denominator)取模，求余
绝对值
mysql> select abs(-100.2);
+-------------+
| abs(-100.2) |
+-------------+
|       100.2 |
+-------------+
1 row in set (0.00 sec)
向上取整
mysql> select ceiling(23.04);
+----------------+
| ceiling(23.04) |
+----------------+
|             24 |
+----------------+
1 row in set (0.00 sec)
向下取整
mysql> select floor(23.04);
+--------------+
| floor(23.04) |
+--------------+
|           23 |
+--------------+
1 row in set (0.00 sec)
保留2位小数位数（小数四舍五入）
mysql> select format(12.456,2);
+------------------+
| format(12.456,2) |
+------------------+
| 12.46            |
+------------------+
1 row in set (0.00 sec)
产生随机数
mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.8124908859026534 |
+--------------------+
1 row in set (0.00 sec)
user()查询当前用户
mysql> select user();
+--------+
| user() |
+--------+
| root@  |
+--------+
1 row in set (0.00 sec)
md5（str）对一个字符串进行md5摘要，摘要后得到一个32位字符串
mysql> select md5('admin');
+----------------------------------+
| md5('admin')                     |
+----------------------------------+
| 21232f297a57a5a743894a0e4a801fc3 |
+----------------------------------+
1 row in set (0.00 sec)
database（）显示当前正在使用的数据库
mysql> select database();
+------------+
| database() |
+------------+
| scott      |
+------------+
1 row in set (0.00 sec)
password（）函数，MySQL数据库使用该函数对用户加密
mysql> select password('root');
+-------------------------------------------+
| password('root')                          |
+-------------------------------------------+
| *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |
+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)
ifnull（val1,val2）如果val1为null,返回val2，否则返回val1
mysql> select ifnull('abc','123');
+---------------------+
| ifnull('abc','123') |
+---------------------+
| abc                 |
+---------------------+
1 row in set (0.00 sec)mysql> select ifnull(null,'123');
+--------------------+
| ifnull(null,'123') |
+--------------------+
| 123                |
+--------------------+
1 row in set (0.00 sec)

复合查询（重点）

查询工资高于500或岗位为manager的雇员，同时还满足他们的名字首字母为大写的J

mysql> select *from emp where(sal>500 or job = 'manager') and ename like 'J%';
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
+--------+-------+---------+------+---------------------+---------+------+--------+
2 rows in set (0.00 sec)
按照部门升序而工资降序排列
mysql> select *from emp order by deptno ,sal desc;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)
使用年薪进行降序排列
mysql> select ename , sal*12+ifnull(comm,0)as'年薪'from emp order by 年薪 desc;
+--------+----------+
| ename  | 年薪     |
+--------+----------+
| KING   | 60000.00 |
| SCOTT  | 36000.00 |
| FORD   | 36000.00 |
| JONES  | 35700.00 |
| BLAKE  | 34200.00 |
| CLARK  | 29400.00 |
| ALLEN  | 19500.00 |
| TURNER | 18000.00 |
| MARTIN | 16400.00 |
| MILLER | 15600.00 |
| WARD   | 15500.00 |
| ADAMS  | 13200.00 |
| JAMES  | 11400.00 |
| SMITH  |  9600.00 |
+--------+----------+
14 rows in set (0.00 sec)
显示工资最高的员工的名字和工作岗位
mysql> select ename ,job from emp where sal = (select max(sal)from emp);
+-------+-----------+
| ename | job       |
+-------+-----------+
| KING  | PRESIDENT |
+-------+-----------+
1 row in set (0.00 sec)
显示工资高于平均工资的员工信息
mysql> select ename ,job from emp where sal = (select max(sal)from emp);
+-------+-----------+
| ename | job       |
+-------+-----------+
| KING  | PRESIDENT |
+-------+-----------+
1 row in set (0.00 sec)
显示每个部门的平均工资和最高工资
mysql> select deptno ,format(avg(sal),2),max(sal) forom emp group by deptno;
+--------+--------------------+----------+
| deptno | format(avg(sal),2) | max(sal) |
+--------+--------------------+----------+
|     10 | 2,916.67           |  5000.00 |
|     20 | 2,175.00           |  3000.00 |
|     30 | 1,566.67           |  2850.00 |
+--------+--------------------+----------+
3 rows in set (0.00 sec)
显示平均工资低于2000的部门号和它的平均工资（--having 和 group by配合使用，对group by 结果进行过滤）
mysql> select deptno ,avg(sal)as avg_sal from emp group by deptno having avg_sal<2000;
+--------+-------------+
| deptno | avg_sal     |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)
显示每种岗位的雇员总数，平均工资
mysql> select job ,count(*),format(avg(sal),2) from emp group by job;
+-----------+----------+--------------------+
| job       | count(*) | format(avg(sal),2) |
+-----------+----------+--------------------+
| ANALYST   |        2 | 3,000.00           |
| CLERK     |        4 | 1,037.50           |
| MANAGER   |        3 | 2,758.33           |
| PRESIDENT |        1 | 5,000.00           |
| SALESMAN  |        4 | 1,400.00           |
+-----------+----------+--------------------+
5 rows in set (0.00 sec)

