KPI Cross-selling

$this->db->query("
    SELECT 
        AVG(sale_count) AS average_sales
    FROM (
        SELECT 
            tbl_sales_product.sale_id,
            COUNT(tbl_sales_product.id) AS sale_count
        FROM 
            isellpt_isystem_new.tbl_sales_product AS tbl_sales_product
        WHERE 
            tbl_sales_product.subtotal > 0
        GROUP BY 
            tbl_sales_product.sale_id
    ) AS sales_counts;
")->row();