七 .数据库(表的查询)

一 .表的查询

1. 表单查询及关键字和查询优先级

语法:
SELECT DISTINCT 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

优先级

from：找到表
where：拿着where指定的约束条件，去文件/表中取出一条条记录
group by：将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
select：执行select
distinct：去重
having：将分组的结果进行having过滤
order by：将结果按条件排序：order by
limit：限制结果的显示条数

mysql> select * from cc;
+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| id | name      | sex | age | hire_date  | post                           | post_comment | salary     | office | depart_id |
+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| 19 | nick      | 男  |  18 | 2017-03-01 | 老男孩驻上海虹桥最帅           | NULL         |    7300.33 |    401 |         1 |
| 20 | jason     | 男  |  78 | 2015-03-02 | teacher                        | NULL         | 1000000.31 |    401 |         1 |
| 21 | sean      | 女  |  81 | 2013-03-05 | teacher                        | NULL         |    8300.00 |    401 |         1 |
| 22 | tank      | 男  |  73 | 2014-07-01 | teacher                        | NULL         |    3500.00 |    401 |         1 |
| 23 | oscar     | 女  |  28 | 2012-11-01 | teacher                        | NULL         |    2100.00 |    401 |         1 |
| 24 | mac       | 男  |  18 | 2011-02-11 | teacher                        | NULL         |    9000.00 |    401 |         1 |
| 25 | rocky     | 女  |  18 | 1900-03-01 | teacher                        | NULL         |   30000.00 |    401 |         1 |
| 26 | 成龙      | 男  |  48 | 2010-11-11 | teacher                        | NULL         |   10000.00 |    401 |         1 |
| 27 | 歪歪      | 女  |  48 | 2015-03-11 | sale                           | NULL         |    3000.13 |    402 |         2 |
| 28 | 丫丫      | 男  |  38 | 2010-11-01 | sale                           | NULL         |    2000.35 |    402 |         2 |
| 29 | 丁丁      | 女  |  18 | 2011-03-12 | sale                           | NULL         |    1000.37 |    402 |         2 |
| 30 | 星星      | 男  |  18 | 2016-05-13 | sale                           | NULL         |    3000.29 |    402 |         2 |
| 31 | 格格      | 男  |  28 | 2017-01-27 | sale                           | NULL         |    4000.33 |    402 |         2 |
| 32 | 张野      | 男  |  28 | 2016-03-11 | operation                      | NULL         |   10000.13 |    403 |         3 |
| 33 | 程咬金    | 女  |  18 | 1997-03-12 | operation                      | NULL         |   20000.00 |    403 |         3 |
| 34 | 程咬银    | 男  |  18 | 2013-03-11 | operation                      | NULL         |   19000.00 |    403 |         3 |
| 35 | 程咬铜    | 男  |  18 | 2015-04-11 | operation                      | NULL         |   18000.00 |    403 |         3 |
| 36 | 程咬铁    | 女  |  18 | 2014-05-12 | operation                      | NULL         |   17000.00 |    403 |         3 |
+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+

1.简单查询

mysql> select sex,age from cc;
+-----+-----+
| sex | age |
+-----+-----+
| 男 | 18 |
| 男 | 78 |
| 女 | 81 |
| 男 | 73 |
| 女 | 28 |
| 男 | 18 |
| 女 | 18 |
| 男 | 48 |
| 女 | 48 |
| 男 | 38 |
| 女 | 18 |
| 男 | 18 |
| 男 | 28 |
| 男 | 28 |
| 女 | 18 |
| 男 | 18 |
| 男 | 18 |
| 女 | 18 |
+-----+-----+

mysql> select distinct age from cc;      distinct避免重复
+-----+
| age |
+-----+
| 18 |
| 78 |
| 81 |
| 73 |
| 28 |
| 48 |
| 38 |
+-----+

mysql> select distinct post from cc;      查出所有的岗位（去掉重复 disinct）

+--------------------------------+
| post |
+--------------------------------+
| 老男孩驻上海虹桥最帅 |
| teacher |
| sale |
| operation |
+--------------------------------+
4 rows in set (0.00 sec)

 

