set.seed(123)
x <- data.frame(id = letters[1:10], valx = rnorm(10))
dim(x)
# [1] 10  2
# 10 rows, 2 columns

y <- data.frame(id = sample(letters[1:10], 5), valy = rnorm(5), valz = LETTERS[3:7])
dim(y)
# [1] 5 3
# 5 rows, 3 columns

merge(x, y, by = "id", all.x = TRUE)

id        valx
1   a -0.56047565
2   b -0.23017749
3   c  1.55870831
4   d  0.07050839
5   e  0.12928774
6   f  1.71506499
7   g  0.46091621
8   h -1.26506123
9   i -0.68685285
10  j -0.44566197

id        valx
1   a -0.56047565
2   b -0.23017749
3   c  1.55870831
4   d  0.07050839
5   e  0.12928774
6   f  1.71506499
7   g  0.46091621
8   h -1.26506123
9   i -0.68685285
10  j -0.44566197

id        valx       valy valz
1   a -0.56047565         NA <NA>
2   b -0.23017749         NA <NA>
3   c  1.55870831         NA <NA>
4   d  0.07050839  0.8255398    G
5   e  0.12928774         NA <NA>
6   f  1.71506499 -1.0488931    E
7   g  0.46091621  0.2382129    D
8   h -1.26506123         NA <NA>
9   i -0.68685285  0.5490967    C
10  j -0.44566197  1.2947633    F