MySQL。 如何组合或匹配两个表中的特定行

Question

First, the title is not very clear. With this example I would like to know if there is a solution with SQL code or if it has to be worked in the other side with C#, Java, PHP, etc.

The principle is this:

1. There is a table of Inputs , like this:

    ID  Name   Amount
    1   AA     10   
    2   BB     9
    3   CC     8
    4   DD     1
    5   ZZ     2

2. And there is a table of Outputs :

    ID  Name   Fouls   
    1   BB     4
    2   ZZ     1

3. What I'm trying to get is a subtraction of the quantities, based on each matching column, hoping to get the following:

    Name   Diff
    AA     10   
    BB     5
    CC     8
    DD     1
    ZZ     1

Can it be done directly with SQL?

Answer 1

You can left join :

select i.name, i.amount - coalesce(o.fouls, 0) diff
from inputs i
left join outputs o on o.name = i.name

Answer 2

I think you just want a left join and arithmetic:

select i.id, i.name, i.amount - coalesce(o.fouls, 0)
from inputs i left join
     outputs o
     using (id)