2.通过四则运算查询

mysql> select name,salary*2 from cc; 
+-----------+------------+
| name | salary*2 |
+-----------+------------+
| nick | 14600.66 |
| jason | 2000000.62 |
| sean | 16600.00 |
| tank | 7000.00 |
| oscar | 4200.00 |
| mac | 18000.00 |
| rocky | 60000.00 |
| 成龙 | 20000.00 |
| 歪歪 | 6000.26 |
| 丫丫 | 4000.70 |
| 丁丁 | 2000.74 |
| 星星 | 6000.58 |
| 格格 | 8000.66 |
| 张野 | 20000.26 |
| 程咬金 | 40000.00 |
| 程咬银 | 38000.00 |
| 程咬铜 | 36000.00 |
| 程咬铁 | 34000.00 |
+-----------+------------+

mysql> select name,salary*2 as bb from cc;
+-----------+------------+
| name | bb |
+-----------+------------+
| nick | 14600.66 |
| jason | 2000000.62 |
| sean | 16600.00 |
| tank | 7000.00 |
| oscar | 4200.00 |
| mac | 18000.00 |
| rocky | 60000.00 |
| 成龙 | 20000.00 |
| 歪歪 | 6000.26 |
| 丫丫 | 4000.70 |
| 丁丁 | 2000.74 |
| 星星 | 6000.58 |
| 格格 | 8000.66 |
| 张野 | 20000.26 |
| 程咬金 | 40000.00 |
| 程咬银 | 38000.00 |
| 程咬铜 | 36000.00 |
| 程咬铁 | 34000.00 |
+-----------+------------+
18 rows in set (0.00 sec

mysql> select name ,salary*2 bb from cc;
+-----------+------------+
| name | bb |
+-----------+------------+
| nick | 14600.66 |
| jason | 2000000.62 |
| sean | 16600.00 |
| tank | 7000.00 |
| oscar | 4200.00 |
| mac | 18000.00 |
| rocky | 60000.00 |
| 成龙 | 20000.00 |
| 歪歪 | 6000.26 |
| 丫丫 | 4000.70 |
| 丁丁 | 2000.74 |
| 星星 | 6000.58 |
| 格格 | 8000.66 |
| 张野 | 20000.26 |
| 程咬金 | 40000.00 |
| 程咬银 | 38000.00 |
| 程咬铜 | 36000.00 |
| 程咬铁 | 34000.00 |
+-----------+------------+

mysql> select name ,salary*2 bb from cc;  查出所有员工名字，以及他们的年薪，年薪的字段名为bb

+-----------+------------+
| name | bb |
+-----------+------------+
| nick | 14600.66 |
| jason | 2000000.62 |
| sean | 16600.00 |
| tank | 7000.00 |
| oscar | 4200.00 |
| mac | 18000.00 |
| rocky | 60000.00 |
| 成龙 | 20000.00 |
| 歪歪 | 6000.26 |
| 丫丫 | 4000.70 |
| 丁丁 | 2000.74 |
| 星星 | 6000.58 |
| 格格 | 8000.66 |
| 张野 | 20000.26 |
| 程咬金 | 40000.00 |
| 程咬银 | 38000.00 |
| 程咬铜 | 36000.00 |
| 程咬铁 | 34000.00 |
+-----------+------------+

3.定义显示格式查询

mysql> select concat("姓名"'name',"年薪","salary") as vv from cc;     CONCAT() 函数用于连接字符串
+------------------------+
| vv |
+------------------------+
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
| 年薪姓名namesalary |
+------------------------+

mysql> select concat('<名字:',name,'> ','<薪资:',salary,'>') from cc;  查出所有员工的名字，薪资，格式为<名字:nick> <薪资:3000>

+-------------------------------------------------------+
| concat('<名字:',name,'> ','<薪资:',salary,'>') |
+-------------------------------------------------------+
| <名字:nick> <薪资:7300.33> |
| <名字:jason> <薪资:1000000.31> |
| <名字:sean> <薪资:8300.00> |
| <名字:tank> <薪资:3500.00> |
| <名字:oscar> <薪资:2100.00> |
| <名字:mac> <薪资:9000.00> |
| <名字:rocky> <薪资:30000.00> |
| <名字:成龙> <薪资:10000.00> |
| <名字:歪歪> <薪资:3000.13> |
| <名字:丫丫> <薪资:2000.35> |
| <名字:丁丁> <薪资:1000.37> |
| <名字:星星> <薪资:3000.29> |
| <名字:格格> <薪资:4000.33> |
| <名字:张野> <薪资:10000.13> |
| <名字:程咬金> <薪资:20000.00> |
| <名字:程咬银> <薪资:19000.00> |
| <名字:程咬铜> <薪资:18000.00> |
| <名字:程咬铁> <薪资:17000.00> |
+-------------------------------------------------------+

