Join diagram tweaking.

Add diagrams for filtering joins.

relational-data.Rmd

@@ -388,7 +388,25 @@ Instead you can use a semi-join, which connects the two tables like a mutating j
 flights %>% semi_join(top_dest)
 ```
 
-The inverse of a semi-join is an anti-join. An anti-join keeps the rows that _don't_ have a match, and are useful for diagnosing join mismatches. For example, when connecting `flights` and `planes`, you might be interested to know that there are many `flights` that don't have a match in `planes`:
+Graphically, a semi-join looks like this:
+
+```{r, echo = FALSE, out.width = "50%"}
+knitr::include_graphics("diagrams/join-semi.png")
+```
+
+Only the existence of a match is important; it doesn't match what observation is matched. This means that filtering joins never duplicate rows like mutating joins do:
+
+```{r, echo = FALSE, out.width = "50%"}
+knitr::include_graphics("diagrams/join-semi-many.png")
+```
+
+The inverse of a semi-join is an anti-join. An anti-join keeps the rows that _don't_ have a match:
+
+```{r, echo = FALSE, out.width = "50%"}
+knitr::include_graphics("diagrams/join-anti.png")
+```
+
+Anti-joins are are useful for diagnosing join mismatches. For example, when connecting `flights` and `planes`, you might be interested to know that there are many `flights` that don't have a match in `planes`:
 
 ```{r}
 flights %>%