Transposing Rows into Columns Based on a Range in T-SQL

This article explains how to transpose rows into columns based on a range using T-SQL. We’ll go through the process step-by-step, starting with understanding the problem and its requirements.

Problem Statement

Suppose you have a table Members containing a list of member IDs. You want to create a query that returns two columns, showing the start and end ID of each range from the list.

For example, if your Members table looks like this:

The expected output would be:

Understanding the Problem

To solve this problem, we need to understand how to transpose rows into columns based on a range. The key concept here is to use a temporary table or Common Table Expression (CTE) to calculate the GroupNumber for each row.

The GroupNumber is calculated by subtracting the RowNumber from the MemberID. This ensures that consecutive rows with different MemberIDs will have different GroupNumbers, while consecutive rows with the same MemberID will have the same GroupNumber.

Solution Overview

To solve this problem, we’ll use a combination of T-SQL features, including:

• Common Table Expressions (CTEs)
• Window functions (ROW_NUMBER)
• Grouping and aggregation

We’ll break down the solution into several steps.

Step 1: Calculate Row Numbers

First, we need to assign row numbers to each record in the Members table. We can use the ROW_NUMBER() window function to achieve this.

declare @Members table (MemberID bigint);
insert @Members values (123), (124), (125), (211), (212), (214), (320), (321), (322);

with RowNumberedMembers as
(
    select
        MemberID,
        RowNumber = ROW_NUMBER() OVER (ORDER BY MemberID)
    from
        @Members
)

In this step, we create a temporary table RowNumberedMembers that contains the original MemberID column and a new RowNumber column.

Step 2: Calculate Group Numbers

Next, we calculate the GroupNumber for each record by subtracting the RowNumber from the MemberID.

with RowNumberedMembers as
(
    select
        MemberID,
        RowNumber = ROW_NUMBER() OVER (ORDER BY MemberID)
    from
        @Members
),
GroupedMembers as
(
    select
        M.MemberID,
        GroupNumber = M.MemberID - RM.RowNumber
    from
        @Members M
    join
        RowNumberedMembers RM on M.MemberID = RM.MemberID
)

In this step, we create a new CTE GroupedMembers that joins the original @Members table with the RowNumberedMembers CTE. We calculate the GroupNumber for each record by subtracting the RowNumber from the MemberID.

Step 3: Transpose Rows into Columns

Finally, we transpose the rows into columns using grouping and aggregation.

with RowNumberedMembers as
(
    select
        MemberID,
        RowNumber = ROW_NUMBER() OVER (ORDER BY MemberID)
    from
        @Members
),
GroupedMembers as
(
    select
        M.MemberID,
        GroupNumber = M.MemberID - RM.RowNumber
    from
        @Members M
    join
        RowNumberedMembers RM on M.MemberID = RM.MemberID
),
FinalResult as
(
    select
        FromID = min(G.GroupNumber),
        ToID = max(G.GroupNumber)
    from
        GroupedMembers G
    group by
        G.GroupNumber
)

In this step, we create a new CTE FinalResult that groups the records in the GroupedMembers CTE based on the GroupNumber. We use the minimum and maximum values of the GroupNumber to get the start and end IDs of each range.

Conclusion

Transposing rows into columns based on a range is a common problem in data analysis and data science. By using T-SQL features like Common Table Expressions, window functions, and grouping, we can solve this problem efficiently and effectively.

In this article, we walked through the solution step-by-step, explaining each part of the process and providing example code to illustrate the concepts.

Last modified on 2024-07-26