 

2.条件约束查询（where）

比较运算符：> < >= <= <> !=
between 80 and 100 值在80到100之间
in(80,90,100) 值是80或90或100
like 'n%'
通配符可以是%或_，
%表示任意多字符
_表示一个字符
逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| id | name      | sex | age | hire_date  | post                           | post_comment | salary     | office | depart_id |
+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| 19 | nick      | 男  |  18 | 2017-03-01 | 老男孩驻上海虹桥最帅           | NULL         |    7300.33 |    401 |         1 |
| 20 | jason     | 男  |  78 | 2015-03-02 | teacher                        | NULL         | 1000000.31 |    401 |         1 |
| 21 | sean      | 女  |  81 | 2013-03-05 | teacher                        | NULL         |    8300.00 |    401 |         1 |
| 22 | tank      | 男  |  73 | 2014-07-01 | teacher                        | NULL         |    3500.00 |    401 |         1 |
| 23 | oscar     | 女  |  28 | 2012-11-01 | teacher                        | NULL         |    2100.00 |    401 |         1 |
| 24 | mac       | 男  |  18 | 2011-02-11 | teacher                        | NULL         |    9000.00 |    401 |         1 |
| 25 | rocky     | 女  |  18 | 1900-03-01 | teacher                        | NULL         |   30000.00 |    401 |         1 |
| 26 | 成龙      | 男  |  48 | 2010-11-11 | teacher                        | NULL         |   10000.00 |    401 |         1 |
| 27 | 歪歪      | 女  |  48 | 2015-03-11 | sale                           | NULL         |    3000.13 |    402 |         2 |
| 28 | 丫丫      | 男  |  38 | 2010-11-01 | sale                           | NULL         |    2000.35 |    402 |         2 |
| 29 | 丁丁      | 女  |  18 | 2011-03-12 | sale                           | NULL         |    1000.37 |    402 |         2 |
| 30 | 星星      | 男  |  18 | 2016-05-13 | sale                           | NULL         |    3000.29 |    402 |         2 |
| 31 | 格格      | 男  |  28 | 2017-01-27 | sale                           | NULL         |    4000.33 |    402 |         2 |
| 32 | 张野      | 男  |  28 | 2016-03-11 | operation                      | NULL         |   10000.13 |    403 |         3 |
| 33 | 程咬金    | 女  |  18 | 1997-03-12 | operation                      | NULL         |   20000.00 |    403 |         3 |
| 34 | 程咬银    | 男  |  18 | 2013-03-11 | operation                      | NULL         |   19000.00 |    403 |         3 |
| 35 | 程咬铜    | 男  |  18 | 2015-04-11 | operation                      | NULL         |   18000.00 |    403 |         3 |
| 36 | 程咬铁    | 女  |  18 | 2014-05-12 | operation                      | NULL         |   17000.00 |    403 |         3 |
+----+-----------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+

1.单条件查询

mysql> select name from cc where post='sale';   查询工作对应的老师名
+--------+
| name   |
+--------+
| 歪歪   |
| 丫丫   |
| 丁丁   |
| 星星   |
| 格格   |
+--------+


 2.多条件查询

mysql> select name,salary from cc where post="teacher" and salary>10000;  查看薪水大于10000 并且职业为老师 对应的名字 和薪水
+-------+------------+
| name | salary |
+-------+------------+
| jason | 1000000.31 |
| rocky | 30000.00 |
+-------+------------+

3.关键字查询(between  and)

mysql> select name,salary from cc where salary between 10000 and 20000;  查看薪水在10000----20000之间 对应的名字
+-----------+----------+
| name | salary |
+-----------+----------+
| 成龙 | 10000.00 |
| 张野 | 10000.13 |
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
6 rows in set (0.28 sec)

