【MySQL】分组汇总实例（面试题入门到拓展）

原文：看似简单的一道SQL面试题，你是否能够很快写出答案？

黄伟呢 数据分析与统计学之美  2020-06-13

 

https://mp.weixin.qq.com/s/BBsN46t1KQ3tW13gLdBmcw

原表数据

目标表输出：

-- 创建表

create table student (
     id varchar(20),
     name varchar(20),
     gender char(1),
     birth varchar(20),
     department varchar(20),
     address varchar(20)
) charset = utf8;

-- 插入数据
insert into student values
("201901","张大佬","男","1985","计算机系","北京市海淀区"),
("201902","郭大侠","男","1986","中文系","北京市昌平区"),
("201903","张三","女","1990","中文系","湖南省永州市"),
("201904","李四","男","1990","英语系","辽宁市阜新市"),
("201905","王五","女","1991","英语系","福建省厦门市"),
("201906","王六","男","1988","计算机系","湖南省衡阳市");

 第一步分组：

select 
    department 院系,
    case gender when "男" then 1 else 0 end 男,
    case gender when "女" then 1 else 0 end 女
from student;

第二部汇总：外面套一层

select 
     院系,
     sum(男) 男,
     sum(女) 女,
     sum(男) + sum(女) as 总计
from
(
    select department 院系,
    case gender when "男" then 1 else 0 end 男,
    case gender when "女" then 1 else 0 end 女
    from student
) a
group by 院系;

原解题思路，详见上述公众号

-----------------------分割线------------------------------

 以上思路比较清晰易懂，但是需要执行两次查询，如果数据量比较大，比如超过50W条记录时，查询效率不高。

以下为个人改进版

-- 分组汇总

select 
    department 院系,
    sum(case gender when "男" then 1 else 0 end) as 男,
    sum(case gender when "女" then 1 else 0 end) as 女
from student
group by department;

 套用公式：

sum(case ...when...then...else...end) as xxx

其中sum，可以改成任意集合函数，如count，max，min等等

有人可能会说，decode也可以啊，decode只能用在oracle

-- Oracle decode写法

select 
    department 院系,
    sum(decode(gender ,”男” ,1 , 0 )) as 男,
    sum(decode(gender ,”女” ,1 , 0)) as 女
from student
group by department;

如果要在MySQL实现上面的方法，除了case when then else end，

还有if和decode相似，感谢群友（深圳-小小明）提供思路

select 
    department 院系,
    sum(if(gender ="男" ,1, 0)) as 男,
    sum(if(gender ="女" ,1, 0)) as 女
from student
group by department;

 未聚合

select 
    department 院系,
    if(gender ="男" ,1, 0) as 男,
    if(gender ="女" ,1, 0) as 女
from student;

---------------------------

再拓展下

加上合计：count（gender），或者sum（1），因为gender字段本身不具备数学运算

如果gender字段为空，合计可能要调整，具体还得看实际业务，不在本次考虑范围内。

 学会了没，是不是很简单