Mutating joins

Source: R/dplyr_methods.R

Mutating joins add columns from y to x, matching observations based on the keys. There are four mutating joins: the inner join, and the three outer joins.

Inner join

An inner_join() only keeps observations from x that have a matching key in y.

The most important property of an inner join is that unmatched rows in either input are not included in the result. This means that generally inner joins are not appropriate in most analyses, because it is too easy to lose observations.

Outer joins

The three outer joins keep observations that appear in at least one of the data frames:

  • A left_join() keeps all observations in x.

  • A right_join() keeps all observations in y.

  • A full_join() keeps all observations in x and y.

# S3 method for class 'SingleCellExperiment'
full_join(x, y, by = NULL, copy = FALSE, suffix = c(".x", ".y"), ...)

Arguments

x, y

A pair of data frames, data frame extensions (e.g. a tibble), or lazy data frames (e.g. from dbplyr or dtplyr). See Methods, below, for more details.

by

A join specification created with join_by(), or a character vector of variables to join by.

If NULL, the default, *_join() will perform a natural join, using all variables in common across x and y. A message lists the variables so that you can check they're correct; suppress the message by supplying by explicitly.

To join on different variables between x and y, use a join_by() specification. For example, join_by(a == b) will match x$a to y$b.

To join by multiple variables, use a join_by() specification with multiple expressions. For example, join_by(a == b, c == d) will match x$a to y$b and x$c to y$d. If the column names are the same between x and y, you can shorten this by listing only the variable names, like join_by(a, c).

join_by() can also be used to perform inequality, rolling, and overlap joins. See the documentation at ?join_by for details on these types of joins.

For simple equality joins, you can alternatively specify a character vector of variable names to join by. For example, by = c("a", "b") joins x$a to y$a and x$b to y$b. If variable names differ between x and y, use a named character vector like by = c("x_a" = "y_a", "x_b" = "y_b").

To perform a cross-join, generating all combinations of x and y, see cross_join().

copy

If x and y are not from the same data source, and copy is TRUE, then y will be copied into the same src as x. This allows you to join tables across srcs, but it is a potentially expensive operation so you must opt into it.

suffix

If there are non-joined duplicate variables in x and y, these suffixes will be added to the output to disambiguate them. Should be a character vector of length 2.

...

Other parameters passed onto methods.

Value

An object of the same type as x (including the same groups). The order of the rows and columns of x is preserved as much as possible. The output has the following properties:

  • The rows are affect by the join type.

    • inner_join() returns matched x rows.

    • left_join() returns all x rows.

    • right_join() returns matched of x rows, followed by unmatched y rows.

    • full_join() returns all x rows, followed by unmatched y rows.

  • Output columns include all columns from x and all non-key columns from y. If keep = TRUE, the key columns from y are included as well.

  • If non-key columns in x and y have the same name, suffixes are added to disambiguate. If keep = TRUE and key columns in x and y have the same name, suffixes are added to disambiguate these as well.

  • If keep = FALSE, output columns included in by are coerced to their common type between x and y.

Many-to-many relationships

By default, dplyr guards against many-to-many relationships in equality joins by throwing a warning. These occur when both of the following are true:

  • A row in x matches multiple rows in y.

  • A row in y matches multiple rows in x.

This is typically surprising, as most joins involve a relationship of one-to-one, one-to-many, or many-to-one, and is often the result of an improperly specified join. Many-to-many relationships are particularly problematic because they can result in a Cartesian explosion of the number of rows returned from the join.

If a many-to-many relationship is expected, silence this warning by explicitly setting relationship = "many-to-many".

In production code, it is best to preemptively set relationship to whatever relationship you expect to exist between the keys of x and y, as this forces an error to occur immediately if the data doesn't align with your expectations.

Inequality joins typically result in many-to-many relationships by nature, so they don't warn on them by default, but you should still take extra care when specifying an inequality join, because they also have the capability to return a large number of rows.

Rolling joins don't warn on many-to-many relationships either, but many rolling joins follow a many-to-one relationship, so it is often useful to set relationship = "many-to-one" to enforce this.

Note that in SQL, most database providers won't let you specify a many-to-many relationship between two tables, instead requiring that you create a third junction table that results in two one-to-many relationships instead.

Methods

These functions are generics, which means that packages can provide implementations (methods) for other classes. See the documentation of individual methods for extra arguments and differences in behaviour.

Methods available in currently loaded packages:

See also

Other joins: cross_join(), filter-joins, nest_join()

Examples

data(pbmc_small)
tt <- pbmc_small
tt |> full_join(tibble::tibble(groups="g1", other=1:4))
#> Joining with `by = join_by(groups)`
#> tidySingleCellExperiment says: This operation lead to duplicated cell names. A data frame is returned for independent data analysis.
#> # A tibble: 212 × 32
#>    .cell orig.ident nCount_RNA nFeature_RNA RNA_snn_res.0.8 letter.idents groups
#>    <chr> <fct>           <dbl>        <int> <fct>           <fct>         <chr> 
#>  1 ATGC… SeuratPro…         70           47 0               A             g2    
#>  2 CATG… SeuratPro…         85           52 0               A             g1    
#>  3 CATG… SeuratPro…         85           52 0               A             g1    
#>  4 CATG… SeuratPro…         85           52 0               A             g1    
#>  5 CATG… SeuratPro…         85           52 0               A             g1    
#>  6 GAAC… SeuratPro…         87           50 1               B             g2    
#>  7 TGAC… SeuratPro…        127           56 0               A             g2    
#>  8 AGTC… SeuratPro…        173           53 0               A             g2    
#>  9 TCTG… SeuratPro…         70           48 0               A             g1    
#> 10 TCTG… SeuratPro…         70           48 0               A             g1    
#> # ℹ 202 more rows
#> # ℹ 25 more variables: RNA_snn_res.1 <fct>, file <chr>, ident <fct>,
#> #   PC_1 <dbl>, PC_2 <dbl>, PC_3 <dbl>, PC_4 <dbl>, PC_5 <dbl>, PC_6 <dbl>,
#> #   PC_7 <dbl>, PC_8 <dbl>, PC_9 <dbl>, PC_10 <dbl>, PC_11 <dbl>, PC_12 <dbl>,
#> #   PC_13 <dbl>, PC_14 <dbl>, PC_15 <dbl>, PC_16 <dbl>, PC_17 <dbl>,
#> #   PC_18 <dbl>, PC_19 <dbl>, tSNE_1 <dbl>, tSNE_2 <dbl>, other <int>