LeetCode力扣数据库题库-简单模式部分题解

196. 删除重复的电子邮箱

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错误写法

delete from Person
where Id not in (
select min(Id) from Person
group by Email)

错误提示
不允许同时查找和删除同一张表
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正确写法
运用子查询

delete from Person
where Id not in (
	select Id
	from(
		select min(Id) Id from Person
		group by Email
		) t
	)

627. 交换工资

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update salary set sex=if(sex='f','m','f')

1083. 销售分析三

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注意：in，not in 在表中的具体范围
分为两部分来考虑，1.买S8的买家，2.在1的基础上，除去买iphone的买家

select distinct buyer_id
from Sales s
join Product p
on p.product_id=s.product_id
where product_name in ('S8') and buyer_id not in (
	select buyer_id
	from Sales s
	join Product p
	on p.product_id=s.product_id
	where product_name in ('iPhone')
	)

1179.重新格式化部门表

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select id,
	sum(case month when 'Jan' then revenue else null end) Jan_Revenue,
	sum(case month when 'Feb' then revenue else null end) Feb_Revenue,
    sum(case month when 'Mar' then revenue else null end)  Mar_Revenue,
    sum(case month when 'Apr' then revenue else null end)  Apr_Revenue,
    sum(case month when 'May' then revenue else null end)  May_Revenue,
    sum(case month when 'Jun' then revenue else null end)  Jun_Revenue,
    sum(case month when 'Jul' then revenue else null end)  Jul_Revenue,
    sum(case month when 'Aug' then revenue else null end)  Aug_Revenue,
    sum(case month when 'Sep' then revenue else null end)  Sep_Revenue,
    sum(case month when 'Oct' then revenue else null end)  Oct_Revenue,
    sum(case month when 'Nov' then revenue else null end)  Nov_Revenue,
    sum(case month when 'Dec' then revenue else null end)  Dec_Revenue
	from Department group by id

1211.查询结果的质量和占比

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select query_name,round(sum(rating/position)/count(*),2) quality,round(sum(rating<3)*100/count(*),2) oor_query_percentage
from Queries  group by query_name

1280.学生们参加各科测试的次数

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1.看结果前三列，每一个student_name 把所有的subject_name都考虑进去，所以考虑笛卡尔乘积，即交叉连接
2.最后一列，可以考虑，在1的基础上左连接Examinations，测试数为null（0）

select a.student_id,student_name,c.subject_name,count( b.subject_name) attended_exams
from Students a cross join Subjects c 
left join Examinations b 
on a.student_id=b.student_id and c.subject_name=b.subject_name
group by a.student_id,c.subject_name
order by a.student_id

1294.不同国家的天气类型

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1.先计算每个国家的月平均weather_state
2.在表1的基础上，利用case，输出天气类型

select country_name,(
case 
when weather_state<=15 then 'Cold'
when weather_state<25 then 'Warm'
else 'Hot' end) weather_type
from (select country_name,sum(weather_state)/count(*) weather_state
	from Countries c join 
	Weather w
	on c.country_id=w.country_id
	where day between '2019-11-01' and '2019-11-30'
	group by c.country_id) t

1435.创建一个分组表格

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利用联合

(select '[0-5>' bin, count(*) from Sessions where duration/60<5)
union
(select '[5-10>' bin,count(*) total 
from Sessions
where  duration/60 >=5 and duration/60 <10 )
union
(select '[10-15>' bin,count(*) total 
from Sessions
where  duration/60 >=10 and duration/60 <15 )
union
(select '15 or more' bin,count(*) total 
from Sessions
where  duration/60 >=15  )

1485.按日期分组

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

concat:将多个字符串连接成一个字符串
group_concat：将group by产生的同一个分组中的值连接起来，返回一个字符串结果。
语法：group_concat( [distinct] 要连接的字段 [order by 排序字段 asc/desc ] [separator ‘分隔符’] )