Rising Temperature | Leetcode 197 | Crack SQL Interviews in 50 Qs #mysql #leetcode

Summary

In this SQL interview series video, Chirag introduces a problem based on finding rising temperatures in a `weather` table. The task involves identifying records where the temperature on a certain day is higher than the previous day's temperature. He explains the solution in detail, using SQL concepts like `INNER JOIN`, date comparison functions like `DATE_DIFF()` and `SUBDATE()`, and the logic behind comparing consecutive days’ temperatures. With step-by-step guidance, Chirag demonstrates how to write the SQL query to solve this problem, ensuring viewers understand key SQL concepts and techniques for handling date-based comparisons in queries.

Takeaways

• 😀 The video is part of an SQL interview series, focusing on solving SQL queries.
• 😀 The topic of the current session is basic joins, specifically discussing a 'Rising Temperature' problem.
• 😀 The task involves working with a 'Weather' table that includes columns for ID, record date, and temperature.
• 😀 The goal is to find all date IDs with higher temperatures compared to their previous days (yesterday).
• 😀 Example data is given to illustrate how to identify records where the temperature on a particular date is higher than the previous day.
• 😀 If the temperature on a given date is higher than the previous day, the ID for that date is returned in the output.
• 😀 The solution requires checking both the consecutive dates and the temperature comparison between them.
• 😀 The SQL query uses an INNER JOIN to compare the 'Weather' table to itself using aliases (w1 and w2) to distinguish the records.
• 😀 The 'DATE_DIFF' function is used to calculate the difference between two dates, ensuring the dates being compared are consecutive.
• 😀 The final SQL code returns the ID of dates where the temperature is higher than the previous date, based on the conditions set in the problem.
• 😀 The video encourages viewers to try the problem themselves before referring to the solution for a deeper understanding.

Q & A

What is the main objective of the SQL interview question discussed in the script?

The main objective is to find all IDs with a higher temperature compared to the previous day's temperature, from a weather table that includes columns for ID, record date, and temperature.

What are the key columns in the 'weather' table used in the SQL question?

The key columns in the 'weather' table are ID, record date, and temperature.

What SQL concept is primarily used to compare consecutive rows in the same table?

The primary SQL concept used to compare consecutive rows is the 'INNER JOIN', where the table is joined with itself to compare each row with the previous day's data.

How does the 'INNER JOIN' work in the SQL solution?

The 'INNER JOIN' is used to join the 'weather' table with itself using aliases (w1 and w2), and the join condition is based on the difference in the record dates. The result includes only rows where the temperature is higher on the current day compared to the previous day.

What SQL function is used to calculate the difference between two dates?

The SQL function 'DATEDIFF' is used to calculate the difference between two dates, returning the number of days between them.

Why is the condition 'DATEDIFF(w1.Record_Date, w2.Record_Date) = 1' used in the query?

This condition ensures that only consecutive days are compared. It checks that the difference in record dates between the two rows is exactly one day.

What happens if we use 'LEFT JOIN' instead of 'INNER JOIN' in this query?

Using 'LEFT JOIN' would return all rows from the first table (w1), even if there is no matching record in the second table (w2). However, in this scenario, 'INNER JOIN' is more appropriate because we only want to return rows where there is a matching consecutive date.

What would be the output for the weather data example if we had a temperature drop from one day to the next?

If there is a temperature drop from one day to the next, the corresponding ID would not be included in the output, as the query filters for rows where the temperature is higher on the current day than the previous day.

Can the query be modified to return the actual temperature difference between consecutive days?

Yes, the query can be modified to return the temperature difference by adding a column in the SELECT statement: 'SELECT w1.ID, (w1.Temperature - w2.Temperature) AS Temp_Difference'. This would display the difference between the current and previous day's temperatures.

What is the purpose of the aliasing in the SQL query (w1 and w2)?

Aliasing the table as 'w1' and 'w2' helps distinguish between the two instances of the same table. This makes it easier to reference each row's data for comparison within the query, especially when joining the table to itself.