Newbie question on how to extract column value

[James Starks]

I am very new to Spark. Just successfully setup Spark SQL connecting to 
postgresql database, and am able to display table with code

    sparkSession.sql("SELECT id, url from table_a where col_b <> '' ").show()

Now I want to perform filter and map function on col_b value. In plain scala it 
would be something like

    Seq((1, "http://a.com/a";), (2, "http://b.com/b";), (3, "unknown")).filter { 
case (_, url) => isValid(url) }.map { case (id, url)  => (id, pathOf(url)) }

where filter will remove invalid url, and then map (id, url) to (id, path of 
url).

However, when applying this concept to spark sql with code snippet

    sparkSession.sql("...").filter(isValid($"url"))

Compiler complains type mismatch because $"url" is ColumnName type. How can I 
extract column value i.e. http://... for the column url in order to perform 
filter function?

Thanks

Java 1.8.0
Scala 2.11.8
Spark 2.1.0