是不是可以只连接，不子查询？
否，因为当每个id（TableA）有多个a_id（TableB和TableC）时，会进行一对多连接，这会创建更多的行（本质上是行的复制，也称为“关系乘法”）。
解决该问题的一种常用方法是将SUM（）隔离在单独的查询（子查询）中

select a.id, B_money_sum , C_money_sum
from tableA a 
join (select a_id,sum(money) as B_money_sum
      from tableB
      group by a_id 
     ) b on a.id = b.a_id
join (select a_id,sum(money) as C_money_sum
      from tableC
      group by a_id 
     ) c on a.id = c.a_id  ;

https://dbfiddle.uk/coZvQsuT

如果表b有一个id而c没有，这就有点棘手了，反之亦然。让我们以这个案例为例。

create table a (id int);
create table b (id int,a_id int,money int);
create table c (id int,a_id int,money int);
insert a values(1),(2),(3),(4);
insert b values(1,1,1000),(2,1,2000),(3,2,3500),(4,4,1000),(5,4,7000);
insert c values(1,1,100),(2,2,200),(3,2,400),(4,3,200),(5,3,600);

在这种情况下，B具有id 4，而c具有id 3。如果我们使用内部连接，id 3和id 4的数据在结果集中都会丢失：

select a_id,sum_a,sum_b 
from
    (select a_id ,sum(money) sum_a
    from a
    join b on a.id=b.a_id
    group by a_id) tb
join
    (select a_id ,sum(money) sum_b
    from a
    join c on a.id=c.a_id
    group by a_id) tc
using(a_id)
;

+------+-------+-------+
| a_id | sum_a | sum_b |
+------+-------+-------+
|    1 |  3000 |   100 |
|    2 |  3500 |   600 |
+------+-------+-------+

但是，如果我们使用单侧外连接，我们仍然会在另一侧丢失一些数据。

select a_id,sum_a,sum_b 
from
    (select a_id ,sum(money) sum_a
    from a
    join b on a.id=b.a_id
    group by a_id) tb
left join
    (select a_id ,sum(money) sum_b
    from a
    join c on a.id=c.a_id
    group by a_id) tc
using(a_id)
;

+------+-------+-------+
| a_id | sum_a | sum_b |
+------+-------+-------+
|    1 |  3000 |   100 |
|    2 |  3500 |   600 |
|    4 |  8000 |  NULL |
+------+-------+-------+

select a_id,sum_a,sum_b 
from
    (select a_id ,sum(money) sum_a
    from a
    join b on a.id=b.a_id
    group by a_id) tb
right join
    (select a_id ,sum(money) sum_b
    from a
    join c on a.id=c.a_id
    group by a_id) tc
using(a_id)
;

+------+-------+-------+
| a_id | sum_a | sum_b |
+------+-------+-------+
|    1 |  3000 |   100 |
|    2 |  3500 |   600 |
|    3 |  NULL |   800 |
+------+-------+-------+

为了弥合所有边差距，需要完全连接。但是由于MySQL还没有实现完全连接，所以需要调用UNION来合并两个外部连接。

select * from
(select a_id,sum_a,sum_b 
from
    (select a_id ,sum(money) sum_a
    from a
    join b on a.id=b.a_id
    group by a_id) tb
left join
    (select a_id ,sum(money) sum_b
    from a
    join c on a.id=c.a_id
    group by a_id) tc
using(a_id)

UNION

select a_id,sum_a,sum_b 
from
    (select a_id ,sum(money) sum_a
    from a
    join b on a.id=b.a_id
    group by a_id) tb
right join
    (select a_id ,sum(money) sum_b
    from a
    join c on a.id=c.a_id
    group by a_id) tc
using(a_id)
) t_a_b
order by a_id
;

+------+-------+-------+
| a_id | sum_a | sum_b |
+------+-------+-------+
|    1 |  3000 |   100 |
|    2 |  3500 |   600 |
|    3 |  NULL |   800 |
|    4 |  8000 |  NULL |
+------+-------+-------+

注意：最外层需要使用ORDER BY子句，这在UNION中无效。