计算每个人的运行总计 [英] Calculate running total for each person

问题描述

我需要显示我的数据库中每个人的运行总计，但我只能得到他们所有的运行总计，所以这些是我的表上的图片

I need to show the running total for each person in my database but I could only get the running total for all of them, so these are my tables on the pic

我已经有这个查询：

SELECT 
    id, 
    studno,
    if(type=0,amount,0)debit,
    if(type=1,amount,0)credit,
    if(type=0,@bal:=@bal+amount,@bal:=@bal-amount) runningTotal
FROM
(SELECT id, studno, amount, 0 type from tblPayables 
UNION ALL 
SELECT id, studno, amount, 1 type from tblPayments)s, (SELECT @bal:=0)b
ORDER BY studno, id, type;

但事情是我只能得到这个结果：

but the thing is I can only come up to this result:

突出显示number应该为50，因为它是一个不同的studno

The highlighted number should be 50 since it's for a different studno

推荐答案

您必须以这样一种方式写入您的查询，

You must write your query in such a way that the variable is initialized every time an Id changes.

假设您可以使用以下列写入查询或视图：

Let's say you can write a query or view with the following columns:

id | studno | debit | credit
---+--------+-------+-------

那么，让我们写下查询：

So, let's write the query:

select id, debit, credit
     , @bal := ( -- If the value of the column `studno` is the same as the
                 -- previous row, @bal is increased / decreased;
                 -- otherwise, @bal is reinitialized
         case 
             when @studno = studno then @bal + debit - credit
             else debit - credit
         end
     ) as balance
     @studno := a.studno as studno -- It's important to update @studno
                                   -- AFTER you update @bal
from 
    (
        select @bal := 0
             , @studno := 0 -- This variable will hold the previous
                            -- value of the `studno` column
    ) as init, -- You must initialize the variables
    ( -- The above mentioned query or view
        select ...
        from ...
    ) as a
order by a.studno, a.id -- It's REALLY important to sort the rows

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