join_by {dplyr}R Documentation

Join specifications


join_by() constructs a specification that describes how to join two tables using a small domain specific language. The result can be supplied as the by argument to any of the join functions (such as left_join()).





Expressions specifying the join.

Each expression should consist of one of the following:

  • Equality condition: ==

  • Inequality conditions: >=, >, <=, or <

  • Rolling helper: closest()

  • Overlap helpers: between(), within(), or overlaps()

Other expressions are not supported. If you need to perform a join on a computed variable, e.g. join_by(sales_date - 40 >= promo_date), you'll need to precompute and store it in a separate column.

Column names should be specified as quoted or unquoted names. By default, the name on the left-hand side of a join condition refers to the left-hand table, unless overridden by explicitly prefixing the column name with either ⁠x$⁠ or ⁠y$⁠.

If a single column name is provided without any join conditions, it is interpreted as if that column name was duplicated on each side of ==, i.e. x is interpreted as x == x.

Join types

The following types of joins are supported by dplyr:

Equality, inequality, rolling, and overlap joins are discussed in more detail below. Cross joins are implemented through cross_join().

Equality joins

Equality joins require keys to be equal between one or more pairs of columns, and are the most common type of join. To construct an equality join using join_by(), supply two column names to join with separated by ==. Alternatively, supplying a single name will be interpreted as an equality join between two columns of the same name. For example, join_by(x) is equivalent to join_by(x == x).

Inequality joins

Inequality joins match on an inequality, such as >, >=, <, or <=, and are common in time series analysis and genomics. To construct an inequality join using join_by(), supply two column names separated by one of the above mentioned inequalities.

Note that inequality joins will match a single row in x to a potentially large number of rows in y. Be extra careful when constructing inequality join specifications!

Rolling joins

Rolling joins are a variant of inequality joins that limit the results returned from an inequality join condition. They are useful for "rolling" the closest match forward/backwards when there isn't an exact match. To construct a rolling join, wrap an inequality with closest().

closest() will always use the left-hand table (x) as the primary table, and the right-hand table (y) as the one to find the closest match in, regardless of how the inequality is specified. For example, closest(y$a >= x$b) will always be interpreted as closest(x$b <= y$a).

Overlap joins

Overlap joins are a special case of inequality joins involving one or two columns from the left-hand table overlapping a range defined by two columns from the right-hand table. There are three helpers that join_by() recognizes to assist with constructing overlap joins, all of which can be constructed from simpler inequalities.

These conditions assume that the ranges are well-formed and non-empty, i.e. x_lower <= x_upper when bounds are treated as "[]", and x_lower < x_upper otherwise.

Column referencing

When specifying join conditions, join_by() assumes that column names on the left-hand side of the condition refer to the left-hand table (x), and names on the right-hand side of the condition refer to the right-hand table (y). Occasionally, it is clearer to be able to specify a right-hand table name on the left-hand side of the condition, and vice versa. To support this, column names can be prefixed by ⁠x$⁠ or ⁠y$⁠ to explicitly specify which table they come from.


sales <- tibble(
  id = c(1L, 1L, 1L, 2L, 2L),
  sale_date = as.Date(c("2018-12-31", "2019-01-02", "2019-01-05", "2019-01-04", "2019-01-01"))

promos <- tibble(
  id = c(1L, 1L, 2L),
  promo_date = as.Date(c("2019-01-01", "2019-01-05", "2019-01-02"))

# Match `id` to `id`, and `sale_date` to `promo_date`
by <- join_by(id, sale_date == promo_date)
left_join(sales, promos, by)

# For each `sale_date` within a particular `id`,
# find all `promo_date`s that occurred before that particular sale
by <- join_by(id, sale_date >= promo_date)
left_join(sales, promos, by)

# For each `sale_date` within a particular `id`,
# find only the closest `promo_date` that occurred before that sale
by <- join_by(id, closest(sale_date >= promo_date))
left_join(sales, promos, by)

# If you want to disallow exact matching in rolling joins, use `>` rather
# than `>=`. Note that the promo on `2019-01-05` is no longer considered the
# closest match for the sale on the same date.
by <- join_by(id, closest(sale_date > promo_date))
left_join(sales, promos, by)

# Same as before, but also require that the promo had to occur at most 1
# day before the sale was made. We'll use a full join to see that id 2's
# promo on `2019-01-02` is no longer matched to the sale on `2019-01-04`.
sales <- mutate(sales, sale_date_lower = sale_date - 1)
by <- join_by(id, closest(sale_date >= promo_date), sale_date_lower <= promo_date)
full_join(sales, promos, by)

# ---------------------------------------------------------------------------

segments <- tibble(
  segment_id = 1:4,
  chromosome = c("chr1", "chr2", "chr2", "chr1"),
  start = c(140, 210, 380, 230),
  end = c(150, 240, 415, 280)

reference <- tibble(
  reference_id = 1:4,
  chromosome = c("chr1", "chr1", "chr2", "chr2"),
  start = c(100, 200, 300, 415),
  end = c(150, 250, 399, 450)

# Find every time a segment `start` falls between the reference
# `[start, end]` range.
by <- join_by(chromosome, between(start, start, end))
full_join(segments, reference, by)

# If you wanted the reference columns first, supply `reference` as `x`
# and `segments` as `y`, then explicitly refer to their columns using `x$`
# and `y$`.
by <- join_by(chromosome, between(y$start, x$start, x$end))
full_join(reference, segments, by)

# Find every time a segment falls completely within a reference.
# Sometimes using `x$` and `y$` makes your intentions clearer, even if they
# match the default behavior.
by <- join_by(chromosome, within(x$start, x$end, y$start, y$end))
inner_join(segments, reference, by)

# Find every time a segment overlaps a reference in any way.
by <- join_by(chromosome, overlaps(x$start, x$end, y$start, y$end))
full_join(segments, reference, by)

# It is common to have right-open ranges with bounds like `[)`, which would
# mean an end value of `415` would no longer overlap a start value of `415`.
# Setting `bounds` allows you to compute overlaps with those kinds of ranges.
by <- join_by(chromosome, overlaps(x$start, x$end, y$start, y$end, bounds = "[)"))
full_join(segments, reference, by)

[Package dplyr version 1.1.4 Index]