problem 1 added

problem1-1225-Report-Contiguos-Dates.sql

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+WITH combine_states AS (
+  SELECT
+    fail_date AS `date`,
+    'failed' AS period_state
+  FROM Failed
+  where year(fail_date) = 2019
+
+  UNION ALL
+
+  SELECT
+    success_date AS `date`,
+    'succeeded' AS period_state
+  FROM Succeeded
+  where year(success_date) = 2019
+)
+
+, cte_based_on_state as (
+ SELECT
+    `date`,
+    period_state,
+    CASE
+      WHEN LAG(period_state) OVER (ORDER BY date) = period_state THEN 0
+      ELSE 1
+    END AS state_change_flag
+  FROM combine_states
+  order by `date`
+)
+
+,grouped_rank AS (
+  SELECT
+    `date`,
+    period_state,
+    SUM(state_change_flag) OVER (ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS rnk
+  FROM cte_based_on_state
+)
+
+select
+period_state,
+min(`date`) as start_date,
+max(`date`) as end_date
+from
+grouped_rank
+group by rnk

problem2-618-student-report-geography.sql

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+WITH am_con AS (
+    SELECT
+        NAME AS America,
+        ROW_NUMBER() OVER (ORDER BY name) AS rnk
+    FROM Student
+    WHERE continent = 'America'
+),
+eu_con AS (
+    SELECT
+        NAME AS Europe,
+        ROW_NUMBER() OVER (ORDER BY name) AS rnk
+    FROM Student
+    WHERE continent = 'Europe'
+),
+as_con AS (
+    SELECT
+        NAME AS Asia,
+        ROW_NUMBER() OVER (ORDER BY name) AS rnk
+    FROM Student
+    WHERE continent = 'Asia'
+)
+
+select
+    am_con.America,
+    as_con.Asia,
+    eu_con.Europe
+from
+    am_con
+left join
+    eu_con
+on am_con.rnk = eu_con.rnk
+left join
+    as_con
+on am_con.rnk = as_con.rnk

problem3-615-average-salary-department-vs-company.sql

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+-- window function
+
+with cte as (
+    select
+        e.department_id,
+        date_format(pay_date, '%Y-%m') as pay_month,
+        case
+            when
+                avg(amount) over (partition by date_format(pay_date, '%Y-%m'))
+                > avg(amount) over (partition by e.department_id, date_format(pay_date, '%Y-%m'))
+                then 'lower'
+            when
+                avg(amount) over (partition by date_format(pay_date, '%Y-%m'))
+                < avg(amount) over (partition by e.department_id, date_format(pay_date, '%Y-%m'))
+                then 'higher'
+            else
+                'same'
+        end as 'comparison'
+    from salary as s
+    inner join
+        employee as e
+        on s.employee_id = e.employee_id
+)
+
+select
+    pay_month,
+    department_id,
+    comparison
+from cte
+group by pay_month, department_id, comparison
+order by pay_month, department_id
+
+
+
+--- with aggregation way
+
+with cte_monthly_avg as (
+    select
+        date_format(pay_date, '%Y-%m') as pay_month,
+        avg(amount) as avg_amount
+    from
+        salary
+    group by date_format(pay_date, '%Y-%m')
+
+),
+
+cte_depart_monthly_avg as (
+    select
+        e.department_id,
+        date_format(pay_date, '%Y-%m') as pay_month,
+        avg(amount) as dpt_avg_salary
+    from salary as s
+    inner join
+        employee as e
+        on s.employee_id = e.employee_id
+    group by date_format(pay_date, '%Y-%m'), e.department_id
+
+)
+
+select
+    cdma.pay_month,
+    cdma.department_id,
+    case
+        when cdma.dpt_avg_salary > cma.avg_amount then 'higher'
+        when cdma.dpt_avg_salary < cma.avg_amount then 'lower'
+        else 'same'
+    end as comparison
+from cte_depart_monthly_avg as cdma
+inner join cte_monthly_avg as cma
+    on cdma.pay_month = cma.pay_month

problem4-511-game-play-analysis.sql

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+select
+    player_id,
+    min(event_date) as first_login
+from activity
+group by player_id
+order by player_id