Boolean using SQL

I have a .MDB database. Some fields are true/false and could be NULL. When i exported it to SQL fields are bit and NOT NULL ALLOW. Of course my APP doesnot work 'cos fields could not be NULL. WHY?

1. How do i can say these fields MUST ALLOW NULL?
2. Why i can not test TRUE or FALSE in VB as i do with ACCESS? I must use -1 or 1 or 0

In Access, the Required property for the field must be 'No'. In SQLServer, the 'Allow Null' property must be 'Yes' (or checked)

You can use a SQL sentece for convert False/True values to 0/1/-1.... :)