How do I select columns that may or may not exist?

You can use any_of() (from the tidyselect package):

df %>% select(any_of(c("year", "boo")))

In the devel version of dplyr

df %>%
   select(year, contains("boo"))
#     year
#1  2000
#2  2001
#3  2002
#4  2003
#5  2004
#6  2005
#7  2006
#8  2007
#9  2008
#10 2009
#11 2010

gives the expected output

Otherwise one option would be to use one_of

df %>%
   select(one_of("year", "boo"))

It returns a warning message if the column is not available

Other option is matches

df %>%
  select(matches("year|boo"))

Here's a slight twist using dplyr::select_if() that will not throw an Unknown columns: warning if you try to select a column name does not exist, in this case 'bad_column':

df %>% 
  select_if(names(.) %in% c('year', 'bar', 'bad_column'))