Combining Rows Based on Time Constraints

In this article, we’ll explore a common problem in data manipulation where rows need to be combined based on specific time constraints.

Problem Statement

Suppose we have a table with three columns: Sr.No, start, and end. The start column represents the start date and time of an activity, while the end column represents the end date and time. We also have another column called Actual_Date which is used to keep track of the actual completion date of each activity.

The task at hand is to combine rows when the end time of one row matches the start time of another row, if not then combine them as they are in SQL.

Example Data

Here’s an example table with some sample data:

SQL Query

We’ve tried the following SQL query to achieve this:

SELECT
    T1.Malf_start,
    T2.MalfEnd,
    T1.COMPARE_DATE
FROM
    #temp T1
INNER JOIN
    #temp T2
    ON T2.DATE_SRNO = T1.DATE_SRNO
    AND T2.Malf_start = T1.MalfEnd

However, the above query doesn’t produce the desired output.

Solution

To solve this problem, we can use a combination of LEFT JOINs and COALESCE function to combine rows based on time constraints. Here’s an example query:

SELECT
    t1.malf_start,
    COALESCE(t2.malf_end, t1.malf_end) AS malf_end,
    t1.actual_date
FROM
    test_date t1
LEFT JOIN
    test_date t2
    ON t1.actual_date = t2.actual_date
    AND t1.malf_end = t2.malf_start
LEFT JOIN
    test_date t3
    ON t1.actual_date = t3.actual_date
    AND t1.malf_start = t3.malf_end
WHERE t3.malf_start IS NULL

Explanation

Let’s break down the query:

• t1 represents the list of all records.
• t2 represents the records where the starting date matches an ending date in t1.
• If there’s no match in t2, we show the original t1 ending date using COALESCE function.
• t3 is to check if any t1 records were used in t2. We don’t want to show the records already attached to t1 records.
• The final WHERE clause filters out the records where t3.malf_start IS NULL, meaning that there’s no matching record in t2.

Result

The query produces the following result:

As expected, the query combines rows based on time constraints.

Conclusion

In this article, we’ve explored a common problem in data manipulation where rows need to be combined based on specific time constraints. We’ve also discussed the importance of using COALESCE function and LEFT JOINs to solve such problems efficiently. By following the steps outlined in this article, you can develop effective solutions for combining rows based on time constraints in your own projects.

Further Reading

For more information on data manipulation and combination techniques, consider the following resources:

• SQL Tutorial
• Data Manipulation with SQL
• COALESCE Function in SQL

By expanding your knowledge of data manipulation techniques, you’ll be better equipped to tackle complex problems and improve your overall data analysis skills.

Last modified on 2024-12-29