mysql数据库之单表查询与多表查询

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;
  
 查询顺序：from----》where---->group by---->having----->distinct----->order by---->limit



def select():
    res1=from()
    res2=where(res1,pattern)
    res3=group(res2,)
    res4=having(res3)
    res5=distinct(res4)
    res6=order(res5)
    limit(res6)


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;


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;
    select post,min(salary) from emp group by post;
    select post,avg(salary) from emp group by post;
    select post,sum(salary) from emp group by post;
    select post,count(id) from emp group by post;



    #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;


    # 查询四则运算
    select name,salary*12 as annual_salary from emp;


    分组练习
11 查询python1班 班级id，学生id最大的人id，所有人的名字，把名字列表的列名命名为names

    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;

// sum() ---- 求所有数据的总和
// max() ----求所有数据中的最大值
// min() -----求所有数据中的最小值
// count() ---求所有数据的个数
// avg() ----　求所有数据的平均值

 select sname, "同学", age+1, avg(age) from student;

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;



五 distinct去重

select distinct post,avg(salary) from emp
            where age >= 30
            group by post
            having avg(salary) > 10000;


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)
    ;


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;    #从0开始显示5条
select * from emp limit 5,5;    #从5开始显示5条



# 正则表达式

select * from emp where name regexp '^jin.*(n|g)$';







二、多表查询

1、内连接：把两张表有对应关系的记录连接成一张虚拟表
select * from emp inner join dep on emp.dep_id = dep.id;

#应用：
    select * from emp,dep where emp.dep_id = dep.id and dep.name = "技术"; # 不要用where做连表的活

    select * from emp inner join dep on emp.dep_id = dep.id
        where dep.name = "技术"
    ;

2、左连接：在内连接的基础上，保留左边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id;


3、右连接：在内连接的基础上，保留右边没有对应关系的记录
select * from emp right join dep on emp.dep_id = dep.id;


4、全连接：在内连接的基础上，保留左、右边没有对应关系的记录
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;



#补充：多表连接可以不断地与虚拟表连接

查找各部门最高工资
select t1.* from emp as t1
inner join
(select post,max(salary) as ms from emp group by post) as t2
on t1.post = t2.post
where t1.salary = t2.ms
;



子查询：
  把一条查询语句用括号括起来，用来当作查询条件去使用，这种查询方式就是子查询。

select emp.name from emp inner join dep on emp.dep_id = dep.id where dep.name="技术";     #多表链接查询

select name from emp where dep_id =
(select id from dep where name="技术");      #子查询