what’s the difference between DATEDIFF and doing subtraction by myself

the leetcode question 197.Rising Temperature

---

Given a Weather table, write a SQL query to find all dates’ Ids with higher temperature compared to its previous (yesterday’s) dates.

+---------+------------------+------------------+
| Id(INT) | RecordDate(DATE) | Temperature(INT) |
+---------+------------------+------------------+
|       1 |       2015-01-01 |               10 |
|       2 |       2015-01-02 |               25 |
|       3 |       2015-01-03 |               20 |
|       4 |       2015-01-04 |               30 |
+---------+------------------+------------------+
For example, return the following Ids for the above Weather table:

+----+
| Id |
+----+
|  2 |
|  4 |
+----+

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i didn’t know the function DATEDIFF(),so i wrote my sql solution:

select w1.Id
from Weather w1,Weather w2
where w1.RecordDate - w2.RecordDate = 1
and w1.Temperature > w2.Temperature

and i went through the testcase but got a wrong submit,the right solution is use funtion DATEDIFF()

select w1.Id
from Weather w1,Weather w2
where DATEDIFF(w1.RecordDate,w2.RecordDate)=1
and w1.Temperature > w2.Temperature

so my question is what’s the difference between DATEDIFF(w1.RecordDate,w2.RecordDate)=1and w1.RecordDate - w2.RecordDate = 1

thank you for your help

Answer

If the datatype of RecordDate is DATETIME rather than DATE, subtracting them returns a large value that contains the difference between the times as well as the dates. E.g.

mysql> select cast('2019-03-21 10:20:30' as datetime) - cast('2019-03-11 9:15:20' as datetime) as difference;
+-----------------+
| difference      |
+-----------------+
| 10010510.000000 |
+-----------------+

But if they’re DATE then subtraction should be the same as DATEDIFF():

mysql> select cast('2019-03-21' as date) - cast('2019-03-11' as date) as difference;
+------------+
| difference |
+------------+
|         10 |
+------------+