title: Conditional INSERT INTO statement in postgres
body: <p>I'm writing a booking procedure for a mock airline booking database and what I really want to do is something like this:</p>
  
  <pre><code>IF EXISTS (SELECT * FROM LeadCustomer 
      WHERE FirstName = 'John' AND Surname = 'Smith') 
  THEN
     INSERT INTO LeadCustomer (Firstname, Surname, BillingAddress, email) 
     VALUES ('John', 'Smith', '6 Brewery close,
              Buxton, Norfolk', 'cmp.testing@example.com');
  </code></pre>
  
  <p>But Postgres doesn't support <code>IF</code> statements without loading the PL/pgSQL extension. I was wondering if there was a way to do some equivalent of this or if there's just going to have to be some user interaction in this step?</p>