Is Second Day Verification easy or hard?

Second Day Verification is classified as an Easy database problem. The main concept is understanding how to join two tables and apply a date difference filter to detect events that occur exactly one day after another event.

Second Day Verification Python/Java solution

This is a database problem, so the primary solution is written in SQL (MySQL). In real applications, Python or Java would execute the SQL query through a database connector, but the core logic remains the JOIN and date comparison in SQL.

How to solve Second Day Verification in O(n)?

Use a direct JOIN between the two tables and filter with a date comparison such as verification_date = DATE_ADD(signup_date, INTERVAL 1 DAY). With proper indexing on the join key and date columns, the database engine can process the join close to linear scan behavior.

What is the best approach for Second Day Verification?

The most effective solution uses a SQL JOIN between the signup table and the verification table. Join the tables on the user identifier and filter rows where the verification date equals the signup date plus one day. This approach lets the database optimizer use indexes and avoids repeated subquery execution.

Is Second Day Verification asked at Google/Amazon/Meta?

Database join and event-timeline questions like this appear frequently in SQL interview rounds at large companies. Variations of this pattern are used in data engineering and analytics interviews at companies such as Amazon, Meta, and Google.

What data structure is used in Second Day Verification?

The problem relies on relational database tables and SQL JOIN operations rather than traditional in-memory data structures. Internally, the database engine may use indexed lookups or hash joins to match rows efficiently.

What is the time complexity of Second Day Verification?

The logical complexity of the SQL solution is typically O(n log n) when indexes are used for joins or lookups. Without indexes, the database may perform full scans that behave closer to O(n^2) in worst cases. Proper indexing on user_id and date columns keeps the query efficient.

3172. Second Day Verification - Practice Online

Table: emails

+-------------+----------+
| Column Name | Type     | 
+-------------+----------+
| email_id    | int      |
| user_id     | int      |
| signup_date | datetime |
+-------------+----------+
(email_id, user_id) is the primary key (combination of columns with unique values) for this table.
Each row of this table contains the email ID, user ID, and signup date.

Table: texts

+---------------+----------+
| Column Name   | Type     | 
+---------------+----------+
| text_id       | int      |
| email_id      | int      |
| signup_action | enum     |
| action_date   | datetime |
+---------------+----------+
(text_id, email_id) is the primary key (combination of columns with unique values) for this table. 
signup_action is an enum type of ('Verified', 'Not Verified'). 
Each row of this table contains the text ID, email ID, signup action, and action date.

Write a Solution to find the user IDs of those who verified their sign-up on the second day.

Return the result table ordered by user_id in ascending order.

The result format is in the following example.

Example:

Input:

emails table:

+----------+---------+---------------------+
| email_id | user_id | signup_date         |
+----------+---------+---------------------+
| 125      | 7771    | 2022-06-14 09:30:00|
| 433      | 1052    | 2022-07-09 08:15:00|
| 234      | 7005    | 2022-08-20 10:00:00|
+----------+---------+---------------------+

texts table:

+---------+----------+--------------+---------------------+
| text_id | email_id | signup_action| action_date         |
+---------+----------+--------------+---------------------+
| 1       | 125      | Verified     | 2022-06-15 08:30:00|
| 2       | 433      | Not Verified | 2022-07-10 10:45:00|
| 4       | 234      | Verified     | 2022-08-21 09:30:00|
+---------+----------+--------------+---------------------+

Output:

+---------+
| user_id |
+---------+
| 7005    |
| 7771    |
+---------+

Explanation:

User with user_id 7005 and email_id 234 signed up on 2022-08-20 10:00:00 and verified on second day of the signup.
User with user_id 7771 and email_id 125 signed up on 2022-06-14 09:30:00 and verified on second day of the signup.