SELECT
MyTable1.ID,
listagg(MyTable2.ID,';') within group(order by MyTable2.ID) tab2_id
listagg(MyTable2.object_ID,';') within 
   group(order by MyTable2.object_ID) tab2_object_id
FROM
MyTable1
LEFT JOIN MyTable2 ON MyTable2.MyTable1_ID = MyTable1.ID
group by MyTable1.ID
order by 1;

但是，要注意，如果连接长度不超过4000个字符，listagg也可以工作。
因此，在Oracle livesql中，我运行了以下命令：

with tab1 (id) as (
  select 567 from dual
)
,tab2 (id,tab1_id,object_id) as (
  select 1, 567, 896 from dual union all
  select 2, 567, 998 from dual union all
  select 3, 567, 777 from dual
)
select tab1.id
 ,listagg(tab2.id,';') within group(order by tab2.id) tab_ids
 ,listagg(tab2.object_id,';') within group(order by tab2.object_id) 
tab_object_ids 
from tab1
left join tab2 on tab1.id=tab2.tab1_id
group by  tab1.id
order by 1

然后运行它只显示了一行。不明白你怎么说它为你获取了三行。

提供了样本数据：

WITH
    t1 AS
        (   Select 567 "ID" From Dual   ), 
    t2 AS
        (
            Select 1 "ID", 567 "MY_T1_ID", 896 "OBJECT_ID" From Dual Union All
            Select 2 "ID", 567 "MY_T1_ID", 967 "OBJECT_ID" From Dual Union All
            Select 3 "ID", 567 "MY_T1_ID", 756 "OBJECT_ID" From Dual 
        )

如果您可以使用不带OVER（Partition By t1.ID）的LISTAGG（）分析函数：

Select  t1.ID, 
        LISTAGG(t2.ID, ';') WITHIN GROUP (Order By t2.ID)  "MY_T2_ID",
        LISTAGG(t2.OBJECT_ID, ';') WITHIN GROUP (Order By t2.ID) "MY_T2_OBJECT_ID"
From    t1
Inner Join t2 ON(t2.MY_T1_ID = t1.ID)

Result:
ID   MY_T2_ID  MY_T2_OBJECT_ID
---  --------  ---------------
567  1;2;3     896;967;756

越有可能需要按t1.ID进行分区，则应使用DISTINCT关键字排除重复行（对于大数据集，性能成本可能较高）：

Select  DISTINCT t1.ID, 
        LISTAGG(t2.ID, ';') WITHIN GROUP (Order By t2.ID)  OVER (Partition By t1.ID) "MY_T2_ID",
        LISTAGG(t2.OBJECT_ID, ';') WITHIN GROUP (Order By t2.ID) OVER (Partition By t1.ID) "MY_T2_OBJECT_ID"
From    t1
Inner Join t2 ON(t2.MY_T1_ID = t1.ID)

如果是这种情况，你可以这样做：

Select ID, Max(MY_T2_ID) "MY_T2_ID", Max(MY_T2_OBJECT_ID) "MY_T2_OBJECT_ID"
From
    (
        Select  t1.ID, 
                LISTAGG(t2.ID, ';') WITHIN GROUP (Order By t2.ID)  OVER (Partition By t1.ID) "MY_T2_ID",
                LISTAGG(t2.OBJECT_ID, ';') WITHIN GROUP (Order By t2.ID) OVER (Partition By t1.ID) "MY_T2_OBJECT_ID"
        From    t1
        Inner Join t2 ON(t2.MY_T1_ID = t1.ID)
    )
Group By ID

Result:
ID   MY_T2_ID  MY_T2_OBJECT_ID
---  --------  ---------------
567  1;2;3     896;967;756