join: Fast Table Joins

In SebKrantz/collapse: Advanced and Fast Data Transformation

Fast Table Joins

Description

Join two data frame like objects x and y on columns. Inspired by polars and by default uses a vectorized hash join algorithm (workhorse function fmatch).

Usage

join(x, y,
     on = NULL,
     how = "left",
     suffix = NULL,
     validate = "m:m",
     multiple = FALSE,
     sort = FALSE,
     keep.col.order = TRUE,
     drop.dup.cols = FALSE,
     verbose = .op[["verbose"]],
     column = NULL,
     attr = NULL,
     ...
)

Arguments

x	a data frame-like object. The result will inherit the attributes of this object.
y	a data frame-like object to join with x.
on	character. vector of columns to join on. NULL uses intersect(names(x), names(y)). Use a named vector to match columns named differently in x and y, e.g. c("x_id" = "y_id").
how	character. Join type: "left", "right", "inner", "full", "semi" or "anti". The first letter suffices.
suffix	character(1 or 2). Suffix to add to duplicate column names. NULL renames duplicate y columns as paste(col, y_name, sep = "_"), where y_name = as.character(substitute(y)) i.e. the name of the data frame as passed into the function. In general, passing suffix length 1 will only rename y, whereas a length 2 suffix will rename both x and y, respectively. If verbose > 0 a message will be printed.
validate	character. (Optional) check if join is of specified type. One of "1:1", "1:m", "m:1" or "m:m". The default "m:m" does not perform any checks. Checks are done before the actual join step and failure results in an error. Note that this argument does not affect the result, it only triggers a check.
multiple	logical. Handling of rows in x with multiple matches in y. The default FALSE takes the first match in y. TRUE returns every match in y (a full cartesian product), increasing the size of the joined table.
sort	logical. TRUE implements a sort-merge-join: a completely separate join algorithm that sorts both datasets on the join columns using radixorder and then matches the rows without hashing. Note that in this case the result will be sorted by the join columns, whereas sort = FALSE preserves the order of rows in x.
keep.col.order	logical. Keep order of columns in x? FALSE places the on columns in front.
drop.dup.cols	instead of renaming duplicate columns in x and y using suffix, this option simply drops them: TRUE or "y" drops them from y, "x" from x.
verbose	integer. Prints information about the join. One of 0 (off), 1 (default, see Details) or 2 (additionally prints the classes of the on columns). Note: verbose > 0 or validate != "m:m" invoke the count argument to fmatch, so verbose = 0 is slightly more efficient.
column	(optional) name for an extra column to generate in the output indicating which dataset a record came from. TRUE calls this column ".join" (inspired by STATA's '_merge' column). By default this column is generated as the last column, but, if keep.col.order = FALSE, it is placed after the 'on' columns. The column is a factor variable with levels corresponding to the dataset names (inferred from the input) or "matched" for matched records. Alternatively, it is possible to specify a list of 2, where the first element is the column name, and the second a length 3 (!) vector of levels e.g. column = list("joined", c("x", "y", "x_y")), where "x_y" replaces "matched". The column has an additional attribute "on.cols" giving the join columns corresponding to the factor levels. See Examples.
attr	(optional) name for attribute providing information about the join performed (including the output of fmatch) to the result. TRUE calls this attribute "join.match". Note: this also invokes the count argument to fmatch.
...	further arguments to fmatch (if sort = FALSE). Notably, overid can bet set to 0 or 2 (default 1) to control the matching process if the join condition more than identifies the records.

Details

If verbose > 0, join prints a compact summary of the join operation using cat. If the names of x and y can be extracted (if as.character(substitute(x)) yields a single string) they will be displayed (otherwise 'x' and 'y' are used) followed by the respective join keys in brackets. This is followed by a summary of the records used from each table. If multiple = FALSE, only the first matches from y are used and counted here (or the first matches of x if how = "right"). Note that if how = "full" any further matches are simply appended to the results table, thus it may make more sense to use multiple = TRUE with the full join when suspecting multiple matches.

If multiple = TRUE, join performs a full cartesian product matching every key in x to every matching key in y. This can considerably increase the size of the resulting table. No memory checks are performed (your system will simply run out of memory; usually this should not terminate R).

In both cases, join will also determine the average order of the join as the number of records used from each table divided by the number of unique matches and display it between the two tables at up to 2 digits. For example "<4:1.5>" means that on average 4 records from x match 1.5 records from y, implying on average 4*1.5 = 6 records generated per unique match. If multiple = FALSE "1st" will be displayed for the using table (y unless how = "right"), indicating that there could be multiple matches but only the first is retained. Note that an order of '1' on either table must not imply that the key is unique as this value is generated from round(v, 2). To be sure about a keys uniqueness employ the validate argument.

Value

A data frame-like object of the same type and attributes as x. "row.names" of x are only preserved in left-join operations.

See Also

fmatch, Data Frame Manipulation, Fast Grouping and Ordering, Collapse Overview

Examples

df1 <- data.frame(
  id1 = c(1, 1, 2, 3),
  id2 = c("a", "b", "b", "c"),
  name = c("John", "Jane", "Bob", "Carl"),
  age = c(35, 28, 42, 50)
)
df2 <- data.frame(
  id1 = c(1, 2, 3, 3),
  id2 = c("a", "b", "c", "e"),
  salary = c(60000, 55000, 70000, 80000),
  dept = c("IT", "Marketing", "Sales", "IT")
)

# Different types of joins
for(i in c("l","i","r","f","s","a"))
    join(df1, df2, how = i) |> print()

# With multiple matches
for(i in c("l","i","r","f","s","a"))
    join(df1, df2, on = "id2", how = i, multiple = TRUE) |> print()

# Adding join column: useful esp. for full join
join(df1, df2, how = "f", column = TRUE)
# Custom column + rearranging
join(df1, df2, how = "f", column = list("join", c("x", "y", "x_y")), keep = FALSE)

# Attaching match attribute
str(join(df1, df2, attr = TRUE))

SebKrantz/collapse documentation built on Dec. 16, 2024, 7:26 p.m.