4. 关键字is null(判断某个字段是否为null不能用等号，需要用is)

mysql> select name ,post_comment from cc where post_comment is null;   查看post_comment为空对应的名字
+-----------+--------------+
| name | post_comment |
+-----------+--------------+
| nick | NULL |
| jason | NULL |
| sean | NULL |
| tank | NULL |
| oscar | NULL |
| mac | NULL |
| rocky | NULL |
| 成龙 | NULL |
| 歪歪 | NULL |
| 丫丫 | NULL |
| 丁丁 | NULL |
| 星星 | NULL |
| 格格 | NULL |
| 张野 | NULL |
| 程咬金 | NULL |
| 程咬银 | NULL |
| 程咬铜 | NULL |
| 程咬铁 | NULL |
+-----------+--------------+
18 rows in set (0.00 sec)

5.关键字in集合查询

mysql> select name,salary from cc where salary=3000 or salary=3500;
+------+---------+
| name | salary |
+------+---------+
| tank | 3500.00 |
+------+---------+
1 row in set (0.00 sec)

mysql> select name,salary from cc where salary in (3000,4000,9000,12000);   查看in集合里面又没符合表里面的薪水 所对应的老师
+------+---------+
| name | salary |
+------+---------+
| mac | 9000.00 |
+------+---------+
1 row in set (0.29 sec)

 6. 关键字like模糊查询 %  __

mysql> select * from cc where name like 'ni%';      查看名字中含有ni对应的老师
+----+------+-----+-----+------------+--------------------------------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+------+-----+-----+------------+--------------------------------+--------------+---------+--------+-----------+
| 19 | nick | 男 | 18 | 2017-03-01 | 老男孩驻上海虹桥最帅 | NULL | 7300.33 | 401 | 1 |
+----+------+-----+-----+------------+--------------------------------+--------------+---------+--------+-----------+
1 row in set (0.30 sec)

通配符’_’ SELECT * FROM employee WHERE emp_name LIKE 'ja__';

通配符’%’ SELECT * FROM employee WHERE emp_name LIKE 'ni%';

7. 练习

select emp_name,age from employee where post = 'teacher';  查看岗位是teacher的员工姓名、年龄

select emp_name,age from employee where post='teacher' and age > 30; 查看岗位是teacher且年龄大于30岁的员工姓名、年龄

select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000; 查看岗位是teacher且薪资在9000-1000范员工姓名、年龄、薪资

select * from employee where post_comment is not null;查看岗位描述不为NULL的员工信息

select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资

select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资

select emp_name,salary*12 from employee where post='teacher' and emp_name like 'mac%';查看岗位是teacher且名字是jin开头的员工姓名、年薪

 

3. 分组查询（group by）

1.单独使用GROUP BY关键字分组

mysql> select post from cc group by post;   意思把所以职业分为一组
+--------------------------------+
| post                           |
+--------------------------------+
| 老男孩驻上海虹桥最帅           |
| teacher                        |
| sale                           |
| operation                      |
+--------------------------------+
4 rows in set (0.00 sec)

2.GROUP BY关键字和GROUP_CONCAT()函数一起使用

select post,group_concat(name) from cc group by post;   并按照组分 查看里面所有成员
+--------------------------------+------------------------------------------------+
| post | group_concat(name) |
+--------------------------------+------------------------------------------------+
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | jason,sean,tank,oscar,mac,rocky,成龙 |
| 老男孩驻上海虹桥最帅 | nick |
+--------------------------------+------------------------------------------------+
4 rows in set (0.31 sec)

 

mysql> select post,group_concat(name) as lover from cc group by post;   # 按照岗位分组，并查看组内成员名
+--------------------------------+------------------------------------------------+
| post | lover |
+--------------------------------+------------------------------------------------+
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | jason,sean,tank,oscar,mac,rocky,成龙 |
| 老男孩驻上海虹桥最帅 | nick |
+--------------------------------+------------------------------------------------+
4 rows in set (0.00 sec)

 3. GROUP BY与聚合函数一起使用

mysql> select post,count(id) as count from cc group by post;# 按照岗位分组，并查看每个组有多少人
+--------------------------------+-------+
| post | count |
+--------------------------------+-------+
| 老男孩驻上海虹桥最帅 | 1 |
| teacher | 7 |
| sale | 5 |
| operation | 5 |
+--------------------------------+-------+
4 rows in set (0.28 sec)

