一、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
二、关键字的执行优先级(重点)
from ——> where ——> group by ——> having ——> select ——> distinct ——> order by ——> limit
1.from
首先要找到定位到要查找的表。
2.where
然后根据where指定的约束条件，去表中取出一条条记录
3.group by
将取出的所有记录进行分组group by，如果没有group by，则整体作为一组
4.having
将分组的结果进行having过滤
5.select
执行select
6.distinct
去重
7.order by
将结果按条件排序
8.limit
限制结果的显示条数
三、简单查询
company.employee
员工id id int
姓名 emp_name varchar
性别 sex enum
年龄 age int
入职日期 hire_date date
岗位 post varchar
职位描述 post_comment varchar
薪水 salary double
办公室 office int
部门编号 depart_id int
创建表：
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male',
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,
depart_id int);
查看表结构：
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(3) unsigned | NO | | 28 | |
| hire_date | date | NO | | NULL | |
| post | varchar(50) | YES | | NULL | |
| post_comment | varchar(100) | YES | | NULL | |
| salary | double(15,2) | YES | | NULL | |
| office | int(11) | YES | | NULL | |
| depart_id | int(11) | YES | | NULL | |
+--------------+-----------------------+------+-----+---------+----------------+
10 rows in set (0.04 sec)
插入记录：
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('Alex0','male',78,'20150302','teacher',1000000,401,1),
('Alex1','male',81,'20130305','teacher',8300,401,1),
('Alex2','male',73,'20140701','teacher',3500,401,1),
('Alex3','male',28,'20121101','teacher',2100,401,1),
('Alex4','male',18,'20110211','teacher',9000,401,1),
('Alex5','male',18,'19000301','teacher',30000,401,1),
('Coco0','female',48,'20150311','sale',3000.13,402,2),
('Coco1','female',38,'20101101','sale',2000.35,402,2),
('Coco2','female',18,'20110312','sale',1000.37,402,2),
('Coco3','female',18,'20160513','sale',3000.29,402,2),
('Coco4','female',28,'20170127','sale',4000.33,402,2),
('Bei0','male',28,'20160311','operation',10000.13,403,3),
('Bei1','male',18,'19970312','operation',20000,403,3),
('Bei2','female',18,'20130311','operation',19000,403,3),
('Bei3','male',18,'20150411','operation',18000,403,3),
('Bei4','female',18,'20140512','operation',17000,403,3);
简单查询：
mysql> select id, name, sex, age, salary from employee;
+----+-------+--------+-----+------------+
| id | name | sex | age | salary |
+----+-------+--------+-----+------------+
| 1 | Alex0 | male | 78 | 1000000.00 |
| 2 | Alex1 | male | 81 | 8300.00 |
| 3 | Alex2 | male | 73 | 3500.00 |
| 4 | Alex3 | male | 28 | 2100.00 |
| 5 | Alex4 | male | 18 | 9000.00 |
| 6 | Alex5 | male | 18 | 30000.00 |
| 7 | Coco0 | female | 48 | 3000.13 |
| 8 | Coco1 | female | 38 | 2000.35 |
| 9 | Coco2 | female | 18 | 1000.37 |
| 10 | Coco3 | female | 18 | 3000.29 |
| 11 | Coco4 | female | 28 | 4000.33 |
| 12 | Bei0 | male | 28 | 10000.13 |
| 13 | Bei1 | male | 18 | 20000.00 |
| 14 | Bei2 | female | 18 | 19000.00 |
| 15 | Bei3 | male | 18 | 18000.00 |
| 16 | Bei4 | female | 18 | 17000.00 |
+----+-------+--------+-----+------------+
16 rows in set (0.00 sec)
避免重复distinct：
mysql> select distinct age from employee;
+-----+
| age |
+-----+
| 78 |
| 81 |
| 73 |
| 28 |
| 18 |
| 48 |
| 38 |
+-----+
7 rows in set (0.01 sec)
通过四则运算查询：
mysql> select name, salary * 12 as annual_salary from employee;
+-------+---------------+
| name | annual_salary |
+-------+---------------+
| Alex0 | 12000000.00 |
| Alex1 | 99600.00 |
| Alex2 | 42000.00 |
| Alex3 | 25200.00 |
| Alex4 | 108000.00 |
| Alex5 | 360000.00 |
| Coco0 | 36001.56 |
| Coco1 | 24004.20 |
| Coco2 | 12004.44 |
| Coco3 | 36003.48 |
| Coco4 | 48003.96 |
| Bei0 | 120001.56 |
| Bei1 | 240000.00 |
| Bei2 | 228000.00 |
| Bei3 | 216000.00 |
| Bei4 | 204000.00 |
+-------+---------------+
16 rows in set (0.10 sec)
定义显示格式（concat函数）：
mysql> select concat("姓名：", name, " 年薪：", salary * 12) as annual_salary from employee;
