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Create a classification function

-- Classification function based on session user name
CREATE FUNCTION dbo.classified_function()
RETURNS SYSNAME
WITH SCHEMABINDING
AS
BEGIN

RETURN
(
	SELECT CASE SUSER_NAME()
		WHEN 'login1' THEN 'group1'
		WHEN 'login2' THEN 'group2'
		WHEN 'login3' THEN 'group3'
		ELSE 'Default'
	END
)
END; 
GO

Binding resource gouvernor with classification function

-- Binding resource gouvernor with the classification function
-- Don't forget to reload the resource governor's configuration after !! 
ALTER RESOURCE GOVERNOR 
WITH
(
 CLASSIFIER_FUNCTION = dbo.classified_function
);
GO