注意：如果我们用unique的字段作为分组的依据，则每一条记录自成一组，这种分组没有意义；多条记录之间的某个字段值相同，该字段通常用来作为分组的依据

 5.聚合函数

   强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组

    SELECT COUNT(*) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    SELECT MAX(salary) FROM employee;
    SELECT MIN(salary) FROM employee;
    SELECT AVG(salary) FROM employee;
    SELECT SUM(salary) FROM employee;
    SELECT SUM(salary) FROM employee WHERE depart_id=3;

1.练习聚合函数

mysql> select post,group_concat(name) from cc group by post;  查询岗位名以及岗位包含的所有员工名字

+--------------------------------+------------------------------------------------+
| post | group_concat(name) |
+--------------------------------+------------------------------------------------+
| operation | 张野,程咬金,程咬银,程咬铜,程咬铁 |
| sale | 歪歪,丫丫,丁丁,星星,格格 |
| teacher | jason,sean,tank,oscar,mac,rocky,成龙 |
| 老男孩驻上海虹桥最帅 | nick |
+--------------------------------+------------------------------------------------+
4 rows in set (0.00 sec)

mysql> select post,count(id) from cc group by post;查询岗位名以及各岗位内包含的员工个数

+--------------------------------+-----------+
| post | count(id) |
+--------------------------------+-----------+
| 老男孩驻上海虹桥最帅 | 1 |
| teacher | 7 |
| sale | 5 |
| operation | 5 |
+--------------------------------+-----------+
4 rows in set (0.00 sec)

 

mysql> select sex,count(id) from cc group by sex;查询公司内男员工和女员工的个数

+-----+-----------+
| sex | count(id) |
+-----+-----------+
| 男 | 11 |
| 女 | 7 |
+-----+-----------+
2 rows in set (0.00 sec)

 

mysql> select post,avg(salary) from cc group by post; 查询岗位名以及各岗位的平均薪资  avg平均

+--------------------------------+---------------+
| post | avg(salary) |
+--------------------------------+---------------+
| 老男孩驻上海虹桥最帅 | 7300.330000 |
| teacher | 151842.901429 |
| sale | 2600.294000 |
| operation | 16800.026000 |
+--------------------------------+---------------+
4 rows in set (0.00 sec)

 

mysql> select post,max(salary) from cc group by post;查询岗位名以及各岗位的最高薪资

+--------------------------------+-------------+
| post | max(salary) |
+--------------------------------+-------------+
| 老男孩驻上海虹桥最帅 | 7300.33 |
| teacher | 1000000.31 |
| sale | 4000.33 |
| operation | 20000.00 |
+--------------------------------+-------------+
4 rows in set (0.00 sec)

 

mysql> select post,min(salary) from cc group by post;查询岗位名以及各岗位的最低薪资

+--------------------------------+-------------+
| post | min(salary) |
+--------------------------------+-------------+
| 老男孩驻上海虹桥最帅 | 7300.33 |
| teacher | 2100.00 |
| sale | 1000.37 |
| operation | 10000.13 |
+--------------------------------+-------------+
4 rows in set (0.00 sec)

 

mysql> select sex,avg(salary) from cc group by sex; 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资

+-----+--------------+
| sex | avg(salary) |
+-----+--------------+
| 男 | 98709.249091 |
| 女 | 11628.642857 |
+-----+--------------+

6.过滤(where having)

执行优先级从高到低：where > group by > having

Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。
Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数

1.简单过滤查询

mysql> select * from cc where salary>10000;   查询薪水大于10000的
+----+-----------+-----+-----+------------+-----------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+-----+-----+------------+-----------+--------------+------------+--------+-----------+
| 20 | jason | 男 | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 |
| 25 | rocky | 女 | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
| 32 | 张野 | 男 | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 |
| 33 | 程咬金 | 女 | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 34 | 程咬银 | 男 | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 |
| 35 | 程咬铜 | 男 | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 36 | 程咬铁 | 女 | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+-----------+-----+-----+------------+-----------+--------------+------------+--------+-----------+
7 rows in set (0.00 sec)

mysql> select @@sql_mode;
+--------------------+
| @@sql_mode         |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
row in set (0.00 sec)

mysql> select * from emp where salary > 100000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | emp_name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | jason | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
row in set (0.00 sec)

mysql> select post,group_concat(emp_name) from emp group by post having salary > 10000;#错误，分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000;
+-----------+-------------------------------------------------------+
| post | group_concat(emp_name) |
+-----------+-------------------------------------------------------+
| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
| teacher | 成龙,rocky,mac,oscar,tank,sean,jason |
+-----------+-------------------------------------------------------+
rows in set (0.00 sec)

