Oracle Date Processing Full edition

To_date format
Day:
DD Number 12
DY abbreviated Fri
Day spelled out Friday
Ddspth spelled out, ordinal twelfth
Month:
MM Number 03
Mon abbreviated Mar
Month spelled out March
Year:
YY two digits 98
YYYY four digits 1998

24-hour format of the time range: 0:00:00-23:59:59 ....
12-hour format of the time range: 1:00:00-12:59:59 ....

1.
Date and character conversion function usage (TO_DATE,TO_CHAR)

2.
Select To_char (to_date (222, ' J '), ' JSP ') from dual

Show two hundred twenty-two

3.
How many days is the day of the week
Select To_char (to_date (' 2002-08-26 ', ' yyyy-mm-dd '), ' Day ') from dual;
Monday
Select To_char (to_date (' 2002-08-26 ', ' yyyy-mm-dd '), ' Day ', ' nls_date_language = American ') from dual;
Monday
Set Date language
ALTER session SET nls_date_language= ' American ';
I can do that.
To_date (' 2002-08-26 ', ' yyyy-mm-dd ', ' nls_date_language = American ')

4.
Days of two days
Select Floor (sysdate-to_date (' 20020405 ', ' YYYYMMDD ')) from dual;

5. The use of time as null
Select ID, active_date from table1
UNION
Select 1, to_date (null) from dual;

Note to use to_date (NULL)

6.
A_date between To_date (' 20011201 ', ' YYYYMMDD ') and to_date (' 20011231 ', ' YYYYMMDD ')
The December 31 is not included in this range until after 12 o'clock noon and December 1 of 12 points.
So, when time needs to be accurate, feel to_char is still necessary
7. Date format conflict issues
The format you enter depends on the type of Oracle character set you have installed, such as: Us7ascii, the date format type is: ' 01-jan-01 '
alter system Set Nls_date_language = American
Alter session Set Nls_date_language = American
or write in To_date.
Select To_char (to_date (' 2002-08-26 ', ' yyyy-mm-dd '), ' Day ', ' nls_date_language = American ') from dual;
Notice I'm just lifting up the nls_date_language, and of course there's plenty,
To view
SELECT * FROM Nls_session_parameters
SELECT * FROM V$nls_parameters