一 语法
select distinct 查询字段1，查询字段2，。。。 from 表名
where 分组之前的过滤条件
group by 分组依据
having 分组之后的过滤条件
order by 排序字段
limit 显示的条数;
上面是语法定义顺序，不是执行顺序；
#使用函数的方式模拟语法顺序；
def from(dir,file):
open('%s\%s' %(dir,file),'r')
return f
def where(f,pattern):
for line in f:
if pattern：
yield line
def group():
pass
def having():
pass
def distinct():
pass
def order():
pass
def limit():
pass
# 语法执行顺序（如果直接写后面的 前面关键字默认值=True）
def select():
res1=from()
res2=where(res1,pattern)
res3=group(res2,)
res4=having(res3)
res5=distinct(res4)
res6=order(res5)
limit(res6) #limit 本质是一个打印操作
？？？
select math as "级别",count(math)as "人数" from stu group by "级别" order by "人数" desc limit 0,3;
？？？？
二 where过滤
select id,name from db39.emp where id >= 3 and id <= 6
select * from db39.emp where id between 3 and 6;
select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
select * from emp where salary in (20000,18000,17000);
要求：查询员工姓名中包含i字母的员工姓名与其薪资
select name,salary from db39.emp where name like '%i%'
要求：查询员工姓名是由四个字符组成的的员工姓名与其薪资
select name,salary from db39.emp where name like '____';
select name,salary from db39.emp where char_length(name) = 4;
select * from db39.emp where id not between 3 and 6;
select * from emp where salary not in (20000,18000,17000);
要求：查询岗位描述为空的员工名与岗位名
select name,post from db39.emp where post_comment is NULL;
select name,post from db39.emp where post_comment is not NULL;
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三 group by分组
#设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据
mysql> set global sql_mode="strict_trans_tables,only_full_group_by";
#每个部门的最高工资
select post,max(salary) from emp group by post; 最大值
select post,min(salary) from emp group by post; 最小值
select post,avg(salary) from emp group by post; 平均值
select post,sum(salary) from emp group by post; 总和
select post,count(id) from emp group by post; 每个分组的人数
#group_concat（分组之后用）
select post,group_concat(name) from emp group by post;
select post,group_concat(name,"_SB") from emp group by post;
select post,group_concat(name,": ",salary) from emp group by post;
select post,group_concat(salary) from emp group by post;
# 补充concat（不分组时用）
select name as 姓名,salary as 薪资 from emp;
select concat("NAME: ",name) as 姓名,concat("SAL: ",salary) as 薪资 from emp;
# 补充as语法
mysql> select emp.id,emp.name from emp as t1; # 报错
mysql> select t1.id,t1.name from emp as t1;
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# 查询四则运算
select name,salary*12 as annual_salary from emp;
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分组练习
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 emp 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 emp group by post;
3. 查询公司内男员工和女员工的个数
select sex,count(id) from emp group by sex;
4. 查询岗位名以及各岗位的平均薪资
select post,avg(salary) from emp group by post;
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
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 emp group by sex;
8、统计各部门年龄在30岁以上的员工平均工资
select post,avg(salary) from emp where age >= 30 group by post;
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四 having过滤
having的语法格式与where一模一样，只不过having是在分组之后进行的进一步过滤
即where不能用聚合函数，而having是可以用聚合函数，这也是他们俩最大的区别
1、统计各部门年龄在30岁以上的员工平均工资，并且保留平均工资大于10000的部门
select post,avg(salary) from emp
where age >= 30
group by post
having avg(salary) > 10000;
#强调：having必须在group by后面使用
select * from emp
having avg(salary) > 10000; #这是错误的
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五 distinct去重
select distinct post,avg(salary) from emp
where age >= 30
group by post
having avg(salary) > 10000;
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六 order by 排序
select * from emp order by salary asc; #默认升序排
select * from emp order by salary desc; #降序排
select * from emp order by age desc; #降序排
select * from emp order by age desc,salary asc; #先按照age降序排，再按照薪资升序排
# 统计各部门年龄在10岁以上的员工平均工资，并且保留平均工资大于1000的部门，
然后对平均工资进行排序
select post,avg(salary) from emp
where age > 10
group by post
having avg(salary) > 1000
order by avg(salary)
;
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七 limit 限制显示条数
select * from emp limit 3;
select * from emp order by salary desc limit 1;
# 分页显示
select * from emp limit 0,5;
select * from emp limit 5,5;
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八 正则表达式
# MYSQL支持正则匹配
select * from emp where name regexp '^jin.*(n|g)$';
# 开启正则
select * from emp where name regexp
