A summary of time-empty processing in SQL Server _mssql

Several main situations are summarized:
First, how to enter a null value
If you do not enter a null value, when the time is empty, the default is written to "1900-01-01", which is cumbersome for business processing.
Ctrl+0 You can enter a null value.
Second, the time field how to judge Null in the SQL statement
Suppose 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 to query, if written:
Select (Case DateTime1 when NULL then ' a ' else ' B ' ") from TestTable
The result of the query is:

B

This is clearly not the desired result;
Select (Case DateTime1 when DateTime1 then ' B ' else ' a ') from TestTable
Its query results are:

A

This is the result of the want.