Add Confirmation Rate Leetcode SQL problem

LeetCode/SQL50/ConfirmationRate.sql

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+-- 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;