Function audience_column_mismatch 

pub fn audience_column_mismatch(
    left_qualifier: &str,
    right_qualifier: &str,
) -> String

True when left_qualifier.audience and right_qualifier.audience are both set but disagree. NULL-tolerant: a NULL audience on either side does not count as a mismatch, so a legacy row on either side of the join passes through cleanly. Both qualifiers must be trusted table names/aliases, never user input.