How Can I Detect and Bound Changes Between Row Values in a SQL Table?

Question

I have a table which records values over time, similar to the following:

RecordId  Time   Name
========================
1         10     Running
2         18         


        

Answer 1

I think you're essentially interested in where the 'Name' changes from one record to the next (in order of 'Time'). If you can identify where this happens you can generate your desired output.

Since you mentioned CTEs I'm going to assume you're on SQL Server 2005+ and can therefore use the ROW_NUMBER() function. You can use ROW_NUMBER() as a handy way to identify consecutive pairs of records and then to find those where the 'Name' changes.

How about this:

WITH OrderedTable AS
(
    SELECT
        *,
        ROW_NUMBER() OVER (ORDER BY Time) AS Ordinal
    FROM
        [YourTable]
),
NameChange AS
(
    SELECT
        after.Time AS Time,
        after.Name AS Name,
        ROW_NUMBER() OVER (ORDER BY after.Time) AS Ordinal
    FROM
        OrderedTable before
        RIGHT JOIN OrderedTable after ON after.Ordinal = before.Ordinal + 1
    WHERE
        ISNULL(before.Name, '') <> after.Name
)

SELECT
    before.Time AS FromTime,
    after.Time AS ToTime,
    before.Name
FROM
    NameChange before
    LEFT JOIN NameChange after ON after.Ordinal = before.Ordinal + 1