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+# 1 Problem 1 : The Number of Seniors and Juniors to Join the Company (https://leetcode.com/problems/the-number-of-seniors-and-juniors-to-join-the-company/)
+
+# Solution
+with cte as
+(
+    select employee_id, experience, sum(salary) over(partition by experience order by salary, employee_id) as running_salary
+    from Candidates
+)
+select 'Senior' as experience, count(employee_id) as accepted_candidates 
+from cte
+where experience = 'Senior' and running_salary <= 70000
+union 
+select 'Junior' as experience, count(employee_id) as accepted_candidates 
+from cte
+where experience = 'Junior' and running_salary <= (
+    select 70000 - ifnull(max(running_salary), 0) 
+    from cte 
+    where experience = 'Senior' and running_salary <= 70000)
+
+# 2 Problem 2 : League Statistics (https://leetcode.com/problems/league-statistics/ )
+
+# Solution
+with cte as
+(
+    select home_team_id as r1 , home_team_goals as g1, away_team_goals as g2
+    from Matches 
+    union all
+    select away_team_id as r1, away_team_goals as g1, home_team_goals as g2
+    from Matches 
+)
+select
+t.team_name, 
+count(c.r1) as matches_played, 
+sum(case 
+        when c.g1 > c.g2 then 3
+        when c.g1 = c.g2 then 1
+        else 0
+    end) as points,
+sum(c.g1) as goal_for, 
+sum(c.g2) as goal_against, 
+sum(c.g1) - sum(c.g2) as goal_diff 
+from Teams t
+join cte c
+on t.team_id = c.r1
+group by c.r1
+order by points desc, goal_diff desc, t.team_name
+
+# 3 Problem 3 : Sales Person (https://leetcode.com/problems/sales-person/ )
+
+# Solution
+select name
+from SalesPerson
+where sales_id not in
+(
+    select s.sales_id
+    from SalesPerson s
+    left join Orders o
+    on s.sales_id = o.sales_id
+    left join Company c
+    on c.com_id = o.com_id
+    where c.name = 'Red'
+)
+
+# 4 Problem 4 : Friend Requests II (https://leetcode.com/problems/friend-requests-ii-who-has-the-most-friends/ )
+
+# Solution
+with cte as
+(
+    select requester_id as id, count(requester_id) as cnt
+    from RequestAccepted
+    group by requester_id  
+    union all
+    select accepter_id as id, count(accepter_id) as cnt
+    from RequestAccepted
+    group by accepter_id  
+)
+select id, sum(cnt) as num
+from cte
+group by id
+order by num desc
+limit 1
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