两个SQL表之间的两列之间的区别

Question

我有 捐助者表

ID  BG  amount                                                            

1   A   7
                                                                          
2   A   5
                                                                          
3   A+  8
                                                                          
4   A+  4
                                                                          
5   A   6

和受体表

ID  BG  amount

1   A+  9

2   A   7

3   A+  10

4   A   5

5   A+  7

我想找出不同血型的受体数量多于供血者，以及缺乏多少

o / p：

A+ 14

Answer 1

SELECT bg,sum(diff) FROM 
(SELECT BG,-amount AS diff FROM donor 
UNION ALL 
SELECT  BG,amount AS diff FROM acceptor) 
GROUP BY bg having sum(diff)>0;