在R中按ID左联接变量

Question

I have two tables df1 and df2. Now I want to left join a variable newVar of df2 to df1 by the same id. But the problem is that there are some ids exist in df1 but not in df2.

So I want that if df1.id exists in df2, then df1.newVar=df2.newVar, else df1.newVar=0 .

In R, I've tried df1$newVar= ifelse((df1$id %in% df2$id), df2$newVar, 0) but the result was not good.

I've tried also sqldf('select df1.*, df2.newVar as newVar from df1 left outer join df2 on df2.id= df1.id') but R gives an error.

Here is an example of df1:

id   var1
A     1
A     2
B     1

df2:

id   newVar
A     1
A     1

I want the result of df1 looks like:

id   var1   newVar
A     1       1
A     2       1
B     1       0

Answer 1

You can use left_join{dplyr} to do that work.

library("dplyr")
df1 <- data_frame(
  id = c("A", "A", "B"),
  var1 = c(1, 2, 1)
)

df2 <- data_frame(
  id = c("A", "A"),
  newvar = c(1, 1)
)

unique(left_join(df1,df2,by="id"))

And the result is :

# A tibble: 3 × 3
     id  var1 newvar
  <chr> <dbl>  <dbl>
1     A     1      1
2     A     2      1
3     B     1     NA