title: Liquibase - How to skip changesets that have been executed
body: <p>I have a dropwizard application backed by a mysql database. I am using the liquibase wrapper for database migrations</p>
  
  <p>To start with I used the ' db dump' command to autogenerate the migrations.xml file for me.</p>
  
  <p>Now I am working on refactoring the database and I want to be able to update specific column names and names of tables. </p>
  
  <p>I use the preconditions to skip the already generated tables to skip the '<code>createTable</code>' commands </p>
  
  <pre><code>&lt;preConditions onFail="MARK_RAN"&gt;
              &lt;not&gt;
                  &lt;tableExists tableName="myTable" /&gt;
              &lt;/not&gt;
          &lt;/preConditions&gt;
  </code></pre>
  
  <p>How do I now skip the execution of the primary key and foreign key contraints? Is there a precondition at the changelog level that I can use to indicate "skip already executed changesets'? Or do I just create a new migrations.xml? Will this delete the existing data?</p>