To_date () and to_char () date and String Conversion

O_date ("string to be converted", "format to be converted") must match the format of the two parameters. Otherwise, an error is returned.
That is, the first parameter is interpreted according to the format of the second parameter.
To_char (date, "conversion format") converts a given date according to the "conversion format.
 
Conversion format:
Y indicates the last yy Of The year. yyy indicates the last two digits of the year. yyyy indicates the last three digits of the year. yyyy indicates the year in four digits.
For month: mm uses two digits to represent the month. mon uses the abbreviation such as January 1, November or nov. month uses the full name such as January 1, November or November.
Dd indicates the day of the month; ddd indicates the day of the year; dy indicates the day of the week, such as Friday or fri; day indicates the day of the week.
For example, Friday or Friday.
Hh 2-digit represents the hour 12-digit; hh24 2-digit represents the hour 24-hour
Indicates minute. mi 2-digit indicates minute.
Indicating second: two-digit ss indicates 60 seconds
For a quarter: q indicates a single digit for a quarter (1-4)
In addition, ww is used to indicate the week w of the current year to indicate the week w of the current month.
Time range in the 24-hour system: 00: 00: 00-23:59:59
Time range in the 12-hour format: 1: 00: 00-12:59:59
For example:
Select to_char (sysdate, yy-mm-dd hh24: mi: ss) from dual // display: 08-11-07 13:22:42
Select to_date (2005-12-25, 13: 25: 59, yyyy-mm-dd, hh24: mi: ss) from dual // display: 13:25:59
However, if the previous writing: select to_date (2005-12-25, 13: 25: 59, yyyy-mm-dd, hh: mi: ss) from dual, an error is returned, because the hour hh is in 12-digit format and the value 13 is invalid, it cannot be matched.
Supplement:
Current Time minus 7 minutes
Select sysdate, sysdate-interval 7 MINUTE from dual
Current Time minus 7 hours
Select sysdate-interval 7 hour from dual
Current Time minus 7 days
Select sysdate-interval '7' day from dual
Current Time minus July
Select sysdate, sysdate-interval 7 month from dual
Current Time minus 7 years
Select sysdate, sysdate-interval 7 year from dual
Time Interval multiplied by a number
Select sysdate, sysdate-8 * interval 7 hour from dual
 
Dual pseudo Column
Description:
Dual is an existing table in Oracle that can be read by any user. It is often used in select statement blocks without a target table.
For example, if you want to obtain the system time, use "select sysdate from dual" to return the current system time: 9:32:49. Different systems may return different date formats. "Select user from dual" returns the user of the current connection. If it is "select 1 + 2 from dual", the returned result is 3.