Understanding the Problem

The question asks how to join two tables, table_1 and table_2, based on common columns (user_id) while ensuring that only one row from each table is selected for each unique combination of date and user_id. The goal is to obtain a single most recent row for each group.

Choosing the Join Type

To achieve this, we can use an inner join with additional filtering based on ranking functions. We’ll start by joining the two tables on their common columns (user_id) and then apply the ranking function to determine which row should be selected for each combination of date and user_id.

Using Ranking Functions

There are several ranking functions available in SQL, including:

• row_number(): assigns a unique number to each row within a partition.
• dense_rank(): assigns a rank to each row within a partition, with gaps only between rows with the same value.

We’ll use row_number() as it ensures that we get exactly one result per input and avoids gaps in ranking.

Joining Tables and Applying Ranking Function

To perform the join, we can follow these steps:

1. Join the two tables on their common columns (user_id).
2. Apply a condition to ensure that only rows with date greater than the corresponding row in table_2 are selected.
3. Use the row_number() function to assign a unique number to each row within the partition of date and user_id.
4. Filter the results to select only the row with row_number() = 1, which corresponds to the most recent row for each group.

SQL Code

Here’s the complete SQL code that performs the described steps:

SELECT t1.date, t1.user_id, t1.purchase, t2.cumulative_items
FROM table_1 as t1
JOIN table_2 as t2 ON t1.user_id = t2.user_id
WHERE t1.date > t2.date
QUALIFY row_number() OVER(PARTITION BY t1.date, t1.user_id ORDER BY t2.date DESC) = 1;

Explanation

This code performs the following steps:

• Joins table_1 and table_2 on their common column (user_id).
• Filters rows to include only those where the date in table_1 is greater than the corresponding row in table_2.
• Applies the row_number() function to assign a unique number to each row within the partition of date and user_id, ordered by the date in descending order.
• Filters the results to select only the row with row_number() = 1, which corresponds to the most recent row for each group.

Final Result

The final result will be a table with columns (date, user_id, purchase, and cumulative_items) containing the desired data, where each row represents the most recent record for each combination of date and user_id.

Example Output

Here’s an example output based on the provided sample data:

This output shows the most recent records for each combination of date and user_id, as calculated using the ranking function.

Last modified on 2024-04-26