submitting solution of problem 4

03-AvgSalaryDepartmentVsCompany.sql

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+-- 3 Problem 3 : Average Salary Department vs Company (https://leetcode.com/problems/average-salary-departments-vs-company/solution/ )
+
+with companyavg as (
+  select 
+    date_format(pay_date, '%y-%m') as pay_month, 
+    avg(amount) as companyavg 
+  from salary 
+  group by pay_month
+),
+deptavg as (
+  select 
+    date_format(pay_date, '%y-%m') as pay_month, 
+    department_id, 
+    avg(amount) as departmentavg 
+  from salary 
+  join employee on salary.employee_id = employee.employee_id 
+  group by department_id, pay_month
+)
+select 
+  deptavg.pay_month, 
+  department_id, 
+  case
+    when departmentavg > companyavg then 'higher'
+    when departmentavg < companyavg then 'lower'
+    else 'same'
+  end as comparison
+from companyavg
+join deptavg on deptavg.pay_month = companyavg.pay_month

04-GamePlayAnalysis-I.sql

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+-- Problem 4 : Game Play Analysis I	(https://leetcode.com/problems/game-play-analysis-i/ )
+
+select player_id, min(event_date) as 'first_login' from activity group by player_id 
+-- since this is group by it will return agg value as in one min 
+
+select distinct player_id, min(event_date) over (partition by player_id) as 'first_login' from activity
+-- for partition by one needs distinct because partition by returns all the rows, it will return 2 rows of player id 1 with same min date
+
+select distinct player_id, first_value(event_date) over (partition by player_id order by event_date) 
+as 'first_login' from activity
+-- since we are using first_value, order by event date is important to pick 1st row
+
+select a.player_id, a.event_date as 'first_login' from (
+    select b.player_id, b.event_date, 
+    rank() over (partition by b.player_id order by b.event_date) as 'rnk' from activity b
+) a where a.rnk = 1