Why MAX is NULL when projecting along the same dimension - ssas

Question

Here is a vanilla "Sales" database:
CREATE TABLE Sales(id INT, category VARCHAR(50), item VARCHAR(50), date DATE, amount DECIMAL(10, 2));
INSERT INTO Sales VALUES
(1, 'Memory', 'Corsair 16GB', '2021-01-01', 200),
(2, 'Storage', 'Seagate BarraCuda 2TB', '2021-01-02', 50),
(3, 'Storage', 'Samsung 980 1TB', '2021-01-02', 150),
(4, 'OS', 'Windows 11', '2021-01-02', 150),
(5, 'OS', 'Ubuntu', '2021-01-03', 15),
(6, 'DBMS', 'MySQL Enterprise Edition 8', '2021-01-03', 5000),
(7, 'DBMS', 'SQL Server 2022', '2021-01-04', 15000),
(8, 'Memory', 'Corsair 16GB', '2021-01-04', 200),
(9, 'Memory', 'G.Skill Trident 32GB', '2021-01-04', 250),
(10, 'OS', 'Ubuntu', '2021-01-05', 15),
(11, 'DBMS', 'SQL Server 2022', '2021-01-06', 15000),
(12, 'DBMS', 'MySQL Enterprise Edition 8', '2021-01-06', 5000);
From it we build an SSAS Multidimensional cube with 3 dimensions:
Category with attribute Name,
Item with attribute Name,
Date with attribute Date.
So far so good.
Now creating some measures to get the maximum amounts along some dimensions:
WITH
MEMBER [Max Daily Sale] AS MAX([Date].[Date].Children, [Amount])
MEMBER [Max Category Sale] AS MAX([Category].[Name].Children, [Amount])
MEMBER [Max Category Daily Sale] AS MAX(([Category].[Name].Children, [Date].[Date].Children), [Amount])
MEMBER [Max Item Daily Sale] AS MAX(([Item].[Name].Children, [Date].[Date].Children), [Amount])
MEMBER [Max Item Sale] AS MAX([Item].[Name].Children, [Amount])
They are working fine except when one of the dimensions used in the MDX query is also used in the measure.
As an example:
SELECT
[Category].[Name].Children ON ROWS,
--[Item].[Name].Children ON ROWS,
--[Date].[Date].Children ON ROWS,
{ Amount, [Max Daily Sale], [Max Category Sale], [Max Category Daily Sale], [Max Item Sale], [Max Item Daily Sale] } ON COLUMNS
FROM Sales
Gives:
Amount Max Daily Sale Max Category Sale Max Category Daily Sale Max Item Sale Max Item Daily Sale
DBMS 40000 20000 (null) (null) 30000 15000
Memory 650 450 (null) (null) 400 250
OS 180 150 (null) (null) 150 150
Storage 200 200 (null) (null) 150 150
All the measures using dimension Category, [Max Category Sale] and [Max Category Daily Sale], result in NULL.
In the same way, using [Item].[Name] as the ROWS axis will "break" the [Max Item Sale] and [Max Item Daily Sale] measures.
And so on for [Max Daily Sale], [Max Category Daily Sale], and [Max Item Daily Sale], when using [Date].[Date].
It's obviously some MDX triviality I'm missing, but which one?

Answer 1

You are correct in what you observe. Each dimension can only be used in one section of the MDX: either on ROWS, on COLUMNS, in the WHERE clause, or within a calculated member. Sometimes an MDX statement will appear to break that rule, but look closely and you'll see that the dimension is only being used in one consistent way.
There are ways around this, often with the use of [Dimension name].currentMember but I'm not sure what to suggest for your particular case. Can this report be broken down into separate reports, to simply matters?