多表查询

实际开发中往往数据来自于不同的表，所以需要多表查询，以下例子根据emp、dept、salgrade 来演示如何进行多表查询。

//deptmysql> select *from dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.00 sec)//empmysql> select *from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)salgrademysql> select *from salgrade;
+-------+-------+-------+
| grade | losal | hisal |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
5 rows in set (0.00 sec)
笛卡尔积：

从第一张表中选出第一条记录，和第二表的所有记录进行组合；然后从第一张表中取第二条记录，和第二张表中所有记录组合；不加过滤条件，得到的结果称为“笛卡尔积”

mysql> select *from emp,dept;
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno | deptno | dname      | loc      |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     10 | ACCOUNTING | NEW YORK |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     20 | RESEARCH   | DALLAS   |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     30 | SALES      | CHICAGO  |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |     40 | OPERATIONS | BOSTON   |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     10 | ACCOUNTING | NEW YORK |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     20 | RESEARCH   | DALLAS   |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     30 | SALES      | CHICAGO  |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |     40 | OPERATIONS | BOSTON   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     10 | ACCOUNTING | NEW YORK |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     20 | RESEARCH   | DALLAS   |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     30 | SALES      | CHICAGO  |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |     40 | OPERATIONS | BOSTON   |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     10 | ACCOUNTING | NEW YORK |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     20 | RESEARCH   | DALLAS   |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     30 | SALES      | CHICAGO  |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |     40 | OPERATIONS | BOSTON   |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     10 | ACCOUNTING | NEW YORK |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     20 | RESEARCH   | DALLAS   |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     30 | SALES      | CHICAGO  |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |     40 | OPERATIONS | BOSTON   |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     10 | ACCOUNTING | NEW YORK |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     20 | RESEARCH   | DALLAS   |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     30 | SALES      | CHICAGO  |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |     40 | OPERATIONS | BOSTON   |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     10 | ACCOUNTING | NEW YORK |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     20 | RESEARCH   | DALLAS   |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     30 | SALES      | CHICAGO  |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |     40 | OPERATIONS | BOSTON   |
+--------+--------+-----------+------+---------------------+---------+---------+--------+--------+------------+----------+
56 rows in set (0.00 sec)

