Thank you for all the answers.
I have another following question.
I have two tables that related to each other. (Relationship). Let's say
that Table A is the mother table and table B is the child table. When I open
table A in the front-end, I couldn't see the plus sign at the first section
of each record. I can see the plus sign when I open the back-end though. Why
I don't see it on the front-end? Am I be able to see it?
This is a bit confusing... Table A is in the mother table? Do you mean
database?
Relationships can only be established and enforced between tables *in the same
database*. Normally all of the tables will be in the backend .mdb file, and
all relationships will be established there. Access cannot enforce
relationships between different databases, because it has no way to control
what's happening in some OTHER database - if you have a relationship between
TableA in Front.mdb and TableB in Back.mdb, established in Front.mdb, there's
no way to prevent someone opening Back.mdb by itself, or from a third
database, and changing the contents of the table in a way that violates the
relationship.
Do note that Subdatasheets - the + sign - are a major drag on performance, and
are *NEVER* necessary. You shouldn't be using Table datasheets to work with
data in any case - you should always use a Form (with subforms) for
interaction with the data.