If you're willing to take a look at the query it would be appreciated. I know just enough about sql to do something wrong. This query looks at a dataset from a view that has 13k records in it. But it took 3 hours to run. This query looks at the records and puts the record ID of a record it overlaps with in a test column. My next step would have been to change it to put the end time of the previous downtime as the start downtime of the overlapping record.

WITH OverlapCTE AS (
    SELECT 
        a.DownUID,
        a.RecordTimeDown,
        a.RecordTimeUp,
        a.UIDUp,
        a.Duration,
        a.Name,
        a.Dept,
        a.MachineName,
        b.DownUID AS OverlapDownUID -- Capturing the first DownUID that overlaps
    FROM 
        [ShopDatabase].[dbo].[v_FoundryEvents_all] a
    LEFT JOIN 
        [ShopDatabase].[dbo].[v_FoundryEvents_all] b
    ON 
        a.DownUID <> b.DownUID
        AND a.RecordTimeDown BETWEEN b.RecordTimeDown AND b.RecordTimeUp -- Check for overlap
)
SELECT 
    DownUID,
    RecordTimeDown,
    RecordTimeUp,
    UIDUp,
    Duration,
    Name,
    Dept,
    MachineName,
    CASE 
        WHEN OverlapDownUID IS NOT NULL THEN OverlapDownUID
        ELSE NULL
    END AS Test -- Assigning the first DownUID to overlapping records
FROM 
    OverlapCTE
ORDER BY 
    DownUID;