title: normalization weak entity with BCNF
body: <p>how to Decompose FDs of weak entity into BCNF when it does not have primary key...
  we could not define any function dependency!
  for example we have these entites:<br></p>
  
  <p><strong>user</strong>(name,user_id)<br>
  <strong>product</strong>(product_type,cost,product_id)<br>
  <strong>laptop</strong>(cpu,time,user_have_id)<br>
  <strong>mobile</strong>(resolution,time,user_have_id)<br>
  <strong>flash</strong>(memory,time,user_have_id)<br></p>
  
  <p>product_type,cost,product_id do not depend on any thing
  I don't know how to work with weak entity
  could't we design it in better way???</p>