oracle sql date comparison

 

I always use the time comparison, I found some information, and found that although there are a lot of things on the Internet, it is quite messy. Then do some experiments yourself and record them here. . . .

1 SELECT
2   sysdate - DEADLINE,
3   DEADLINE
4 FROM T_FUNC_MISSION;

sysdate is the oracle system time;

The output result is: -6 8:1:34.0 2018-04-30 23:50:00.000000 Obviously, this method can compare the time, the method is simple, but a bit low.

Continue to find information:

　　https://www.cnblogs.com/xuxm2007/archive/2010/12/28/1918652.html

　　This introduction is very detailed, but I found that many of them are not available for Oracle, and they are basically functions in SQL Server. . . . -_-||,

　　

　　http://liwx.iteye.com/blog/1236382

　　The oracles introduced in this article can be used, but there is still a pit. .

　　

1 select sysdate,to_date('2009-2-20 17:45:39','yyyy-mm-dd hh24:mi:ss'),  
2 ROUND(TO_NUMBER(sysdate - to_date('2009-2-20 17:45:39','yyyy-mm-dd hh24:mi:ss')) * 24 * 60)  
3 from dual;

Such sql can be run, but if it is set to its own table, it will report an error, an invalid number.

 At first, I thought that the format in to_number was wrong, and the converted string must conform to the numeric type format.

to_number is one of the commonly used type conversion functions in Oracle. It is a format for converting a string to a numeric type, which is the opposite of the TO_CHAR() function.

https://blog.csdn.net/dongdong9223/article/details/48972815 has an introduction

 

But why is there no error when querying the dual table? ? ? Weird. . . .

SELECT
  sysdate - DEADLINE
FROM T_FUNC_MISSION;

SELECT sysdate - to_date('2018-4-20 17:45:39', 'yyyy-mm-dd hh24:mi:ss')
FROM dual;

The field type found from the dual table is the number type, but the field found in the T_FUNC_MISSION table is the intervalds type (time difference);

 

Still have some questions:

　　Is the dual table any different?

Make a note. . . . .