Calculate the difference between the total sales in the East superstore and the total sales in the West superstore.
East superstore refers to Region = 'East'; West superstore refers to Region = 'West'; difference = subtract(sum(Sales) when Region = 'East', sum(Sales) when Region = 'West')
SELECT SUM(T1.Sales) - SUM(T2.Sales) AS difference FROM east_superstore AS T1 INNER JOIN west_superstore AS T2 ON T1.`Customer ID` = T2.`Customer ID`