Select sum until a set amount and then update fields in mysql database

问题

 item_id      rate            status
 ---------    -----------     ------
   1           12              credit
   2           10              credit
   3           10              credit
   4           20              cash
   5           55              credit

I have the above table, A user inputs and amount of 25. Now I want to update the status of the rows having credit as status from credit to cash until the sum of rate is 25, so in the above table the top 1 rows having a sum of 22 should get a status of cash. Since the user input is 25, I still have a balance of 3 (25-22), this balance should be deducted from the third row making the third row rate 7. The result I want is tabular form with the changes highlighted:

 item_id      rate            status
 ---------    -----------     ------
   1           12              **cash**
   2           10              **cash**
   3           **7**           credit
   4           20              cash
   5           55              credit

回答1:

You can use window functions to identify the rows that need to be changed:

select item_id, 
    case when sum_rate >= 25 then 'credit' else 'cash' end as status, 
    case when sum_rate >= 25 then sum_rate - 25 else rate end as rate
from (
    select t.*, sum(rate) over(order by item_id) sum_rate
    from mytable t
    where status = 'credit'
) t
where sum_rate - rate < 25

You can put that logic in an update statement if you prefer:

update mytable t
inner join (
    select item_id, sum(rate) over(order by item_id) sum_rate
    from mytable t
    where status = 'credit'
) t1 on t1.item_id = t.item_id
set 
    t.status = case when sum_rate >=  25 then 'credit' else 'cash' end,
    t.rate =   case when t1.sum_rate >= 25 then t1.sum_rate - 25 else t.rate end
where t1.sum_rate - t.rate < 25

Demo on DB Fiddle

来源：https://stackoverflow.com/questions/65135819/select-sum-until-a-set-amount-and-then-update-fields-in-mysql-database