Extra Rows After Merging

After merging data, you will generate extra rows if one source has repeated IDs. See below.

No ID repetitions = join successful.

df1 <- data.frame(
  'id' = c(1, 2, 3),
  'y' = c(20, 30, 40)
)

df2 <- data.frame(
  'id' = c(1, 2, 3),
  'x' = c(rnorm(3, 30, 5))
)

library(tidyverse)

df <- left_join(df1, df2)
df

  id  y        x
1  1 20 30.09988
2  2 30 29.41230
3  3 40 23.84746

ID repetitions = join unsuccessful

data1 <- data.frame(
  'id' = c(1, 2, 3),
  'y' = c(20, 23, 40)
)

data2 <- data.frame(
  'id' = c(1, 2, 2), # repeated id
  'x' = c(60, 70, 80)
)

library(tidyverse)

dataf <- left_join(data1, data2)
dataf # has 4 rows rather than 3

  id  y  x
1  1 20 60
2  2 23 70
3  2 23 80
4  3 40 NA

Bo\(^2\)m =)