Subquerying and Joining: Retrieving Recent Mobile Requests with Specific Conditions

Introduction

As a database professional, it’s not uncommon to encounter complex queries that involve joining multiple tables and applying various conditions. In this article, we’ll delve into a specific problem involving two tables: MobileRequest and MobileRequestAnswers. We’ll explore how to use subqueries and joins to retrieve recent mobile requests with certain conditions.

The Problem

The problem at hand involves retrieving the most recent mobile requests for each job number that do not have question ID 4 in the set of records from MobileRequestAnswers. In other words, we want to get the latest mobile request IDs for jobs that don’t have any answers associated with question ID 4.

Let’s take a closer look at the tables involved:

MobileRequest Table

MobileRequestAnswers Table

Solution

The solution involves using a subquery with a WITH clause to identify jobs that have no answers associated with question ID 4. We’ll then join this subquery with the original MobileRequest table to get the most recent mobile requests for these jobs.

Subquery: Identifying Jobs with No Question ID 4

WITH
    jobs_checked_question_4 AS
(
    SELECT
        mr.*,
        MAX(
            CASE WHEN mra.QuestionID = 4 THEN 1 ELSE 0 END
           )
               OVER (PARTITION BY mr.job_number)   AS HasQuestionID4,
        RANK() OVER (PARTITION BY mr.job_number
                         ORDER BY mr.CreatedOn )   AS RequestRank
    FROM
        mobile_request            AS mr
    LEFT JOIN
        mobile_request_answer     AS mra
            ON  mra.MobileRequestID = mr.MobileRequestID
)
SELECT
    *
FROM
    jobs_checked_question_4
WHERE
        HasQuestionID4 = 0     -- The job never associates to a QuestionID 4
    AND RequestRank    = 1     -- The most recent Request for each Job

This subquery works as follows:

• We use a WITH clause to define the subquery.
• We select all columns from the mobile_request table and perform two types of aggregations: one for checking if there are any answers associated with question ID 4 (HasQuestionID4) and another for ranking the requests by their creation date (RequestRank).
• We join the mobile_request_answer table to the mobile_request table on the foreign key relationship.
• We use a left join because we want to include all records from the mobile_request table, even if there are no matching records in the mobile_request_answer table.
• In the outer query, we select all columns from the subquery and filter the results to only include rows where HasQuestionID4 is 0 (i.e., jobs with no answers associated with question ID 4) and RequestRank is 1 (the most recent request for each job).

Joining with Original Mobile Request Table

SELECT
    mr.*
FROM
    (
        SELECT
            *
        FROM
            jobs_checked_question_4
        WHERE
            HasQuestionID4 = 0     -- The job never associates to a QuestionID 4
            AND RequestRank    = 1     -- The most recent Request for each Job
    ) AS subquery
JOIN
    mobile_request AS mr
ON
    mr.ID = subquery.MobileRequestID;

In this final step, we join the filtered results from the subquery with the original mobile_request table on the foreign key relationship. This allows us to retrieve all columns from both tables.

Conclusion

Retrieving recent mobile requests with specific conditions requires careful planning and execution of a well-crafted query. By using a subquery with aggregations and joins, we can efficiently identify jobs that meet our criteria and retrieve the most recent mobile requests for those jobs. This approach enables us to effectively manage complex data relationships in databases.

Example Use Cases

• Identifying high-priority tasks based on creation date and assignment status
• Retrieving top-performing employees based on sales performance and review ratings
• Analyzing website traffic patterns by region and device type

Last modified on 2024-07-02