MS Access / SQL子查询的语法，包括聚合函数

Question

I am trying to produce a database to manage maintenance of equipment. I have two tables:

1. One ( Inventory ) containing details of each piece of equipment, including Purchase Date and Service Period ,
2. One containing details of work done ( WorkDone ), including the date the work was carried out ( Work Date ).

I would like a query that displays the date that it should be next serviced. So far I have:

SELECT Max(DateAdd('m', [Inventory].[Service Period], 
                        [WorkDone].[Work Date])) AS NextServiceDate, 
       Inventory.Equipement
FROM Inventory INNER JOIN WorkDone ON Inventory.ID = WorkDone.Equipment
GROUP BY Inventory.Equipement

This works well as long as some work done has been registered for a given piece of equipment. If no work has been carried out I would like the NextServiceDat e to also show

DateAdd('m',[Inventory].[Service Period], [Inventory].[Purchase Date])

However, I cannot work out how to get SQL/MS access to compare two values and only display the greater of the two. From reading around I think I should be able to do a sub-query, but I cannot work out how to phase it.

I've been trying to adapt @MikeTeeVee's answer from here: Is there a Max function in SQL Server that takes two values like Math.Max in .NET? . But I keep getting errors saying that query is not part of an aggregate function and I'm not certain what I doing wrong. For example, I tried:

SELECT Inventory.Equipement,
       (SELECT MAX(NSD_proxy) 
        FROM (VALUES 
             (Max(DateAdd('m', Inventory.[Service Period], WorkDone.[Work Date]))), 
                 (DateAdd('m', Inventory.[Service Period], Inventory.[Purchase Date]))) 
         AS FUNCTION(NSD_proxy)
        ) AS NextServiceDate,
FROM Inventory INNER JOIN WorkDone ON Inventory.ID = WorkDone.Equipment
GROUP BY Inventory.Equipement

which has some syntax error.

Answer 1

You don't have to compare the two dates, just check if a WorkDone record exists to match the Inventory record.

You can use:

IIF(ISNULL(WorkDone.Equipment),
DateAdd('m',[Inventory].[Service Period],[Inventory].[Purchase Date]),
Max(DateAdd('m',[Inventory].[Service Period],[WorkDone].[Work Date])))
AS NextServiceDate

The rest of your query can remain as is.

Answer 2

Consider a LEFT JOIN to return matched or unmatched records where latter is filled with NULLs, and then run your aggregate, MAX , with an NZ() :

SELECT Max(NZ(DateAdd('m', i.[Service Period], w.[Work Date]),
              DateAdd('m', i.[Service Period], i.[Purchase Date]))
          ) AS NextServiceDate, i.Equipement
FROM Inventory i LEFT JOIN WorkDone w ON i.ID = w.Equipment
GROUP BY i.Equipement