由不同用户运行时，SQL查询中的MAX和COALESCE导致奇怪的结果

Question

I have a SQL Server query that tries to choose NULL if there is at least one NULL in column PredComplDate or the latest date stored as NVARCHAR(30)

The weird thing is that when I run the query it returns correct results, but when my co-worker runs the query on the same computer it returns different results.

I saved the table created when my co-worker ran the query under a different name and compared it to the one that was created when I run the query. everything is the same, number of records, data type etc, but the below part of the query does not return NULL for some reason:

SELECT [Reference Number],
    CASE 
        WHEN LEN(Predecessors)>0 AND MAX(COALESCE(PredComplDate,'31/12/2099'))='31/12/2099' THEN NULL
        ELSE MAX(PredComplDate)
    END AS LatestPredComplDate
    FROM mytable33
    GROUP BY [Reference Number],Predecessors

what is the problem with this query?

mytable33 has been created by me and mytable22 has been created by my co-worker. when I query the results are identical:

SELECT Predecessors,[Reference Number], PredRefNo,PredComplDate  FROM mytable22

SELECT Predecessors,[Reference Number], PredRefNo,PredComplDate  FROM mytable33

when I run the above-mentioned query on both tables, the result is different:

SELECT [Reference Number],
    CASE 
        WHEN LEN(Predecessors)>0 AND MAX(COALESCE(PredComplDate,'31/12/2099'))='31/12/2099' THEN NULL
        ELSE MAX(PredComplDate)
    END AS LatestPredComplDate
    FROM mytable22
    GROUP BY [Reference Number],Predecessors

SELECT [Reference Number],
    CASE 
        WHEN LEN(Predecessors)>0 AND MAX(COALESCE(PredComplDate,'31/12/2099'))='31/12/2099' THEN NULL
        ELSE MAX(PredComplDate)
    END AS LatestPredComplDate
    FROM mytable33
    GROUP BY [Reference Number],Predecessors

The expected result is NULL for LastestPredComplDate

Answer 1

也许您可以将两个子句放在窗口函数中，如下所示（如果只是为了清晰起见）：

  MAX(CASE WHEN LEN(Predecessors)>0 THEN NULL ELSE PredComplDate END) AS LatestPredComplDate

Answer 2

Eventually, I found out the problem. The date (stored as NVARCHAR) format was the culprit. If you have another look at the first image that I posted you can see that the same dates have different formats. One is 04/12/2019 and the other is 4/12/2019 . It turns out the MAX function would consider 4/12/2019 larger than 04/12/2019 and that would generate different results.