Added four problems

[hinduja97]

No description provided.

[super30admin]

1. Correctness:

   • Problem1.sql: The solution correctly identifies consecutive dates with the same state (failed/succeeded) and groups them into periods. It handles the date range filtering and grouping appropriately.
   • Problem2.sql: The solution correctly pivots the student table to show names in columns by continent, handling the row numbering and grouping well.
   • Problem3.sql: The solution correctly compares department averages with company averages by month, though it could be simplified by using GROUP BY directly in the CTE.
   • Problem4.sql: All three approaches (commented out and final) correctly find the first login date for each player, with the final solution being the most elegant using FIRST_VALUE().

2. Time Complexity:

   • Problem1.sql: O(n log n) due to the window function and sorting.
   • Problem2.sql: O(n log n) due to the window function and grouping.
   • Problem3.sql: O(n log n) due to the window functions and grouping.
   • Problem4.sql: O(n log n) for the window function approach, O(n) for the GROUP BY approach.

3. Space Complexity:

   • All solutions have O(n) space complexity due to storing intermediate results in CTEs or subqueries.

4. Code Quality:

   • The code is generally well-structured and readable.
   • Problem1.sql could benefit from comments explaining the logic of grouping consecutive dates.
   • Problem4.sql shows good exploration of different approaches but keeps commented-out code which should be removed in production.
   • Consistent indentation and formatting are maintained throughout.

5. Efficiency:

   • Problem3.sql could be optimized by moving the GROUP BY into the CTE to avoid redundant calculations.
   • Problem4.sql's final solution is the most efficient among the presented options.
   • All solutions handle edge cases well (empty tables, single records, etc.).