Solutions:

1.
SELECT 
  p.Category,
  EXTRACT(MONTH FROM s.Date) AS Month,
  SUM(s.Gross_Amount) AS Total_Sales,
  GROUPING(p.Category, EXTRACT(MONTH FROM s.Date)) AS Grouping_Level
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
GROUP BY GROUPING SETS (
  (p.Category),
  (EXTRACT(MONTH FROM s.Date)),
  ()
);

2.
SELECT 
  p.Category,
  p.Size,
  p.Color,
  SUM(s.Gross_Amount) AS Total_Sales
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
GROUP BY CUBE (p.Category, p.Size, p.Color);

3.
SELECT 
  EXTRACT(YEAR FROM Date) AS Year,
  EXTRACT(QUARTER FROM Date) AS Quarter,
  EXTRACT(MONTH FROM Date) AS Month,
  SUM(Gross_Amount) AS Total_Sales
FROM Sales_Transactions
GROUP BY ROLLUP (
  EXTRACT(YEAR FROM Date),
  EXTRACT(QUARTER FROM Date),
  EXTRACT(MONTH FROM Date)
);

4.
SELECT *
FROM (
  SELECT 
    EXTRACT(MONTH FROM s.Date) AS Month,
    p.Category,
    s.Gross_Amount
  FROM Sales_Transactions s
  JOIN Products p ON s.SKU_Code = p.SKU_Code
)
PIVOT (
  SUM(Gross_Amount)
  FOR Category IN ('Men', 'Women', 'Kids', 'Accessories')
)
ORDER BY Month;

5.
SELECT SKU_Code, Warehouse, Stock_Qty
FROM Inventory_Wide
UNPIVOT (
  Stock_Qty FOR Warehouse IN (Warehouse_A, Warehouse_B, Warehouse_C, Warehouse_D)
);

6.
SELECT 
  p.Category,
  c.Region,
  SUM(s.Gross_Amount) AS Total_Sales,
  GROUPING_ID(p.Category, c.Region) AS Grouping_ID
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
JOIN Customers c ON s.Customer_ID = c.Customer_ID
GROUP BY GROUPING SETS (
  (p.Category),
  (c.Region),
  (p.Category, c.Region),
  ()
);

7.
SELECT 
  p.Category,
  p.Size,
  CASE 
    WHEN SUM(s.Gross_Amount) > 5000 THEN 'Premium'
    WHEN SUM(s.Gross_Amount) > 2000 THEN 'Gold'
    ELSE 'Standard'
  END AS Customer_Tier,
  SUM(s.Gross_Amount) AS Total_Sales
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
JOIN Customers c ON s.Customer_ID = c.Customer_ID
GROUP BY CUBE (p.Category, p.Size, 
  CASE 
    WHEN SUM(s.Gross_Amount) > 5000 THEN 'Premium'
    WHEN SUM(s.Gross_Amount) > 2000 THEN 'Gold'
    ELSE 'Standard'
  END
);

8.
SELECT 
  EXTRACT(YEAR FROM Date) AS Year,
  EXTRACT(QUARTER FROM Date) AS Quarter,
  EXTRACT(MONTH FROM Date) AS Month,
  EXTRACT(DAY FROM Date) AS Day,
  SUM(Gross_Amount) AS Total_Sales
FROM Sales_Transactions
GROUP BY ROLLUP (
  EXTRACT(YEAR FROM Date),
  EXTRACT(QUARTER FROM Date),
  EXTRACT(MONTH FROM Date),
  EXTRACT(DAY FROM Date)
);

9.
SELECT *
FROM (
  SELECT 
    EXTRACT(YEAR FROM s.Date) AS Year,
    p.Category,
    s.Gross_Amount
  FROM Sales_Transactions s
  JOIN Products p ON s.SKU_Code = p.SKU_Code
)
PIVOT (
  SUM(Gross_Amount)
  FOR Category IN ('Men', 'Women', 'Kids', 'Accessories', 'Home')
)
ORDER BY Year;

10.
SELECT SKU_Code, Platform, Price
FROM Pricing_Wide
UNPIVOT (
  Price FOR Platform IN (Amazon_MRP, Flipkart_MRP, Myntra_MRP, Snapdeal_MRP)
);

11.
SELECT 
  COALESCE(p.Category, 'All Categories') AS Category,
  COALESCE(p.Size, 'All Sizes') AS Size,
  COALESCE(c.Region, 'All Regions') AS Region,
  SUM(s.Gross_Amount) AS Total_Sales,
  CASE 
    WHEN GROUPING(p.Category) = 1 AND GROUPING(p.Size) = 1 AND GROUPING(c.Region) = 1 THEN 'Grand Total'
    WHEN GROUPING(p.Category) = 1 AND GROUPING(p.Size) = 1 THEN 'Region Total'
    WHEN GROUPING(p.Category) = 1 AND GROUPING(c.Region) = 1 THEN 'Size Total'
    WHEN GROUPING(p.Size) = 1 AND GROUPING(c.Region) = 1 THEN 'Category Total'
    ELSE 'Detail'
  END AS Grouping_Text
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
JOIN Customers c ON s.Customer_ID = c.Customer_ID
GROUP BY GROUPING SETS (
  (p.Category, p.Size, c.Region),
  (p.Category, p.Size),
  (p.Category, c.Region),
  (p.Size, c.Region),
  (p.Category),
  (p.Size),
  (c.Region),
  ()
);

12.
SELECT 
  p.Category,
  p.Size,
  p.Color,
  c.Region,
  SUM(s.Gross_Amount) AS Total_Sales
FROM Sales_Transactions s
JOIN Products p ON s.SKU_Code = p.SKU_Code
JOIN Customers c ON s.Customer_ID = c.Customer_ID
GROUP BY CUBE (p.Category, p.Size, p.Color, c.Region);

13.
SELECT 
  c.Country,
  c.Region,
  c.City,
  SUM(s.Gross_Amount) AS Total_Sales
FROM Sales_Transactions s
JOIN Customers c ON s.Customer_ID = c.Customer_ID
GROUP BY ROLLUP (c.Country, c.Region, c.City);

14.
SELECT *
FROM (
  SELECT 
    s.Customer_ID,
    p.Category,
    s.Gross_Amount
  FROM Sales_Transactions s
  JOIN Products p ON s.SKU_Code = p.SKU_Code
)
PIVOT (
  SUM(Gross_Amount)
  FOR Category IN ('Men', 'Women', 'Kids', 'Accessories')
)
ORDER BY Customer_ID;

15.
SELECT SKU_Code, Quarter, Sales_Amount
FROM Seasonal_Sales_Wide
UNPIVOT (
  Sales_Amount FOR Quarter IN (Q1_Sales, Q2_Sales, Q3_Sales, Q4_Sales)
);