Can I define fields that do not get stored in a separate table?

I'm using hook_field_info to define my new fields. I need to use this field
within a couple of entities and I dont need to have these fields in
separate tables. However when I try to define the schema of a field using
hook_field_schema I see that I need to mention the columns which means the
table for the field will be created when I create an instance of the field
with some entity. Is there a way out of this? Can I have my field data in
the same table as that of my entity (which is not a node but a custom
entity)?

-- 
Cheers,
Mukesh Agarwal
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