feat:problem 1 and problem 2 solution

problem1-1407-Top-Travellers.sql

@@ -0,0 +1,10 @@
+select
+    u.name,
+    IFNULL(sum(r.distance),0) as travelled_distance
+from
+Users u
+left join
+Rides r
+on u.id = r.user_id
+group by r.user_id
+order by travelled_distance desc,name

problem2-1445-Apples-Oranges.sql

@@ -0,0 +1,30 @@
+--self join way
+
+select
+    s.sale_date,
+    s.sold_num - s1.sold_num AS diff
+from
+Sales s
+join
+Sales s1
+on s.sale_date = s1.sale_date
+and s.fruit != s1.fruit
+and s.fruit = 'apples' and s1.fruit = 'oranges'
+
+-- group by
+SELECT
+    sale_date,
+    SUM(CASE WHEN fruit = 'apples' THEN sold_num ELSE 0 END) -
+    SUM(CASE WHEN fruit = 'oranges' THEN sold_num ELSE 0 END) AS diff
+FROM Sales
+GROUP BY sale_date
+ORDER BY sale_date;
+
+--window function
+
+SELECT DISTINCT
+    sale_date,
+    SUM(CASE WHEN fruit = 'apples' THEN sold_num ELSE 0 END) OVER (PARTITION BY sale_date) -
+    SUM(CASE WHEN fruit = 'oranges' THEN sold_num ELSE 0 END) OVER (PARTITION BY sale_date) AS diff
+FROM Sales
+ORDER BY sale_date;