SQL 问题：内连接给我空数据集

Question

Please have a look at this question - https://leetcode.com/problems/rising-temperature/

I went with the approach of joining the table with itself and then putting my where condition to get the data I want. This is the query I wrote :-

SELECT wt1.Id FROM Weather wt1 
    inner join 
Weather wt2 on wt1.id = wt2.id
WHERE wt1.Temperature > wt2.Temperature AND 
      TO_DAYS(wt1.recordDate)-TO_DAYS(wt2.recordDate)=1;

Which gives me an empty data set.

However if I don't use join, and just use the Weather table twice, it works. My query :-

SELECT wt1.Id 
FROM Weather wt1, Weather wt2
WHERE wt1.Temperature > wt2.Temperature AND 
      TO_DAYS(wt1.recordDate)-TO_DAYS(wt2.recordDate)=1;

I want to know what difference the inner join makes and why it doesn't work.

Answer 1

The correct version of your query, which neither of the two variants is actually doing, is this:

SELECT wt1.Id
FROM Weather wt1 
INNER JOIN Weather wt2
    ON wt1.Temperature > wt2.Temperature AND 
       TO_DAYS(wt1.recordDate) - TO_DAYS(wt2.recordDate) = 1;

This uses a proper explicit self-join with your conditions. Note that the wt1.id = wt2.id join condition is dubious, since assuming id is a primary key or unique column, a given record can never have two different temperature values from the same column.