Global aggregation on non-default measure

Question

To aggregate along the default measure we use:

SELECT FROM [Sales]

It returns a "scalar", a value without any header:

+------+
| 1000 |
+------+

How to do the same with non default measure?

This raises an error:

SELECT [Measures].[Quantity] FROM [Sales]

This works:

SELECT [Measures].[Quantity] ON 0 FROM [Sales]

But instead of having a "scalar" we get an additional header with the name of the measure:

+----------+
| Quantity |
+----------+
|   1000   |
+----------+

Answer 1

This kind of query

SELECT FROM [Sales]

is so called query with zero axes. Because no members were assigned to any (non-slicer) axis, the result is considered to have zero axes and by convention would be single unlabeled cell, or at least cell with no distinct row or column headers, and that is what you see.

With this query

SELECT [Measures].[Quantity] ON 0 FROM [Sales]

you have specified that you want to see Quantity measure on axes 0 (to see Quantity measure on Columns).

To achieve "zero axes" query result you should put your measure on slicer (specified with Where):

SELECT  FROM [Sales]
WHERE [Measures].[Quantity]