SQL doesn't let you use variables that way.

What I would do is (a) figure out if a cursor is really needed for this or if it can be replaced with set-based code, (b) if a cursor is needed, create it with a Union All query based on a Where, instead of using If.

DECLARE RandomUser_CURSOR CURSOR

LOCAL FORWARD_ONLY KEYSET SCROLL_LOCKS

FOR

select *

from TableA

where @bCustom = 0

union all

select *

from TableB

where @bCustom != 0

FOR UPDATE

OPEN RandomUser_CURSOR

FETCH NEXT FROM RandomUser_CURSOR INTO

@Col1, @Col2, @Col3

WHILE @@FETCH_STATUS = 0

BEGIN