表1中列值A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')