1661-AverageTimeOfProcessPerMachine.sql

-- 1661. Average Time of Process per Machine
-- Table: Activity
-- +----------------+---------+
-- | Column Name    | Type    |
-- +----------------+---------+
-- | machine_id     | int     |
-- | process_id     | int     |
-- | activity_type  | enum    |
-- | timestamp      | float   |
-- +----------------+---------+

-- The table shows the user activities for a factory website.
-- (machine_id, process_id, activity_type) is the primary key (combination of columns with unique values) of this table.
-- machine_id is the ID of a machine.
-- process_id is the ID of a process running on the machine with ID machine_id.
-- activity_type is an ENUM (category) of type ('start', 'end').
-- timestamp is a float representing the current time in seconds.
-- 'start' means the machine starts the process at the given timestamp and 'end' means the machine ends the process at the given timestamp.
-- The 'start' timestamp will always be before the 'end' timestamp for every (machine_id, process_id) pair.
 
-- There is a factory website that has several machines each running the same number of processes. Write a solution to find the average time each machine takes to complete a process.
-- The time to complete a process is the 'end' timestamp minus the 'start' timestamp. The average time is calculated by the total time to complete every process on the machine divided by the number of processes that were run.
-- The resulting table should have the machine_id along with the average time as processing_time, which should be rounded to 3 decimal places.
-- Return the result table in any order.

-- The result format is in the following example.

-- Example 1:

-- Input: 
-- Activity table:
-- +------------+------------+---------------+-----------+
-- | machine_id | process_id | activity_type | timestamp |
-- +------------+------------+---------------+-----------+
-- | 0          | 0          | start         | 0.712     |
-- | 0          | 0          | end           | 1.520     |
-- | 0          | 1          | start         | 3.140     |
-- | 0          | 1          | end           | 4.120     |
-- | 1          | 0          | start         | 0.550     |
-- | 1          | 0          | end           | 1.550     |
-- | 1          | 1          | start         | 0.430     |
-- | 1          | 1          | end           | 1.420     |
-- | 2          | 0          | start         | 4.100     |
-- | 2          | 0          | end           | 4.512     |
-- | 2          | 1          | start         | 2.500     |
-- | 2          | 1          | end           | 5.000     |
-- +------------+------------+---------------+-----------+
-- Output: 
-- +------------+-----------------+
-- | machine_id | processing_time |
-- +------------+-----------------+
-- | 0          | 0.894           |
-- | 1          | 0.995           |
-- | 2          | 1.456           |
-- +------------+-----------------+
-- Explanation: 
-- There are 3 machines running 2 processes each.
-- Machine 0's average time is ((1.520 - 0.712) + (4.120 - 3.140)) / 2 = 0.894
-- Machine 1's average time is ((1.550 - 0.550) + (1.420 - 0.430)) / 2 = 0.995
-- Machine 2's average time is ((4.512 - 4.100) + (5.000 - 2.500)) / 2 = 1.456

-- Create table If Not Exists Activity (machine_id int, process_id int, activity_type ENUM('start', 'end'), timestamp float)
-- Truncate table Activity
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('0', '0', 'start', '0.712')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('0', '0', 'end', '1.52')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('0', '1', 'start', '3.14')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('0', '1', 'end', '4.12')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('1', '0', 'start', '0.55')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('1', '0', 'end', '1.55')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('1', '1', 'start', '0.43')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('1', '1', 'end', '1.42')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('2', '0', 'start', '4.1')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('2', '0', 'end', '4.512')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('2', '1', 'start', '2.5')
-- insert into Activity (machine_id, process_id, activity_type, timestamp) values ('2', '1', 'end', '5')

-- Write your MySQL query statement below
SELECT 
    a.machine_id,
    ROUND(
        (
            SELECT 
                avg(a1.timestamp) 
            FROM 
                Activity a1 
            WHERE 
                a1.activity_type = 'end' AND a1.machine_id = a.machine_id
        )
        - 
        (
            SELECT 
                avg(a1.timestamp) 
            FROM 
                Activity a1 
            WHERE 
                a1.activity_type = 'start' AND a1.machine_id = a.machine_id)
    ,3) AS processing_time
FROM 
    Activity a
GROUP BY 
    a.machine_id

-- best solution
SELECT 
    a1.machine_id, 
    round(avg(a2.TIMESTAMP - a1.TIMESTAMP ), 3) AS processing_time
FROM 
    activity a1, activity a2
WHERE 
    a1.machine_id = a2.machine_id AND 
    a1.process_id = a2.process_id AND 
    a1.activity_type = 'start' AND 
    a2.activity_type = 'end'
GROUP BY 
    a1.machine_id;