表1中列值A求和 - 表2中列值B求和

求和：

select sum(A) from table1 where id = \'1\';

select sum(B) from table2 where id = \'1\';

 

若求和值为Null,需要转换，否组会报错

通过IFNULL转换，转换格式：IFNULL(字段,0)

调整后如下：

select IFNULL(sum(A),0)  from table1 where id = \'1\';

select IFNULL(sum(B),0)  from table2 where id = \'1\';

 

相减：

select (select IFNULL(sum(A),0)  from table1 where id = \'1\') - (select IFNULL(sum(B),0)  from table2 where id = \'1\')