Convert query into Relational Algebra
<p>I have the following relational database schema:</p>

<p><a href="https://i.stack.imgur.com/hoiYa.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hoiYa.jpg" alt="schema"></a></p>

<p>I want to write the following query in relational algebra without using inequalities, disjunctions or conjunctions in the selection operator:</p>

<blockquote>
  <p>List the names of items that had bids of $50 that did not sell.</p>
</blockquote>

<p>I've attempted to write it myself and have come up with the following (though unsure if it is correct or not):</p>

<pre><code>πiname(σprice=50((ITEM⨝BID) - πiid(SALE)))
</code></pre>

<p>I don't know how to write relational algebra with the correct symbols using Stackoverflow so I just used an external calculator. If my attempt is unclear, let me know and I will try to explain (or even better, if someone can edit the question to make it clearer that is also great). </p>