A character vector of variables to join by.
If NULL, the default, *_join() will perform a natural join, using all
variables in common across x and y. A message lists the variables so that you
can check they're correct; suppress the message by supplying by explicitly.
To join by different variables on x and y, use a named vector.
For example, by = c("a" = "b") will match x$a to y$b.
To join by multiple variables, use a vector with length > 1.
For example, by = c("a", "b") will match x$a to y$a and x$b to
y$b. Use a named vector to match different variables in x and y.
For example, by = c("a" = "b", "c" = "d") will match x$a to y$b and
x$c to y$d.
To perform a cross-join, generating all combinations of x and y,
use by = character().