其实我们只想要emp表中的deptno = dept表中的deptno字段的记录；

mysql> select emp.ename , emp.sal , dept.dname  fromm emp,dept where emp.deptno = dept.deptno;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| SMITH  |  800.00 | RESEARCH   |
| ALLEN  | 1600.00 | SALES      |
| WARD   | 1250.00 | SALES      |
| JONES  | 2975.00 | RESEARCH   |
| MARTIN | 1250.00 | SALES      |
| BLAKE  | 2850.00 | SALES      |
| CLARK  | 2450.00 | ACCOUNTING |
| SCOTT  | 3000.00 | RESEARCH   |
| KING   | 5000.00 | ACCOUNTING |
| TURNER | 1500.00 | SALES      |
| ADAMS  | 1100.00 | RESEARCH   |
| JAMES  |  950.00 | SALES      |
| FORD   | 3000.00 | RESEARCH   |
| MILLER | 1300.00 | ACCOUNTING |
+--------+---------+------------+
14 rows in set (0.00 sec)
显示部门号为10的部门名，员工名和工资
mysql> select emp.ename , emp.sal , dept.dname  from emp,dept where emp.deptno = dept.deptno and dept.deptno = 10;
+--------+---------+------------+
| ename  | sal     | dname      |
+--------+---------+------------+
| CLARK  | 2450.00 | ACCOUNTING |
| KING   | 5000.00 | ACCOUNTING |
| MILLER | 1300.00 | ACCOUNTING |
+--------+---------+------------+
3 rows in set (0.00 sec)
显示各个员工的姓名，工资，及工资级别
mysql> select ename ,sal,grade from emp ,salgrade where emp.sal between losal and hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.00 sec)
自连接
//使用子查询
mysql> select empno ,ename from emp where emp.empno = (select mgr from emp where emp.ename = 'FORD');
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)//使用自连接（多表查询）--from emp leader,emp worker, 给自己的表起别名，因为要做笛卡尔积，所以别名可以先识别
mysql> select leader.empno , leader.ename from emp
leader,emp worker where leader.empno = worker.mgr and worker.ename = 'FORD';
+--------+-------+
| empno  | ename |
+--------+-------+
| 007566 | JONES |
+--------+-------+
1 row in set (0.00 sec)
子查询（其中的单行、多行、多列指的是条件的个数）

子查询是指嵌入其他sql语句中的select语句，也叫嵌套语句

单行子查询——返回一行记录的子查询

显示SMITH同一部门的员工

mysql> select *from emp where deptno = (select deptno from emp where ename = 'smith');
+--------+-------+---------+------+---------------------+---------+------+--------+
| empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
+--------+-------+---------+------+---------------------+---------+------+--------+
| 007369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
| 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
| 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
| 007876 | ADAMS | CLERK   | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
| 007902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
+--------+-------+---------+------+---------------------+---------+------+--------+
5 rows in set (0.00 sec)
多行子查询——返回多行记录的子查询
1）in关键字；查询和10号部门的工作岗位相同雇员的名字，岗位，工资，部门号，但是不包含10自己的
mysql> select ename ,job ,sal,deptno from emp where  job in (select job from emp where deptno = 10)anddeptno<>10;
+-------+---------+---------+--------+
| ename | job     | sal     | deptno |
+-------+---------+---------+--------+
| JONES | MANAGER | 2975.00 |     20 |
| BLAKE | MANAGER | 2850.00 |     30 |
| SMITH | CLERK   |  800.00 |     20 |
| ADAMS | CLERK   | 1100.00 |     20 |
| JAMES | CLERK   |  950.00 |     30 |
+-------+---------+---------+--------+
5 rows in set (0.00 sec)
2）all关键字；显示工资比部门30的所有员工的工资高的员工的姓名，工资和部门号
mysql> select ename ,sal,deptno from emp where sal>all(select sal from emp where deptno = 30);
+-------+---------+--------+
| ename | sal     | deptno |
+-------+---------+--------+
| JONES | 2975.00 |     20 |
| SCOTT | 3000.00 |     20 |
| KING  | 5000.00 |     10 |
| FORD  | 3000.00 |     20 |
+-------+---------+--------+
4 rows in set (0.00 sec)
3）any关键字;显示工资比部门30的任意员工的工资高的员工的姓名，工资个部门号（包含自己部门的员工）
mysql> select ename ,sal,deptno from emp where sal>any(select sal from emp where deptno = 30);
+--------+---------+--------+
| ename  | sal     | deptno |
+--------+---------+--------+
| ALLEN  | 1600.00 |     30 |
| WARD   | 1250.00 |     30 |
| JONES  | 2975.00 |     20 |
| MARTIN | 1250.00 |     30 |
| BLAKE  | 2850.00 |     30 |
| CLARK  | 2450.00 |     10 |
| SCOTT  | 3000.00 |     20 |
| KING   | 5000.00 |     10 |
| TURNER | 1500.00 |     30 |
| ADAMS  | 1100.00 |     20 |
| FORD   | 3000.00 |     20 |
| MILLER | 1300.00 |     10 |
+--------+---------+--------+
12 rows in set (0.00 sec)
多列子查询

