Formatted question description: https://leetcode.ca/all/1809.html

Level

Easy

Description

Table: Playback

+-------------+------+
| Column Name | Type |
+-------------+------+
| session_id  | int  |
| customer_id | int  |
| start_time  | int  |
| end_time    | int  |
+-------------+------+
session_id is the primary key 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 primary key 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 an SQL query to report all the sessions that did not get shown any ads.

Return the result table in any order.

The query result format is in the following example:

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           | 15        |
| 3     | 2           | 20        |
+-------+-------------+-----------+

Result table:
+------------+
| session_id |
+------------+
| 2          |
| 3          |
| 5          |
+------------+
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 15 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

First get the sessions that got shown any ads. This can be done by joining tables Playback and Ads using customer_id and selecting the sessions where timestamp is between start_time and end_time. Then select the sessions not in the previous selection to get the ad-free sessions.

# Write your MySQL query statement below
select session_id from Playback where session_id not in (
    select distinct session_id from Playback join Ads on Playback.customer_id = Ads.customer_id
    where timestamp between start_time and end_time);

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