+-------------------------------------+
| annual_salary |
+-------------------------------------+
| 姓名：Alex0 年薪：12000000.00 |
| 姓名：Alex1 年薪：99600.00 |
| 姓名：Alex2 年薪：42000.00 |
| 姓名：Alex3 年薪：25200.00 |
| 姓名：Alex4 年薪：108000.00 |
| 姓名：Alex5 年薪：360000.00 |
| 姓名：Coco0 年薪：36001.56 |
| 姓名：Coco1 年薪：24004.20 |
| 姓名：Coco2 年薪：12004.44 |
| 姓名：Coco3 年薪：36003.48 |
| 姓名：Coco4 年薪：48003.96 |
| 姓名：Bei0 年薪：120001.56 |
| 姓名：Bei1 年薪：240000.00 |
| 姓名：Bei2 年薪：228000.00 |
| 姓名：Bei3 年薪：216000.00 |
| 姓名：Bei4 年薪：204000.00 |
+-------------------------------------+
16 rows in set (0.00 sec)
四、where约束
where可以使用简单的表达式：
1.比较运算符：><>= <= <> !=
2.between 80 and 100 值在80到100之间
3.in(80,90,100) 值是80或90或100
4.like 'egon%'
pattern可以是%或_，
%表示任意多字符
_表示一个字符
5.逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not
1.单条件查询
mysql> select name from employee where post = "sale";
+-------+
| name |
+-------+
| Coco0 |
| Coco1 |
| Coco2 |
| Coco3 |
| Coco4 |
+-------+
5 rows in set (0.00 sec)
2.多条件查询：
mysql> select name, salary from employee where post = "teacher" and salary > 10000;
+-------+------------+
| name | salary |
+-------+------------+
| Alex0 | 1000000.00 |
| Alex5 | 30000.00 |
+-------+------------+
2 rows in set (0.02 sec)
3.关键字between and：
mysql> select name, salary from employee where salary between 10000 and 20000;
+------+----------+
| name | salary |
+------+----------+
| Bei0 | 10000.13 |
| Bei1 | 20000.00 |
| Bei2 | 19000.00 |
| Bei3 | 18000.00 |
| Bei4 | 17000.00 |
+------+----------+
5 rows in set (0.00 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 from employee where post is null;
Empty set (0.00 sec)
mysql> select name, post from employee where post is not null;
+-------+-----------+
| name | post |
+-------+-----------+
| Alex0 | teacher |
| Alex1 | teacher |
| Alex2 | teacher |
| Alex3 | teacher |
| Alex4 | teacher |
| Alex5 | teacher |
| Coco0 | sale |
| Coco1 | sale |
| Coco2 | sale |
| Coco3 | sale |
| Coco4 | sale |
| Bei0 | operation |
| Bei1 | operation |
| Bei2 | operation |
| Bei3 | operation |
| Bei4 | operation |
+-------+-----------+
16 rows in set (0.00 sec)
5.关键字in集合查询：
mysql> select name, salary from employee where salary in (2100, 3500, 4000, 5000, 9000);
+-------+---------+
| name | salary |
+-------+---------+
| Alex2 | 3500.00 |
| Alex3 | 2100.00 |
| Alex4 | 9000.00 |
+-------+---------+
3 rows in set (0.00 sec)
5.关键字like模糊查询：
mysql> select * from employee where name like "Al%";
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
| 1 | Alex0 | male | 78 | 2015-03-02 | teacher | NULL | 1000000.00 | 401 | 1 |
| 2 | Alex1 | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 3 | Alex2 | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 4 | Alex3 | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 5 | Alex4 | male | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 6 | Alex5 | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
6 rows in set (0.37 sec)
mysql> select * from employee where name like "Al_";
Empty set (0.00 sec)
mysql> select * from employee where name like "Al__";
Empty set (0.00 sec)
mysql> select * from employee where name like "Al___";
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
| 1 | Alex0 | male | 78 | 2015-03-02 | teacher | NULL | 1000000.00 | 401 | 1 |
| 2 | Alex1 | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 3 | Alex2 | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 4 | Alex3 | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 5 | Alex4 | male | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 6 | Alex5 | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |
+----+-------+------+-----+------------+---------+--------------+------------+--------+-----------+
