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Rising Temperature

Problem Description​

Given a Weather table, write a SQL query to find all dates' Id with higher temperatures compared to its previous dates (yesterday).

Assume the table Weather has the following structure:

+---------+------------+------------------+
| 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 |
+---------+------------+------------------+

Example​

Example 1:

Assume that the `Weather` table has the following content:
+---------+------------+------------------+
| 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 the `Weather` table above, your query should return:
+----+
| Id |
+----+
|  2 |
|  4 |
+----+

Constraints​

  • The Weather table will have at most 10^4 rows.
  • The RecordDate values are unique.

Solution Approach​

Approach Overview​

To find all dates' Id with higher temperatures compared to its previous dates, we need to compare each row with the row corresponding to the previous date.

Detailed Steps​

  1. Join the Table with Itself:

    • Use a JOIN operation to join the Weather table with itself, where the RecordDate of the first instance is one day after the RecordDate of the second instance.

  2. Filter Rows with Higher Temperature:

    • In the WHERE clause, compare the Temperature of the joined rows to ensure we only select rows where the current day's temperature is higher than the previous day's temperature.

Code Examples​

SQL Query​

SELECT w1.Id
FROM Weather w1
JOIN Weather w2
ON DATEDIFF(w1.RecordDate, w2.RecordDate) = 1
WHERE w1.Temperature > w2.Temperature;

Explanation​

  • JOIN Weather w2 ON DATEDIFF(w1.RecordDate, w2.RecordDate) = 1: This joins each row in the Weather table with the row from the previous day.
  • WHERE w1.Temperature > w2.Temperature: This filters the rows to only include those where the temperature is higher than the previous day's temperature.

Complexity​

  • Time Complexity: O(n log n) if indexed properly. The JOIN operation on dates can be efficient with indexing on RecordDate.

  • Space Complexity: O(n), as we might need additional space for the result set depending on the number of rows that satisfy the condition.