Added four problems

problem1.sql

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+WITH all_dates AS (
+  SELECT fail_date AS date, 'failed' AS state FROM Failed
+  WHERE fail_date BETWEEN '2019-01-01' AND '2019-12-31'
+  UNION ALL
+  SELECT success_date AS date, 'succeeded' AS state FROM Succeeded
+  WHERE success_date BETWEEN '2019-01-01' AND '2019-12-31'
+),
+date_with_rank AS (
+  SELECT 
+    date,
+    state,
+    ROW_NUMBER() OVER (PARTITION BY state ORDER BY date) AS rk
+  FROM all_dates
+),
+grouped AS (
+  SELECT 
+    date,
+    state,
+    DATE_SUB(date, INTERVAL rk DAY) AS group_id
+  FROM date_with_rank
+)
+SELECT 
+  state AS period_state,
+  MIN(date) AS start_date,
+  MAX(date) AS end_date
+FROM grouped
+GROUP BY state, group_id
+ORDER BY start_date;

problem2.sql

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+select min(America) as America, min(Asia) as Asia, min(Europe) as Europe from
+(select
+case when continent= "America" then name else null end as "America",
+case when continent = "Asia" then name else null end as "Asia",
+case when continent = "Europe" then name end as "Europe",
+row_number() over (partition by continent order by name) as rnk
+from student
+) t1
+group by rnk

problem3.sql

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+WITH stats AS (
+  SELECT
+    DATE_FORMAT(s.pay_date, '%Y-%m') AS pay_month,
+    e.department_id,
+    AVG(s.amount) OVER (PARTITION BY DATE_FORMAT(s.pay_date, '%Y-%m')) AS company_avg,
+    AVG(s.amount) OVER (PARTITION BY DATE_FORMAT(s.pay_date, '%Y-%m'), e.department_id) AS dept_avg
+  FROM Salary s
+  JOIN Employee e ON s.employee_id = e.employee_id
+)
+SELECT
+  pay_month,
+  department_id,
+  CASE
+    WHEN dept_avg > company_avg THEN 'higher'
+    WHEN dept_avg < company_avg THEN 'lower'
+    ELSE 'same'
+  END AS comparison
+FROM stats
+GROUP BY pay_month, department_id, dept_avg, company_avg
+ORDER BY pay_month, department_id;

problem4.sql

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+# Write your MySQL query statement below
+-- select player_id,event_date as first_login
+-- from (
+--     select player_id,event_date,
+--     rank() over (partition by player_id order by event_date) as second_login 
+--     from Activity ) t
+-- where second_login=1;
+
+-- select distinct(player_id), min(event_date) over(partition by player_id) as 'first_login'
+-- from activity;
+
+-- select player_id,min(event_date) as 'first_login'
+-- from activity
+-- group by player_id;
+
+select distinct(player_id), first_value(event_date) over(partition by player_id order by event_date) as 'first_login'
+from activity;
+