9.3 many to one OR one to many

In a many-to-one merge, one of the tables have duplicate keys

(x <- tribble(
    ~key, ~val_x,
    1, "x1",
    2, "x2",
    3, "x3",
    1, "x4",
    1, "x5",
    1, "x6"
))
## # A tibble: 6 x 2
##     key val_x
##   <dbl> <chr>
## 1     1 x1   
## 2     2 x2   
## 3     3 x3   
## 4     1 x4   
## 5     1 x5   
## 6     1 x6

(y <- tribble(
    ~key, ~val_y,
    1, "y1",
    2, "y2",
    4, "y3"
))
## # A tibble: 3 x 2
##     key val_y
##   <dbl> <chr>
## 1     1 y1   
## 2     2 y2   
## 3     4 y3

x %>%
    left_join(y, by = 'key')
## # A tibble: 6 x 3
##     key val_x val_y
##   <dbl> <chr> <chr>
## 1     1 x1    y1   
## 2     2 x2    y2   
## 3     3 x3    <NA> 
## 4     1 x4    y1   
## 5     1 x5    y1   
## 6     1 x6    y1