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)
关键字的执行优先级 重点中的重点！！！
from--->where--->group by--->having--->select--->distinct--->order by--->limit
1.找到表:from
2.拿着where指定的约束条件，去文件/表中取出一条条记录
3.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
4.将分组的结果进行having过滤
5.执行select
6.去重
7.将结果按条件排序：order by
8.限制结果的显示条
三、简单查询
1、造表
创建表
mysql> create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex 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
);
查看表结构
mysql> desc employee;
插入记录（三个部门：教学，销售，运营）
mysql> insert into employee(name,sex,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)
;
2、查询
简单查询
SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee;
SELECT * FROM employee;
SELECT name,salary FROM employee;
避免重复DISTINCT
SELECT DISTINCT post FROM employee; #distinct意思是输出后面 列 去重后的内容；
通过四则运算查询
SELECT name, salary*12 FROM employee;
SELECT name, salary*12 AS Annual_salary FROM employee;
SELECT name, salary*12 Annual_salary FROM employee;
定义显示格式
#CONCAT() 函数用于连接字符串
SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary FROM employee;
#CONCAT_WS() 第一个参数为分隔符
SELECT CONCAT_WS(':',name,salary*12) AS Annual_salary FROM employee;
结合CASE语句：
SELECT
(
CASE
WHEN NAME = 'egon' THEN
NAME
WHEN NAME = 'alex' THEN
CONCAT(name,'_BIGSB')
ELSE
concat(NAME, 'SB')
END
) as new_name
FROM employee;
3、小练习
1）查出所有员工的名字，薪资,格式为
<名字:egon> <薪资:3000>
SELECT CONCAT('<名字:',name,'> ','<年薪:',salary*12,'>') AS annual_year FROM employee;
2）查出所有的岗位（去掉重复）
SELECT DISTINCT depart_id FROM employee;
3）查出所有员工名字，以及他们的年薪,年薪的字段名为annual_year
SELECT name, salary*12 AS annual_year FROM employee;
四、WHERE约束
where字句中可以使用：
1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在10到20之间
3. in(80,90,100) 值是10或20或30
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
单条件查询：
SELECT name FROM employee WHERE post='sale';
多条件查询：
SELECT name,salary FROM employee WHERE post='teacher' AND salary>10000;
关键字BETWEEN AND：
SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000;
SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000;
关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)：
SELECT name,post_comment FROM employee WHERE post_comment IS NULL;
SELECT name,post_comment FROM employee WHERE post_comment IS NOT NULL;
SELECT name,post_comment FROM employee WHERE post_comment=''; 注意''是空字符串，不是null
ps：
执行update employee set post_comment='' where id=2;再用上条查看，就会有结果了
关键字IN集合查询：
SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ;
SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ;
关键字LIKE模糊查询：
通配符’%’：SELECT * FROM employee WHERE name LIKE 'eg%';
通配符’_’：SELECT * FROM employee WHERE name LIKE 'al__';
小练习：
1. 查看岗位是teacher的员工姓名、年龄
SELECT name,age FROM employee WHERE post='teacher';
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
SELECT name,age FROM employee WHERE post='teacher' AND age>30;
3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
SELECT name,age,salary FROM employee WHERE post='teacher' AND salary between 9000 and 10000;
4. 查看岗位描述不为NULL的员工信息
select * from employee where post_comment is not null;
5. 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 name,age,salary FROM employee WHERE post='teacher' AND salary in (10000,9000,30000);
6. 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 name,age,salary FROM employee WHERE post='teacher' AND salary not in (10000,9000,30000);
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
SELECT name,salary FROM employee WHERE post='teacher' AND age like 'jin%';
五、分组查询:GROUP BY
1、什么是分组？为什么要分组？
1）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
2）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
3）为何要分组呢？
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数
小窍门：‘每’这个字后面的字段，就是我们分组的依据
4）大前提：
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
2、ONLY_FULL_GROUP_BY
查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
！！！注意
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。
设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
3、GROUP BY
单独使用GROUP BY关键字分组
SELECT post FROM employee GROUP BY post;
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
SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;
GROUP BY与聚合函数一起使用
select post,count(id) as count from employee group by post;#按照岗位分组，并查看每个组有多少人
4、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
示例：
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;
5、小练习：
1）查询岗位名以及岗位包含的所有员工名字
SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;
2）查询岗位名以及各岗位内包含的员工个数
SELECT post,count(id) FROM employee GROUP BY post;
3）查询公司内男员工和女员工的个数
SELECT sex,count(id) FROM employee GROUP BY sex;
4）查询岗位名以及各岗位的平均薪资
SELECT post,AVG(salary) FROM employee GROUP BY post;
5）查询岗位名以及各岗位的最高薪资
SELECT post,MAX(salary) FROM employee GROUP BY post;
6）查询岗位名以及各岗位的最低薪资
SELECT post,MIN(salary) FROM employee GROUP BY post;
7）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
SELECT sex,AVG(salary) FROM employee GROUP BY sex;
六、HAVING过滤
HAVING与WHERE不一样的地方在于!!!!!!
！！！执行优先级从高到低：where > group by > having
1. 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。
2. 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：
1. 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
SELECT post,group_concat(name),count(id) FROM employee GROUP BY post having count(id)<2;
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
SELECT post,AVG(salary) FROM employee GROUP BY post having AVG(salary)>10000;
4. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
SELECT post,AVG(salary) FROM employee GROUP BY post having AVG(salary)>10000 and AVG(salary)<20000;
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;
小练习：
1. 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
SELECT * from employee ORDER BY age,hire_date DESC;
2. 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
SELECT post,avg(salary) from employee group by post having avg(salary)>10000 ORDER BY avg(salary);
3. 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
SELECT post,avg(salary) from employee group by post having avg(salary)>10000 ORDER BY avg(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
小练习：
1. 分页显示，每页5条
SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5;
SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5;
SELECT * FROM employee ORDER BY salary DESC LIMIT 10,5;
九、使用正则表达式查询
SELECT * FROM employee WHERE name REGEXP '^ale';
SELECT * FROM employee WHERE name REGEXP 'on$';
SELECT * FROM employee WHERE name REGEXP 'm{2}';
小结：对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';
小练习：
查看所有员工中名字是jin开头，n或者g结果的员工信息
SELECT * FROM employee WHERE name REGEXP '^jin.*[gn]';
