!= 会排除null数据

select name from Customer where referee_id != 2 or referee_id is null;

交叉联结

交叉连接（CROSS JOIN）-CSDN博客

197. 上升的温度

select a.id

from weather

as a

cross join weather

as b

on datediff(a.recordDate  ,b.recordDate  ) = 1

where a.temperature  > b.temperature ;

SELECT b.Id FROM Weather as a,Weather as b WHERE a.Temperature < b.Temperature and DATEDIFF(a.RecordDate,b.RecordDate) = -1; 

1661. 每台机器的进程平均运行时间

SELECT a1.machine_id AS 'machine_id',

    ROUND(AVG(a2.timestamp - a1.timestamp)

        ,3) AS 'processing_time'

FROM Activity AS a1

    INNER JOIN Activity AS a2

    ON a1.machine_id = a2.machine_id

        AND a1.process_id = a2.process_id  

        AND a1.activity_type = 'start' AND a2.activity_type = 'end'

GROUP BY a1.machine_id