Deal with null values ​​from join

[GianluigiBI]

I am extremely struggling to deal with nulls caused by Join.
In a table I have a lot of nulls, but needing to count them I would like indicate  them with a flag like 'null'.

This is my join: 

OriginalTransaction:
LOAD
KeyIdNum,
original_row_id,
if(Not Exists(original_transaction_date),'not exist',original_transaction_date) as original_transaction_date,
row_id
resident Transaction_temp2;
inner Join
Load
row_id,
transaction_date
resident Header;

[Dalton_Ruer]

An Inner Join will only keep rows in the final table that existed in both tables. 

I'm not sure exactly what your goal is based on the code above and not sure others will understand. It's always best if you show sample date in the form of INLINE statements and then illustrate what you are hoping for in terms of the output from the data model. Such as below

SET NullInterpret='';    // Tells Qlik Sense to treat the blank entries from Inline statements as NULL

Source1:
Load * Inline [
Field1, Field2, Field3
1, 1, 399
2, 1, 300
3, 1, 100
4,, 0
5,, 5
];

Source2:
Load * Inline [
Field1, Field4, Field5
3,1,200
4,1,500
5,1,150
6,1,100
];

Given something like this then others will be able to quickly try and reproduce your example.