title: Spark Scala join Seq and lit
body: <p>I'm learning Spark Scala programming, and have a prototype like this</p>
  
  <pre><code>val varible_a = varible_b.join(varible_c, Seq("column_a"))
      .withColumn ("column_b" , lit(50))
  </code></pre>
  
  <p>I don't know what's meaning of "<code>Seq</code>" and "<code>lit(50)</code>"</p>
  
  <p>Looks like <em>varible_b</em> to join <em>varible_c</em>, but there is no join condition
  Not sure what is meaning for "Seq"</p>
  
  <p>Append a column "<code>column_b</code>" with "<code>lit(50)</code>", What is <code>lit(50)</code>?</p>
  
  <p>Thank you very much,
  Tom</p>