Ad-Free Sessions

Problem

Table: Playback

+-------------+------+
| Column Name | Type |
+-------------+------+
| session_id  | int  |
| customer_id | int  |
| start_time  | int  |
| end_time    | int  |
+-------------+------+
session_id is the column with unique values for this table.
customer_id is the ID of the customer watching this session.
The session runs during the **inclusive** interval between start_time and end_time.
It is guaranteed that start_time <= end_time and that two sessions for the same customer do not intersect.

Table: Ads

+-------------+------+
| Column Name | Type |
+-------------+------+
| ad_id       | int  |
| customer_id | int  |
| timestamp   | int  |
+-------------+------+
ad_id is the column with unique values for this table.
customer_id is the ID of the customer viewing this ad.
timestamp is the moment of time at which the ad was shown.

Write a solution to report all the sessions that did not get shown any ads.

Return the result table in any order.

The result format is in the following example.

Examples

Example 1

Input: 
Playback table:
+------------+-------------+------------+----------+
| session_id | customer_id | start_time | end_time |
+------------+-------------+------------+----------+
| 1          | 1           | 1          | 5        |
| 2          | 1           | 15         | 23       |
| 3          | 2           | 10         | 12       |
| 4          | 2           | 17         | 28       |
| 5          | 2           | 2          | 8        |
+------------+-------------+------------+----------+
Ads table:
+-------+-------------+-----------+
| ad_id | customer_id | timestamp |
+-------+-------------+-----------+
| 1     | 1           | 5         |
| 2     | 2           | 17        |
| 3     | 2           | 20        |
+-------+-------------+-----------+
Output: 
+------------+
| session_id |
+------------+
| 2          |
| 3          |
| 5          |
+------------+
Explanation: 
The ad with ID 1 was shown to user 1 at time 5 while they were in session 1.
The ad with ID 2 was shown to user 2 at time 17 while they were in session 4.
The ad with ID 3 was shown to user 2 at time 20 while they were in session 4.
We can see that sessions 1 and 4 had at least one ad. Sessions 2, 3, and 5 did not have any ads, so we return them.

Solution

Method 1 - Using max

Code

Sql

SELECT user_id,
       MAX(time_stamp) last_stamp
FROM Logins
WHERE YEAR(time_stamp) = '2020'
GROUP BY user_id;