19. Ad-Free Sessions

Grokking SQL for Tech Interviews

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19. Ad-Free Sessions

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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.

Problem Definition

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

Return the result table in any order.

Example

Output

Try It Yourself

MYSQL

. . . .

Solution

To solve this problem, the approach involves using SQL queries to identify sessions that did not display any ads. The data is spread across two tables, Playback and Ads, with the former containing information about customer sessions and the latter containing details about ads viewed by customers during specific timestamps.

The solution employs the WHERE NOT IN clause, utilizing a subquery to identify the session_id values that correspond to sessions where ads were shown. The subquery involves an INNER JOIN between the Playback and Ads tables, where the customer_id in both tables match and the timestamp of the ad falls within the inclusive interval of the session (start_time to end_time). This subquery returns distinct session_id values for sessions with ads.

The main query then selects session_id values from the Playback table where the session_id is not present in the result of the subquery. This effectively identifies sessions that did not have any corresponding ads.

The final result is presented as a table of session IDs that did not show any ads. The order of the result table is not specified, as indicated in the problem statement.

SELECT session_id
FROM   Playback
WHERE  session_id NOT IN (SELECT DISTINCT session_id
                            FROM   Playback P
                                   INNER JOIN Ads A
                                           ON
                                   P.customer_id = A.customer_id
                            WHERE  A.timestamp >= P.start_time
                                   AND A.timestamp <= P.end_time);

Let's break down the query step by step:

Step 1: Identify sessions with ads

We need to find sessions that had at least one ad shown during the playback.

SELECT DISTINCT session_id
FROM   Playback P
       INNER JOIN Ads A
       ON P.customer_id = A.customer_id
WHERE  A.timestamp >= P.start_time
       AND A.timestamp <= P.end_time;

Output After Step 1:

+------------+
| session_id |
+------------+
| 1          |
| 4          |
+------------+

Step 2: Exclude sessions with ads

We use NOT IN to exclude sessions with ads from the Playback table.

SELECT session_id
FROM   Playback
WHERE  session_id NOT IN (SELECT DISTINCT session_id
                            FROM   Playback P
                                   INNER JOIN Ads A
                                           ON
                                   P.customer_id = A.customer_id
                            WHERE  A.timestamp >= P.start_time
                                   AND A.timestamp <= P.end_time);

Final Output:

+------------+
| session_id |
+------------+
| 2          |
| 3          |
| 5          |
+------------+

.....

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