LeetCode 高频 SQL 50 题（基础版）之 【连接】部分 · 上

题目：1378. 使用唯一标识码替换员工ID

题解：

select eu.unique_id,e.name from Employees e left join EmployeeUNI eu on e.id=eu.id

题目：1068. 产品销售分析 I

题解：

select p.product_name,s.year,s.price from Sales s,Product p 
where s.product_id = p.product_id 

题目：1581. 进店却未进行过交易的顾客

题解：

select customer_id,count(customer_id) as count_no_trans from Visits v left join Transactions t on v.visit_id=t.visit_id
where t.transaction_id is null 
group by customer_id

select customer_id,count(customer_id) as count_no_trans  from Visits 
where visit_id not in (select distinct visit_id from Transactions )
group by customer_id

题目：197. 上升的温度

题解：

select b.id as id from Weather a,Weather b 
where datediff(b.recordDate,a.recordDate)=1 and a.Temperature < b.Temperature

题目：1661. 每台机器的进程平均运行时间

题解：

select a.machine_id, round(avg(b.timestamp-a.timestamp),3) as processing_time  from Activity a,Activity b 
where a.machine_id=b.machine_id and a.process_id=b.process_id and a.activity_type='start' and b.activity_type='end'
group by a.machine_id