Merge pull request #5 from gymgle/master

add database sql 50 solutions

Author: stackcats

database/easy/percentage_of_users_attended_a_contest.sql

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+--https://leetcode.com/problems/percentage-of-users-attended-a-contest/description/
+
+# Write your MySQL query statement below
+SELECT
+    r.contest_id,
+    ROUND(COUNT(r.user_id) * 100.0 / (SELECT COUNT(*) FROM Users), 2) AS percentage
+FROM
+    Register r
+GROUP BY
+    r.contest_id
+ORDER BY
+    percentage DESC,
+    r.contest_id ASC;

database/medium/game_play_analysis_iv.sql

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+--https://leetcode.com/problems/game-play-analysis-iv/description/
+
+# Write your MySQL query statement below
+SELECT
+  ROUND(COUNT(DISTINCT player_id) / (SELECT COUNT(DISTINCT player_id) FROM Activity), 2) AS fraction
+FROM
+  Activity
+WHERE
+  (player_id, DATE_SUB(event_date, INTERVAL 1 DAY))
+  IN (
+    SELECT player_id, MIN(event_date) AS first_login FROM Activity GROUP BY player_id
+  )

database/medium/immediate_food_delivery_ii.sql

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+--https://leetcode.com/problems/immediate-food-delivery-ii/description
+
+# Write your MySQL query statement below
+SELECT ROUND(100 * SUM(
+    CASE
+        WHEN b.min_order_date = b.min_delivery_date THEN 1
+        ELSE 0
+    END
+) / COUNT(*), 2) AS immediate_percentage
+FROM (
+    SELECT
+        MIN(order_date) AS min_order_date,
+        MIN(customer_pref_delivery_date) AS min_delivery_date
+    FROM delivery
+    GROUP BY customer_id
+) b;

database/medium/monthly_transactions_i.sql

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+--https://leetcode.com/problems/monthly-transactions-i/description/
+
+# Write your MySQL query statement below
+SELECT
+    SUBSTR(trans_date,1,7) AS month,
+    country,
+    COUNT(*) AS trans_count,
+    COUNT(CASE WHEN state = 'approved' THEN 1 END) AS approved_count,
+    SUM(amount) AS trans_total_amount,
+    SUM(CASE WHEN state = 'approved' THEN amount else 0 END) AS approved_total_amount
+FROM
+    Transactions
+GROUP BY
+    month,
+    country;