cc @tarekauel and @davies

LGTM

@rxin Could you do this little fix as well?
https://github.com/apache/spark/pull/7505/files

Why do we switch from day_of_month to dayofmonth? Most SQL implementations use underscores:
MySQL SAP HANA Oracle
I would prefer underscores, because they improve the readability, if you write all SQL stuff in caps, like:
SELECT name, age, DAY_OF_MONTH(birthday) AS birthday FROM people WHERE age > 15 compared to SELECT name, age, DAYOFMONTH(birthday) AS birthday FROM people WHERE age > 15
I'm not a Python pro, but I thought that underscores are 'pythonic', aren't they?

Both MySQL and HANA use dayofmonth, without the underscore?

Test build #37759 has finished for PR 7506 at commit e44a4a0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class Hour(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class Minute(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class Second(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class DayOfYear(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class Year(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class Quarter(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class Month(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class DayOfMonth(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class WeekOfYear(child: Expression) extends UnaryExpression with ImplicitCastInputTypes
  • case class DateFormatClass(left: Expression, right: Expression) extends BinaryExpression

I've merged this.