6 rows in set (0.00 sec)
五、分组查询
首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的。
分组指的是：将所有记录按照某个相同字段进行归类。
可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数。
ONLY_FULL_GROUP_BY
查看sql_mode的默认配置：
mysql> select @@global.sql_mode;
+-----------------------------------------------------------------------------------------------------------------------+
| @@global.sql_mode |
+-----------------------------------------------------------------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION |
+-----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.11 sec)
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。
set global sql_mode='ONLY_FULL_GROUP_BY';
quit
重新启动MySQL。
mysql> select post, count(id) from employee group by post;
+-----------+-----------+
| post | count(id) |
+-----------+-----------+
| teacher | 6 |
| sale | 5 |
| operation | 5 |
+-----------+-----------+
3 rows in set (0.02 sec)
GROUP BY
单独使用GROUP BY关键字分组
mysql> select post from employee group by post;
+-----------+
| post |
+-----------+
| teacher |
| sale |
| operation |
+-----------+
3 rows in set (0.00 sec)
注意：我们按照post字段分组，那么select查询的字段只能是post，想要获取组内的其他相关信息，需要借助函数
GROUP BY关键字和GROUP_CONCAT()函数一起使用
mysql> select post, group_concat(name) from employee group by post;
+-----------+-------------------------------------+
| post | group_concat(name) |
+-----------+-------------------------------------+
| operation | Bei0,Bei1,Bei2,Bei3,Bei4 |
| sale | Coco0,Coco1,Coco2,Coco3,Coco4 |
| teacher | Alex0,Alex1,Alex2,Alex3,Alex4,Alex5 |
+-----------+-------------------------------------+
3 rows in set (0.00 sec)
GROUP BY与聚合函数一起使用
mysql> select post, count(id) as count from employee group by post;
+-----------+-------+
| post | count |
+-----------+-------+
| teacher | 6 |
| sale | 5 |
| operation | 5 |
+-----------+-------+
3 rows in set (0.00 sec)
聚合函数
SELECT COUNT(*) FROM employee; # 统计所有员工数量
SELECT MAX(salary) FROM employee; # 筛选最高工资
SELECT MIN(salary) FROM employee; # 筛选最低工资
SELECT AVG(salary) FROM employee; # 计算平均工资
SELECT SUM(salary) FROM employee; # 计算所有工资和
六、having 过滤
**！！！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
小练习
- 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
mysql> select post, group_concat(name), count(id) from employee group by post having count(id) < 6;
+-----------+-------------------------------+-----------+
| post | group_concat(name) | count(id) |
+-----------+-------------------------------+-----------+
| operation | Bei0,Bei1,Bei2,Bei3,Bei4 | 5 |
| sale | Coco0,Coco1,Coco2,Coco3,Coco4 | 5 |
+-----------+-------------------------------+-----------+
2 rows in set (0.00 sec)
- 查询各岗位平均薪资大于10000的岗位名、平均工资
mysql> select post, avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| teacher | 175483.333333 |
| operation | 16800.026000 |
+-----------+---------------+
2 rows in set (0.00 sec)
- 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
mysql> select post, avg(salary) from employee group by post having avg(salary) between 10000 and 20000;
+-----------+--------------+
| post | avg(salary) |
+-----------+--------------+
| operation | 16800.026000 |
+-----------+--------------+
1 row in set (0.00 sec)
七、查询排序 order by
按单列排序
SELECT * FROM employee ORDER BY salary;
SELECT * FROM employee ORDER BY salary ASC;
SELECT * FROM employee ORDER BY salary DESC;
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;
八、限制查询的记录数 limit
示例：
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，即先查询出第一条，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，即先查询出第6条，然后包含这一条在内往后查5条
九、使用正则表达式查询
SELECT * FROM employee WHERE name REGEXP '^ale';
SELECT * FROM employee WHERE name REGEXP 'on$';
SELECT * FROM employee WHERE name REGEXP 'm{2}';