查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
查询各岗位平均薪资大于10000的岗位名、平均工资
查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
题目1
mysql> select post,group_concat(emp_name),count(id) from employee group by post having count(id) < 2;
+-----------------------------------------+--------------------+-----------+
| post                                    | group_concat(emp_name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻上海虹桥最帅              | nick               |         1 |
+-----------------------------------------+--------------------+-----------+

题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+

题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
+-----------+--------------+
| post      | avg(salary)  |
+-----------+--------------+
| operation | 16800.026000 |

 7. 排序查询（order by）

按单列排序
    SELECT * FROM employee ORDER BY salary; 默认是升序
    SELECT * FROM employee ORDER BY salary ASC; 升序
    SELECT * FROM employee ORDER BY salary DESC; 降序

按多列排序:先按照age排序，如果年纪相同，则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;

查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序
查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
题目1
mysql> select * from employee ORDER BY age asc,hire_date desc;

题目2
mysql> select post,avg(salary) from cc group by post having avg(salary) > 10000 order by avg(salary) asc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+

题目3
mysql> select post,avg(salary) from cc group by post having avg(salary) > 10000 order by avg(salary) desc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| teacher   | 151842.901429 |
| operation |  16800.026000 |
+-----------+---------------+

8.限制条数查询（limit）

SELECT * FROM employee ORDER BY salary DESC 
    LIMIT 3;                    #默认初始位置为0 

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 0,5; #从第0开始，即先出第一条，然后包含这一条在内往后查5条

SELECT * FROM employee ORDER BY salary DESC
    LIMIT 5,5; #从第5开始，即先出第6条，然后包含这一条在内往后查5条

mysql> select * from cc limit 3; 查看3条数据
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| id | name  | sex | age | hire_date  | post                           | post_comment | salary     | office | depart_id |
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| 19 | nick  | 男  |  18 | 2017-03-01 | 老男孩驻上海虹桥最帅           | NULL         |    7300.33 |    401 |         1 |
| 20 | jason | 男  |  78 | 2015-03-02 | teacher                        | NULL         | 1000000.31 |    401 |         1 |
| 21 | sean  | 女  |  81 | 2013-03-05 | teacher                        | NULL         |    8300.00 |    401 |         1 |
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
3 rows in set (0.00 sec)

mysql> select * from cc limit 0,5;  查看0到五条数据
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| id | name  | sex | age | hire_date  | post                           | post_comment | salary     | office | depart_id |
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
| 19 | nick  | 男  |  18 | 2017-03-01 | 老男孩驻上海虹桥最帅           | NULL         |    7300.33 |    401 |         1 |
| 20 | jason | 男  |  78 | 2015-03-02 | teacher                        | NULL         | 1000000.31 |    401 |         1 |
| 21 | sean  | 女  |  81 | 2013-03-05 | teacher                        | NULL         |    8300.00 |    401 |         1 |
| 22 | tank  | 男  |  73 | 2014-07-01 | teacher                        | NULL         |    3500.00 |    401 |         1 |
| 23 | oscar | 女  |  28 | 2012-11-01 | teacher                        | NULL         |    2100.00 |    401 |         1 |
+----+-------+-----+-----+------------+--------------------------------+--------------+------------+--------+-----------+
5 rows in set (0.00 sec

mysql> select * from  employee limit 5,5;  从第五个位置显示五条数据
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | emp_name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | mac | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | rocky     | 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 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
rows in set (0.00 sec)

mysql> select * from  employee limit 10,5; 从第10条数 据显示5条数据
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | emp_name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 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 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
rows in set (0.00 sec)

9. 正则查询

SELECT * FROM employee WHERE emp_name REGEXP '^jas';

SELECT * FROM employee WHERE emp_name REGEXP 'on$';

SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';


小结：对字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';