SQL Server processing summary with time Blank

We will summarize several main cases:
I. How to enter a NULL value
If you do not enter a null value, if the time is null, "" is written by default, which is troublesome for business processing.
Ctrl + 0 to enter a NULL value.
Ii. How to judge the time field as NULL in an SQL statement
Assume that the table is TestTable.
SN DateTime1 DateTime2
1 2011-10-24 2011-10-25
2 NULL 2011-10-26
3 2011-10-25 NULL
Use case for query. If it is written:
Select (case DateTime1 when NULL then 'a 'else' end) from TestTable
The query result is:

B

This is obviously not the expected result; you need to write it:
Select (case DateTime1 when DateTime1 then 'B' else' end) from TestTable
The query result is:

A

This is the expected result.