base::merge()とdplyr::*_join()

対応表 おおむねこんな感じ？

merge	dplyr
merge (x, y)	inner_join(x, y)
merge (x, y, all.x = TRUE)	left_join(x, y)
merge (x, y, all.y = TRUE)	right_join(x,y)
merge (x, y, all = TRUE)	full_join(x,y)

実行例

> (x <- data.frame (a = 1:3, b = c ("A", "B", "C"), x1 = 11:13))
  a b x1
1 1 A 11
2 2 B 12
3 3 C 13
> (y <- data.frame (a = 2:4, b = c ("B", "C", "D"), y1 = 21:23))
  a b y1
1 2 B 21
2 3 C 22
3 4 D 23
> # デフォルトでは重複する要素のある行のみ出力
> merge (x, y)
  a b x1 y1
1 2 B 12 21
2 3 C 13 22
> dplyr::inner_join(x, y, by = c("a", "b"))#に相当
  a b x1 y1
1 2 B 12 21
2 3 C 13 22
Warning message:
In inner_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector

ただしb列は、それぞれの水準が異なるので文字列化されている

「x」列を基準に結合

> merge (x, y, all.x = TRUE)
  a b x1 y1
1 1 A 11 NA
2 2 B 12 21
3 3 C 13 22
> dplyr::left_join(x, y, by = c("a", "b"))#に相当
  a b x1 y1
1 1 A 11 NA
2 2 B 12 21
3 3 C 13 22
Warning message:
In left_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector

「y」列を基準に結合

> merge (x, y, all.y = TRUE)
  a b x1 y1
1 2 B 12 21
2 3 C 13 22
3 4 D NA 23
> dplyr::right_join(x, y, by = c("a", "b"))#に相当
  a b x1 y1
1 2 B 12 21
2 3 C 13 22
3 4 D NA 23
Warning message:
In right_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector

両方の列要素を網羅した結合

> merge (x, y, all = TRUE)
  a b x1 y1
1 1 A 11 NA
2 2 B 12 21
3 3 C 13 22
4 4 D NA 23
> dplyr::full_join(x, y, by = c("a", "b"))#に相当
  a b x1 y1
1 1 A 11 NA
2 2 B 12 21
3 3 C 13 22
4 4 D NA 23
Warning message:
In outer_join_impl(x, y, by$x, by$y) :
  joining factors with different levels, coercing to character vector