现有表atable和表btable，数据如下：

table atable

id    anumber

1       500

2       300

3       400

table btable

id    bnumber

1        501

2        301

3        401

总数据相减：

sql:

select  (select  sum(a.number) from atable a) - (select sum(b.number) from btable  b)  as '总数据'

每条字段相减:

sql:

select sum(anumber),sum(bnumber),sum(anumber-bnumber) from

(select  sum(a.number) from atable a)

union all

(select sum(b.number) from btable b)

想要相加的话符号改一下就可以了