If condition in Where Clause in SQL Query(Please help)

Question

Hi all...
I have a table like below in the database.

ID	Username	Date1	Date2
101	abc	2012-11-01	NULL
102	def	2012-12-03	2013-01-05
103	ghi	2011-02-04	2012-03-04
104	jkl	2012-02-03	2013-01-01
105	mno	NULL	NULL
106	pqr	NULL	2013-04-01

Now, I want to use the "IF-Condition" in my sql query such that -

select * from table where Date1<Date2 and Date1 is not null and Date2 is not null, and Total Days between Date1 and Date2.

The result i want is -

ID	Username	Date1		Date2	Total Datys
101	abc	2012-11-01		NULL	Null
102	def	2012-12-03		2013-01-05	10
103	ghi	2011-02-04		2012-03-04	10
104	jkl	2012-02-03		2013-01-01	0 (Date1> Date2)
105	mno	NULL		NULL	Null
106	pqr	NULL		2013-04-01	Null

How do I get this?? Please help...

Thanks a ton in advance....