关于字符型转换成日期型的问题

15:13:46 SQL>
15:13:46 SQL> CREATE TABLE ycz_test_1
2 (row_id varchar2(100),
3 start_dt varchar2(100),
4 end_dt varchar2(100));

Table created

Executed in 0.02 seconds
15:13:47 SQL> CREATE TABLE ycz_test_2
2 (row_id varchar2(100),
3 start_dt date,
4 end_dt date);

Table created

Executed in 0.03 seconds
15:13:47 SQL> insert into ycz_test_1
2 values('1','','');

1 row inserted

Executed in 0.01 seconds
15:13:47 SQL> commit;

Commit complete

Executed in 0.01 seconds
15:13:47 SQL> insert into ycz_test_2
2 values
3 ('1',to_date('2007-7-1', 'yyyy-mm-dd'), to_date('2008-8-1', 'yyyy-mm-dd'));

1 row inserted

Executed in 0.01 seconds
15:13:47 SQL> commit;

Commit complete

Executed in 0.01 seconds
15:13:47 SQL> update ycz_test_1 a
2 set (a.start_dt,a.end_dt)=
3 (select b.start_dt,b.end_dt
4 from ycz_test_2 b
5 where a.row_id = b.row_id);

1 row updated

Executed in 0 seconds

15:13:47 SQL> commit;

Commit complete

Executed in 0.01 seconds

15:13:54 SQL> select * from ycz_test_1;

ROW_ID START_DT END_DT
-------------------------------------------------------------------------------- -------------------------------------------------------------------------------- --------------------------------------------------------------------------------
1 01-JUL-07 01-AUG-08

Executed in 0.03 seconds

15:14:02 SQL>
15:18:07 SQL>
15:18:08 SQL> update ycz_test_1 a
2 set (a.start_dt,a.end_dt)=
3 (select to_char(b.start_dt,'yyyy-mm-dd'),to_char(b.end_dt,'yyyy-mm-dd')
4 from ycz_test_2 b
5 where a.row_id = b.row_id);

1 row updated

Executed in 0.01 seconds

15:18:08 SQL> select * from ycz_test_1;

ROW_ID START_DT END_DT
-------------------------------------------------------------------------------- -------------------------------------------------------------------------------- --------------------------------------------------------------------------------
1 2007-07-01 2008-08-01

Executed in 0.03 seconds
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