title: impleminting unique _id through whole sqlite DB
body: <p>I'm developing an android app of a library ,divide books into categories containing books,
  implemented categories as sqlite tables and books as records ,i need to make _id of a book unique 
  through whole library ,
  i tried this
  category 1 _id 1 to 1000<br>
  category 2 _id 1001 to 2000<br>
  category 3 _id 2001 to 3000<br>
  but this limits each category to 1000 item
  any solutions???</p>