title: sql select orders with similar items
body: <p>I need to select those orders in pairs who have the same products in them. <code>ORDER_ITEMS</code> contain the product and a foreign key to reference the parent <code>ORDER</code> row.  Order rows need to be different.</p>
  
  <p>I've managed to list out pairs with count how many matching products they have in them, but that's only a similarity count. I need to exclude orders from pairs who have different products in them.</p>
  
  <p>Can have Oracle specific stuff in it.</p>
  
  <p>The two tables are: </p>
  
  <pre><code>Order(order_id, customer_id...) 
  Order_Item(item_id, order_id FK, product_id,...)
  </code></pre>
  
  <p>I need tose order_id-s that have all Order_Item childs with matching product_id-s. </p>
  
  <p>Ex. in  Orders  </p>
  
  <pre><code>{ (ord1, cust1)  
    (ord2, cust2)}  
  </code></pre>
  
  <p>and in Order_Items   </p>
  
  <pre><code>{ (item1, ord1, product_id=3),  
    (item2, ord1, product_id=6),   
    (item3, ord2, product_id=3),   
    (item4, ord2, product_id=6) } 
  </code></pre>
  
  <p>So basically, two people bought exactly the same two things. They are a pair. Those orders whose ordered products don't match exactly are not listed.</p>