单行子查询是指子查询只返回单列，单行数据，多行子查询是指返回单列多行数据，都是针对单列而言的，而多列子查询则是指查询返回多个列的子查询语句。

例子:查询和SMITH的部门和岗位完全相同的所有雇员，不含SMITH本人

mysql> select ename from emp where (deptno, job)=(seelect deptno, job from emp where ename='SMITH') and ename <> 'SMITH';
+-------+
| ename |
+-------+
| ADAMS |
+-------+
1 row in set (0.00 sec)
在from子句中使用子查询

子查询语句出现在from子句中，这里要用到数据查询的技巧，把一个子查询当做一个临时表使用。

显示每个高于自己部门平均工资的员工的名字、部门、工资、平均工资。

mysql> select ename, deptno, sal, format(asal,2) from emp, (select avg(sal) asal, deptno dt from emp guroup by deptno) tmp where emp.sal > tmp.asal and emPp.deptno=tmp.dt;
+-------+--------+---------+----------------+
| ename | deptno | sal     | format(asal,2) |
+-------+--------+---------+----------------+
| KING  |     10 | 5000.00 | 2,916.67       |
| JONES |     20 | 2975.00 | 2,175.00       |
| SCOTT |     20 | 3000.00 | 2,175.00       |
| FORD  |     20 | 3000.00 | 2,175.00       |
| ALLEN |     30 | 1600.00 | 1,566.67       |
| BLAKE |     30 | 2850.00 | 1,566.67       |
+-------+--------+---------+----------------+
6 rows in set (0.00 sec)

查找每个部门工资最高人的姓名，工资、部门、最高工资

mysql> select emp.ename, emp.sal, emp.deptno, ms froom emp, (select max(sal) ms, deptno from emp groupby deptno) tmp where emp.deptno=tmp.deptno and emp.ssal=tmp.ms;
+-------+---------+--------+---------+
| ename | sal     | deptno | ms      |
+-------+---------+--------+---------+
| BLAKE | 2850.00 |     30 | 2850.00 |
| SCOTT | 3000.00 |     20 | 3000.00 |
| KING  | 5000.00 |     10 | 5000.00 |
| FORD  | 3000.00 |     20 | 3000.00 |
+-------+---------+--------+---------+
4 rows in set (0.00 sec)

显示每个部门的信息（部门名，编号，地址）和人员数量

mysql> select dept.dname, dept.deptno, dept.loc,counnt(*) '部门人数' from emp, dept where emp.deptno=deppt.deptno group by dept.deptno,dept.dname,dept.loc;+------------+--------+----------+--------------+
| dname      | deptno | loc      | 部门人数     |
+------------+--------+----------+--------------+
| ACCOUNTING |     10 | NEW YORK |            3 |
| RESEARCH   |     20 | DALLAS   |            5 |
| SALES      |     30 | CHICAGO  |            6 |
+------------+--------+----------+--------------+
3 rows in set (0.00 sec)
合并查询

在实际应用中，为了合并多个select的执行结果，可以使用集合操作符 union ，union all

union

该操作符用于取的两个结果集的并集，当使用该操作符时，会自动去掉集中的重复行。

案例：将工资大于2500或职位是manager的人找出来

mysql> select ename, sal, job from emp where sal>25000 job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
8 rows in set (0.00 sec)
union all

该操作符用于取得两个结果集的并集，当使用该操作符时，不会去掉结果集中的重复行

mysql> select ename, sal, job from emp where sal>2500 union all select ename, sal, job from emp where
job='MANAGER';
+-------+---------+-----------+
| ename | sal     | job       |
+-------+---------+-----------+
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| SCOTT | 3000.00 | ANALYST   |
| KING  | 5000.00 | PRESIDENT |
| FORD  | 3000.00 | ANALYST   |
| JONES | 2975.00 | MANAGER   |
| BLAKE | 2850.00 | MANAGER   |
| CLARK | 2450.00 | MANAGER   |
+-------+---------+-----------+
8 rows in set (0.00 sec)

over

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