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The error that you posted was by the expansion returning null. You cannot evaluate a Max() in this context.
Is not clear what you are trying to do - (1) you want a to use a field NumDate already loaded into QS, or (2) a field NumDate in the source database?
If (1), then you need to compute the Max value in a temp table, and Peek() the value into a variable.
LOAD Max(NumDate) - 29 as MaxDate Resident Data;
Let vMaxDate = Peek('MaxDate ');
Drop Table T_MaxDate;
SQL SELECT ....
Where NumBase > $(vMaxDate);
If (2), you will need to modify the SQL expression with a nested subquery or a join to compare NumDate and Max(NumDate)
>>when I searched I find in a post that is recommended to use it #
This is what that probably meant
Where NumBase > $(#vMaxDate);
The # forces the vMaxDate dollar expansion to be numerical, which is not really necessary here.
What I actually want to do is to extract the data where the date > max(date)-29
so it didnt work when I did such a comparison and now I am stuck
Could you explain to me more what I should be doing in this case
You are tying to use the alias in where condition
try using the actual field name
basically numDate will not be visible to the where condition. Try something like below
Date#(deliverd#Date) as numDate
From Your Excel document;
Check if the numdate has the correct numeric date values
if it does
LOAD Max(NumDate) as MaxDate
Let vMaxDate = Peek('MaxDate ')-29;
Drop Table T1;