| 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.
解答：
SELECT s.user_id , ROUND(AVG(IF(c.action = 'confirmed' ,1 ,0)),2) AS confirmation_rate
FROM Signups s
LEFT JOIN Confirmations c
ON s.user_id = c.user_id
GROUP BY s.user_id
知識點：
1.IF(bollen表達,true時的值,false時的值)
2.GROUP BY用c.user_id會沒有6的資料 因為c沒有user_id = 6
