ConfirmationRate.sql

-- Table: Signups

-- +----------------+----------+
-- | Column Name    | Type     |
-- +----------------+----------+
-- | user_id        | int      |
-- | time_stamp     | datetime |
-- +----------------+----------+
-- user_id is the column of unique values for this table.
-- Each row contains information about the signup time for the user with ID user_id.
 

-- Table: Confirmations

-- +----------------+----------+
-- | Column Name    | Type     |
-- +----------------+----------+
-- | user_id        | int      |
-- | time_stamp     | datetime |
-- | action         | ENUM     |
-- +----------------+----------+
-- (user_id, time_stamp) is the primary key (combination of columns with unique values) for this table.
-- user_id is a foreign key (reference column) to the Signups table.
-- action is an ENUM (category) of the type ('confirmed', 'timeout')
-- Each row of this table indicates that the user with ID user_id requested a confirmation message at time_stamp and that confirmation message was either confirmed ('confirmed') or expired without confirming ('timeout').
 

-- The confirmation rate of a user is the number of 'confirmed' messages divided by the total number of requested confirmation messages. The confirmation rate of a user that did not request any confirmation messages is 0. Round the confirmation rate to two decimal places.

-- Write a solution to find the confirmation rate of each user.

-- Return the result table in any order.

-- The result format is in the following example.

 

-- Example 1:

-- Input: 
-- Signups table:
-- +---------+---------------------+
-- | user_id | time_stamp          |
-- +---------+---------------------+
-- | 3       | 2020-03-21 10:16:13 |
-- | 7       | 2020-01-04 13:57:59 |
-- | 2       | 2020-07-29 23:09:44 |
-- | 6       | 2020-12-09 10:39:37 |
-- +---------+---------------------+
-- Confirmations table:
-- +---------+---------------------+-----------+
-- | user_id | time_stamp          | action    |
-- +---------+---------------------+-----------+
-- | 3       | 2021-01-06 03:30:46 | timeout   |
-- | 3       | 2021-07-14 14:00:00 | timeout   |
-- | 7       | 2021-06-12 11:57:29 | confirmed |
-- | 7       | 2021-06-13 12:58:28 | confirmed |
-- | 7       | 2021-06-14 13:59:27 | confirmed |
-- | 2       | 2021-01-22 00:00:00 | confirmed |
-- | 2       | 2021-02-28 23:59:59 | timeout   |
-- +---------+---------------------+-----------+
-- Output: 
-- +---------+-------------------+
-- | user_id | confirmation_rate |
-- +---------+-------------------+
-- | 6       | 0.00              |
-- | 3       | 0.00              |
-- | 7       | 1.00              |
-- | 2       | 0.50              |
-- +---------+-------------------+
-- Explanation: 
-- User 6 did not request any confirmation messages. The confirmation rate is 0.
-- User 3 made 2 requests and both timed out. The confirmation rate is 0.
-- User 7 made 3 requests and all were confirmed. The confirmation rate is 1.
-- User 2 made 2 requests where one was confirmed and the other timed out. The confirmation rate is 1 / 2 = 0.5.

-- Solution:
-- Write your PostgreSQL query statement below

select user_id, max(confirmation_rate) as confirmation_rate from 
(
    select tb1.user_id, round(tb1.cnt1/tb2.cnt2::numeric,2) as confirmation_rate 
    from 
    (
        select user_id,
            case 
            when action='confirmed' 
            then count(user_id)
            when action is null
            then 0
            else 0 end as cnt1 from 
        (
            select Signups.user_id,
            Confirmations.time_stamp,
            Confirmations.action
            from Signups 
                left join Confirmations 
                on (Signups.user_id = Confirmations.user_id)
        )
        group by user_id,action
    ) tb1 left join (
        select user_id, count(user_id) as cnt2 from 
        (
            select Signups.user_id,
            Confirmations.time_stamp,
            Confirmations.action
            from Signups 
                left join Confirmations 
                on (Signups.user_id = Confirmations.user_id)
        )
    group by user_id
    ) tb2 on (
        tb1.user_id = tb2.user_id
    )
) group by user_id;