Summing different tables values limiting the results

Question

I have two tables (a and b) which have both one date field of type TIMESTAMP. They represent two different type of actions (records): table a's actions are $200 worth each, while table b's ones worth $500.

I want to calculate the total count of dollars worth of both tables actions limiting the maximum number or dollars per day to $10.000. Is there a more clean or efficient way to do this other than:

SELECT SUM(daily_sum_total) AS total
FROM (
    SELECT CASE WHEN SUM(daily_sum) > 10000 THEN 10000 ELSE SUM(daily_sum) END AS daily_sum_total, day
    FROM (
        SELECT COUNT(date) * 200 AS daily_sum, DATE(date) AS day
        FROM a 
        GROUP BY day
        UNION ALL
        SELECT COUNT(date) * 500 AS daily_sum, DATE(date) AS day
        FROM b
        GROUP BY day
    ) AS daily_tmp_table
    GROUP BY day
) as total_tmp_table;

Answer 1

You might try this:

select
  sum(amount) amount
from(
  select
    least(sum(amount),10000) amount
  from (
    select date,
           200 amount
    from   a
    union all
    select date,
           500 amount
    from   b) list_of_all
  group by
    date(date)) list_by_day