title: What's a "single sign on" table used for and why is this column list necessary?
body: <p>I was looking at a schema and trying to figure out what this table could be used for:</p>
  
  <pre><code>CREATE TABLE "single_sign_ons" 
  "token" VARCHAR(64) NOT NULL, 
  "ip" VARCHAR(32) NOT NULL, 
  "expired_at" DATETIME NOT NULL, 
  "one_time" VARCHAR(64), 
  "created_at" DATETIME, 
  "updated_at" DATETIME, 
  "user_id" INTEGER, 
  PRIMARY KEY("token"));
  </code></pre>
  
  <p>What is "single sign on" and why did the author of this table include these columns for this purpose?</p>