数据库字符串字段类型:是否可以为空?

I have several tables that contain several strings for fields. Some of these fields may or may not always contain data. If I create a record with a string field that's null at the time of the record's creation, does it become a problem when I do a read query of that record because the field is null or does the query return an empty string? What's considered based? Non-nullabe strings or nullable strings, and when to know the difference?I have several tables that contain several stri