Sql3 Solutions

consecutive-numbers.sql

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+"""
++-------------+---------+
+| Column Name | Type    |
++-------------+---------+
+| id          | int     |
+| num         | varchar |
++-------------+---------+
+
+In SQL, id is the primary key for this table.
+id is an autoincrement column starting from 1.
+Find all numbers that appear at least three times consecutively.
+
+Return the result table in any order.
+
+The result format is in the following example.
+
+Input: 
+Logs table:
++----+-----+
+| id | num |
++----+-----+
+| 1  | 1   |
+| 2  | 1   |
+| 3  | 1   |
+| 4  | 2   |
+| 5  | 1   |
+| 6  | 2   |
+| 7  | 2   |
++----+-----+
+Output: 
++-----------------+
+| ConsecutiveNums |
++-----------------+
+| 1               |
++-----------------+
+
+"""
+
+-- method 1 : using LEAD()
+
+with cte as
+    (
+        select 
+            num, 
+            lead(num, 1) over() num1, 
+            lead(num, 2) over() num2 
+        from 
+            logs
+    )
+
+select 
+    distinct num as ConsecutiveNums 
+from 
+    cte 
+where 
+    num = num1 and num = num2
+
+
+-- method 2: using DISTINCT and WHERE
+
+select 
+    distinct l1.num as ConsecutiveNums
+from 
+    logs l1, logs l2, logs l3
+where 
+    l1.id = l2.id - 1
+    and 
+    l2.id = l3.id - 1
+    and 
+    l1.num = l2.num 
+    and 
+    l1.num = l3.num

the-number-of-passengers-in-each-bus-i.sql

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+"""
+Table: Buses
++--------------+------+
+| Column Name  | Type |
++--------------+------+
+| bus_id       | int  |
+| arrival_time | int  |
++--------------+------+
+bus_id is the column with unique values for this table.
+Each row of this table contains information about the arrival time of a bus at the LeetCode station.
+No two buses will arrive at the same time.
+
+Table: Passengers
++--------------+------+
+| Column Name  | Type |
++--------------+------+
+| passenger_id | int  |
+| arrival_time | int  |
++--------------+------+
+passenger_id is the column with unique values for this table.
+Each row of this table contains information about the arrival time of a passenger at the LeetCode station.
+
+Buses and passengers arrive at the LeetCode station. 
+If a bus arrives at the station at time tbus and a passenger arrived at time tpassenger where tpassenger <= tbus and the passenger did not catch any bus, the passenger will use that bus.
+
+Write a solution to report the number of users that used each bus.
+
+Return the result table ordered by bus_id in ascending order.
+
+The result format is in the following example.
+
+Input: 
+Buses table:
++--------+--------------+
+| bus_id | arrival_time |
++--------+--------------+
+| 1      | 2            |
+| 2      | 4            |
+| 3      | 7            |
++--------+--------------+
+Passengers table:
++--------------+--------------+
+| passenger_id | arrival_time |
++--------------+--------------+
+| 11           | 1            |
+| 12           | 5            |
+| 13           | 6            |
+| 14           | 7            |
++--------------+--------------+
+Output: 
++--------+----------------+
+| bus_id | passengers_cnt |
++--------+----------------+
+| 1      | 1              |
+| 2      | 0              |
+| 3      | 3              |
++--------+----------------+
+"""
+
+-- method 1 : using LEFT JOIN and GROUP BY
+
+select 
+    b.bus_id, 
+    count(used_bus.t) as passengers_cnt 
+from 
+    buses b 
+    left join
+    (
+        select 
+            p.passenger_id, min(b.arrival_time) as t 
+        from 
+            Passengers p join buses b
+            on 
+            p.arrival_time <= b.arrival_time
+        group by 
+            p.passenger_id
+    ) as used_bus
+
+    on 
+        b.arrival_time = used_bus.t
+    group by
+        b.bus_id
+    order by 
+        b.bus_id

user-activity-for-the-past-30-days-i.sql

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+"""
+Table: Activity
++---------------+---------+
+| Column Name   | Type    |
++---------------+---------+
+| user_id       | int     |
+| session_id    | int     |
+| activity_date | date    |
+| activity_type | enum    |
++---------------+---------+
+This table may have duplicate rows.
+The activity_type column is an ENUM (category) of type ('open_session', 'end_session', 'scroll_down', 'send_message').
+The table shows the user activities for a social media website. 
+Note that each session belongs to exactly one user.
+
+Write a solution to find the daily active user count for a period of 30 days ending 2019-07-27 inclusively. 
+A user was active on someday if they made at least one activity on that day.
+
+Return the result table in any order.
+
+The result format is in the following example.
+
+Input: 
+Activity table:
++---------+------------+---------------+---------------+
+| user_id | session_id | activity_date | activity_type |
++---------+------------+---------------+---------------+
+| 1       | 1          | 2019-07-20    | open_session  |
+| 1       | 1          | 2019-07-20    | scroll_down   |
+| 1       | 1          | 2019-07-20    | end_session   |
+| 2       | 4          | 2019-07-20    | open_session  |
+| 2       | 4          | 2019-07-21    | send_message  |
+| 2       | 4          | 2019-07-21    | end_session   |
+| 3       | 2          | 2019-07-21    | open_session  |
+| 3       | 2          | 2019-07-21    | send_message  |
+| 3       | 2          | 2019-07-21    | end_session   |
+| 4       | 3          | 2019-06-25    | open_session  |
+| 4       | 3          | 2019-06-25    | end_session   |
++---------+------------+---------------+---------------+
+Output: 
++------------+--------------+ 
+| day        | active_users |
++------------+--------------+ 
+| 2019-07-20 | 2            |
+| 2019-07-21 | 2            |
++------------+--------------+
+
+"""
+
+-- method 1: using DATEDIFF and COUNT
+
+select 
+    activity_date as 'day',
+    count(distinct user_id) as active_users
+from 
+    Activity
+where 
+    datediff('2019-07-27', activity_date) between 0 and 29
+group by 
+    activity_date