title: Relational Algebra, join between 3 tables
body: <p>I've three tables , employees, purchases and products.
  I need to find all the employees who did not sell a product with price > 200 which is mentioned in products table.
  Purchases table contains eid referencing employees and pid referencing products.
  My relational algebra query is like this, am I right ?</p>
  
  <pre><code> π employees.ename( σ price &lt;&gt; (σ price &gt;= 200(products)) ⨝ (purchases) ⨝ (employees))
  </code></pre>
  
  <p>or</p>
  
  <pre><code>π ename(employees) - π employees.ename( σ price &gt;= 200(products) ⨝ (purchases) ⨝ (employees))
  </code></pre>
  
  <p>purchases.pid= products.pid and purchases.eid= employees.eid and ename is from table employees.</p>