is there other method to tag “many-to-many、one-to-many and many-to-one” three relationship, like the example below.

df1 <- data.frame(
  id1 = c(1, 1, 3, 4),
  value1 = c("A", "B", "C", "D")
)

df2 <- data.frame(
  id2 = c(1, 1, 3, 3, 4),
  value2 = c("X", "Y", "Z1", "Z2", "W")
)

# Performing a left join to tag the many-to-many relationship and adding a tag variable
result <- df1 %>%
  left_join(df2, by = c("id1" = "id2")) %>%
  group_by(id1) %>%
  mutate(
    tag = if_else(n() > 1, "many-to-many", "one-to-many")
  ) %>%
  ungroup()

result
# A tibble: 7 × 5
    id1 value1   id2 value2 tag        
  <dbl> <chr>  <dbl> <chr>  <chr>      
1     1 A          1 X      many-to-many
2     1 A          1 Y      many-to-many
3     1 B          1 X      many-to-many
4     1 B          1 Y      many-to-many
5     3 C          3 Z1     many-to-many
6     3 C          3 Z2     many-to-many
7     4 D          4 W      one-to-many

like the example above, I have taged the “many-to-many、one-to-many” but no many-to-one