Select info from previous row in SQL Server

My table mixes (the interesting cols):

How do I get the previous article of ordernr based on start_date. I have tried LAG and that works fine until I include the current ordernr (say 10004) then it just return NULL.

This works, I get a list with the correct previous article and ordernr.

SELECT TOP (1) 
    article, ordernr,
    LAG(ordernr) OVER (ORDER BY start_date) AS prev_ordernr,
    LAG(article) OVER (ORDER BY start_date) AS prev_article
FROM 
    mixes
ORDER BY 
    start_date DESC

But then I want a specific row, and add: where ordernr = '10004', it just returns NULL on in the prev_article and prev_ordernr.

How do I solve this?

You could outer join the previous values.

DATA USED

IF(OBJECT_ID('tempdb..#tmp_mixes') is not null)
    DROP TABLE #tmp_mixes

CREATE TABLE #tmp_mixes (id int, article char(1), ordenr int, start_date datetime)

INSERT INTO #tmp_mixes
VALUES
(1, 'a', 10001, '2024-01-01'),
(2, 'b', 10002, '2024-01-03'),
(3, 'c', 10004, '2024-01-02'),
(4, 'a', 10003, null)

Something like:

SELECT
    m.article,
    m.ordenr,
    m.start_date,
    prev.ordenr AS prev_ordernr,
    prev.article AS prev_article
FROM #tmp_mixes m
OUTER APPLY (
    SELECT TOP 1
        ordenr, article
    FROM #tmp_mixes prev 
    WHERE prev.start_date < m.start_date
    order by m.start_date desc
    ) prev
WHERE m.ordenr = 10004
order by m.start_date desc

Results