测试环境：
SQL> select * from t1;

        ID NAME
---------- --------------------
         1 A
         2 B
         3
         4 D

SQL> select * from t2;

        ID NAME
---------- --------------------
         1 A
         1 E

SQL> set autot on exp
--使用in
SQL> select id,name from t1 where t1.id in (select t2.id from t2);

        ID NAME
---------- --------------------
         1 A


Execution Plan
----------------------------------------------------------
Plan hash value: 1713220790

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    16 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN SEMI    |      |     1 |    16 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1   |     4 |    12 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T2   |     2 |    26 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."ID"="T2"."ID")

Note
-----
   - dynamic sampling used for this statement

--两个表等值连接
SQL> select t1.id,t1.name from t1,(select id from t2) t2 where t1.id=t2.id;

        ID NAME
---------- --------------------
         1 A
         1 A


Execution Plan
----------------------------------------------------------
Plan hash value: 2959412835

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     2 |    32 |     7  (15)| 00:00:01 |
|*  1 |  HASH JOIN         |      |     2 |    32 |     7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T2   |     2 |    26 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T1   |     4 |    12 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."ID"="ID")

Note
-----
   - dynamic sampling used for this statement
   
in括号里子查询会走join-semi，等值连接会走hash join；in括号内的子查询有虑重功能，相当于子查询中有distinct。