Method diagnostic_message

datafusion/sql/src/utils.rs:126–130

(&self, expr: &Expr)

    }

    fn diagnostic_message(&self, expr: &Expr) -> String {
        format!(
            "'{expr}' must appear in GROUP BY clause because it's not an aggregate expression"
        )
    }
}

/// Determines if the set of `Expr`'s are a valid projection on the input