Question on Row Level Security / null mean allow to view all data

Hi tomcch ,

According to your statement, I think your requirement is that the User whose Customer ID is null could see all value.

I suggest you to try code as below in Manage Roles 'Main Table'[Custom ID].

[Customer_ID] = 
VAR _LIST = CALCULATETABLE(VALUES('User Table'[Customer ID]),FILTER(ALL('User Table'),'User Table'[User_Email] = USERPRINCIPALNAME()))
RETURN
IF("null" in _LIST,CALCULATE(MAX('Main Table'[Customer_ID])),CALCULATE(MAX('Main Table'[Customer_ID]),FILTER('Main Table','Main Table'[Customer_ID] in _LIST)))

Result is as below.

Best Regards,
Rico Zhou

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