NVARCHR2怎么转换成VARCHAR2

自动转换的，但是NVARCHAR2优先级高点，所以遇到where varchar2 =nvarchar2的，会将varchar2自动转为nvarchar2

SQL> drop table t;

Table dropped

SQL> create table t(name nvarchar2(100));

Table created

SQL> insert into t values('abc');

1 row inserted

SQL> explain plan for
2 select * from t where name='abc';

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 102 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 102 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("NAME"=U'abc')
Note
-----
- dynamic sampling used for this statement (level=2)

17 rows selected

SQL> drop table t;

Table dropped

SQL> create table t(name varchar2(100));

Table created

SQL> insert into t values('abc');

1 row inserted

SQL> select * from t where name=u'abc';

NAME
--------------------------------------------------------------------------------
abc

SQL> explain plan for
2 select * from t where name=u'abc';

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 52 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 52 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(SYS_OP_C2C("NAME")=U'abc')
Note
-----
- dynamic sampling used for this statement (level=2)

17 rows selected

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