Question

Why the join family of data verbs always have a data table as one of the arguments inside the parentheses？（In R）

Answer #1

lmost all joins between 2 `data.table`

s use a
notation where one of them is used as `i`

in a frame
applied to the other, and the joining columns are specified with
the `on`

parameter. However, in addition to the “basic”
joins, `data.table`

allows for special cases like
rolling joins, summarizing while joining, non-equi joins, etc. This
vignette will describe the notation to apply these joins with verbs
defined in `table.express`

, which, like the single-table
verbs, build `data.table`

expressions.

Basic joins

We’ll consider most of the `dplyr`

joining verbs in
this section:

`inner_join`

`left_join`

`right_join`

`anti_join`

`semi_join`

`full_join`

```
A <- data.table::data.table(x = rep(c("b", "a", "c"), each = 3),
y = c(1, 3, 6),
v = 1:9)
B <- data.table::data.table(x = c("c", "b"),
v2 = 8:7,
foo = c(4, 2))
A
```

```
#> x y v
#> 1: b 1 1
#> 2: b 3 2
#> 3: b 6 3
#> 4: a 1 4
#> 5: a 3 5
#> 6: a 6 6
#> 7: c 1 7
#> 8: c 3 8
#> 9: c 6 9
```

B

```
#> x v2 foo
#> 1: c 8 4
#> 2: b 7 2
```

The methods defined in `table.express`

accept the
`on`

part of the expression in their ellipsis:

```
A %>%
inner_join(B, x)
```

```
#> x y v v2 foo
#> 1: c 1 7 8 4
#> 2: c 3 8 8 4
#> 3: c 6 9 8 4
#> 4: b 1 1 7 2
#> 5: b 3 2 7 2
#> 6: b 6 3 7 2
```

```
A %>%
inner_join(B, x, v = v2)
```

```
#> x y v foo
#> 1: c 3 8 4
```

An important thing to note in the second example above is the
order in which the columns are given, i.e. that `v`

is
written before `v2`

, since the order is relevant for
`data.table`

. We can remember the correct order simply
by looking at which `data.table`

appears first in the
expression, and knowing that said `data.table`

’s columns
must appear first in the `on`

expressions. In this case,
`A`

appears before `B`

, so writing ```
v2 =
v
```

would not work.

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