title: How to describe the specific cardinality constraints in e-r diagram?
body: <p>I'm learning database this semester. The homework on e-r model really confuses me.<br>
  <br>
  The homework is drawing the e-r diagram about a vegetable market: A Vegetable Market has a collection of numbered stalls. Each stall has a name (unique) and sells at least one kind of vegetables (each kind of which has a name). Each stall sells each vegetable at its own particular price. Each farm grows some <strong><em>(at least one, but not all)</em></strong> of the vegetables, but a vegetable must be grown by at least one farm. Each farm has one name (unique) and address. Each stall buys each vegetable from only one farm at a particular price they have agreed. <br>
  <br>
  My question is that how to describe the constraint "(at least one, but not all)" in the requirement?<br>
  Another question is that since vegetables don't have primary key or discriminator, should they be treated as entities? Or should they be treated as somethings else like attributes?</p>
  
  <p>Here's my answer: 
  <a href="https://i.stack.imgur.com/oPmrQ.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/oPmrQ.jpg" alt=""></a><br>
  <sub>(source: <a href="http://fmn.rrimg.com/fmn056/20120919/2325/large_xnSN_416500023eb6125e.jpg" rel="nofollow noreferrer">rrimg.com</a>)</sub>  </p>