title: Updating in relational algebra
body: <p>Consider a relation <code>Human(name, age, height, weight)</code>. To update an attribute, e.g. age for the whole relation, I can write <code>Human &lt;- π name,age*2,height,weight (Human)</code>. But what if I want to update just some specific tuple? From SQL: <code>UPDATE Human SET age = 20 WHERE name = &quot;John&quot;</code>. Where do I put sigma/σ (restriction/selection)?</p>