1. 相关知识点

• 相关函数

  函数	含义
  concat()	字符串拼接
  upper（）	转大写
  lower()	转小写
  left(name,1)	从左边截取
  right(name,1)	从有边截取
  substr(name,2)	从右边第二个位置开始截取
  group_concat(name separator ‘,’)	分组后所有的name根据’,'拼接
  regexp	正则表达式筛选条件

• 指定2020年2月的方法

  • order_date between ‘2020-02-01’ and ‘2020-02-29’
  • order_date like ‘2020-02%’
  • DATE_FORMAT(order_date, “%Y-%m”) = “2020-02”
  • LEFT(order_date, 7) 或 substr(1, 7)

2.例子

2.44 - 修复表中的名字

-- concat(upper(left(name,1)),lower(right(name,length(name)-1))) 
-- concat(upper(left(name,1)),lower(substr(name,2)))

select
    user_id,
    concat(upper(left(name,1)),lower(right(name,length(name)-1))) as name
from
    users
order by
    user_id;

2.45 - 患某种疾病的患者

select
    *
from
    Patients
where
    conditions  like 'DIAB1%' or conditions  like '% DIAB1%'

2.46 - 删除重复的电子邮箱

delete 
    p1
from 
    Person p1,Person p2
where 
    p1.email=p2.email and p1.id>p2.id;

2.47 - 第二高的薪水

-- 方法一
select(
    select
        salary
    from   
        Employee
    order by
        salary
    limit 1 offset 1) as SecondHighestSalary;

-- 方法二
select
    (select
        salary 
    from
        (
            select
                *,
                row_number() over(order by salary desc) number
            from
                Employee 
            
        ) as t
    where
        number=2) as SecondHighestSalary ;

2.48 - 按日期分组销售产品

-- group_concat 分组拼接

select
    sell_date,
    count(distinct product) num_sold,
    group_concat(distinct product order by product  separator ',') products       
from
    Activities
group by
    sell_date;      

2.49 - 列出指定时间段内所有的下单产品

-- 指定2020年2月的方法
-- (1) order_date between '2020-02-01' and '2020-02-29'  
-- (2) order_date like '2020-02%'  
-- (3) DATE_FORMAT(order_date, "%Y-%m") = "2020-02"   
-- (4) LEFT(order_date, 7) 或 substr(1, 7)

select
    p.product_name,sum(unit) unit
from
    Orders o left join Products p on p.product_id=o.product_id
where
    o.order_date like '%2020-02%'
group by
    o.product_id
having
    sum(unit)>=100

2.50 - 查找拥有有效邮箱的用户

select
    *
from
    Users 
where
    mail regexp '^[A-Za-z][a-zA-Z0-9\\_\\.\\-\\/]*@leetcode\.com$'