data.table (version 1.9.6)

merge: Merge Two Data Tables

Description

Fast merge of two data.tables.

This merge method for data.table behaves very similarly to that of data.frames with one major exception: By default, the columns used to merge the data.tables are the shared key columns rather than the shared columns with the same names. Set the by, or by.x, by.y arguments explicitly to override this default.

Usage

"merge"(x, y, by = NULL, by.x = NULL, by.y = NULL,
all = FALSE, all.x = all, all.y = all, sort = TRUE, suffixes = c(".x", ".y"),
allow.cartesian=getOption("datatable.allow.cartesian"),  # default FALSE
...)

Arguments

x, y

data tables. y is coerced to a data.table if it isn't one already.

by

A vector of shared column names in x and y to merge on. This defaults to the shared key columns between the two tables. If y has no key columns, this defaults to the key of x.

by.x, by.y

Vectors of column names in x and y to merge on.

all

logical; all = TRUE is shorthand to save setting both all.x = TRUE and all.y = TRUE.

all.x

logical; if TRUE, then extra rows will be added to the output, one for each row in x that has no matching row in y. These rows will have 'NA's in those columns that are usually filled with values from y. The default is FALSE, so that only rows with data from both x and y are included in the output.

all.y

logical; analogous to all.x above.

sort

logical. If TRUE (default), the merged data.table is sorted by setting the key to the by / by.x columns. If FALSE, the result is not sorted.

suffixes

A character(2) specifying the suffixes to be used for making non-by column names unique. The suffix behavior works in a similar fashion as the merge.data.frame method does.

allow.cartesian

See allow.cartesian in [.data.table.

...

Not used at this time.

Value

A new data.table based on the merged data tables, sorted by the columns set (or inferred for) the by argument.

Details

merge is a generic function in base R. It dispatches to either the merge.data.frame method or merge.data.table method depending on the class of its first argument. In versions < v1.9.6, if the specified columns in by was not the key (or head of the key) of x or y, then a copy is first rekeyed prior to performing the merge. This was less performant and memory inefficient.

In version v1.9.4 secondary keys was implemented. In v1.9.6, the concept of secondary keys has been extended to merge. No deep copies are made anymore and therefore very performant and memory efficient. Also there is better control for providing the columns to merge on with the help of newly implemented by.x and by.y arguments.

For a more data.table-centric way of merging two data.tables, see [.data.table; e.g., x[y, ...]. See FAQ 1.12 for a detailed comparison of merge and x[y, ...].

Merges on numeric columns: Columns of numeric types (i.e., double) have their last two bytes rounded off while computing order, by defalult, to avoid any unexpected behaviour due to limitations in representing floating point numbers precisely. For large numbers (integers > 2^31), we recommend using bit64::integer64. Have a look at setNumericRounding to learn more.

See Also

data.table, [.data.table, merge.data.frame

Examples

(dt1 <- data.table(A = letters[1:10], X = 1:10, key = "A"))
(dt2 <- data.table(A = letters[5:14], Y = 1:10, key = "A"))
merge(dt1, dt2)
merge(dt1, dt2, all = TRUE)

(dt1 <- data.table(A = letters[rep(1:3, 2)], X = 1:6, key = "A"))
(dt2 <- data.table(A = letters[rep(2:4, 2)], Y = 6:1, key = "A"))
merge(dt1, dt2, allow.cartesian=TRUE)

(dt1 <- data.table(A = c(rep(1L, 5), 2L), B = letters[rep(1:3, 2)], X = 1:6, key = "A,B"))
(dt2 <- data.table(A = c(rep(1L, 5), 2L), B = letters[rep(2:4, 2)], Y = 6:1, key = "A,B"))
merge(dt1, dt2)
merge(dt1, dt2, by="B", allow.cartesian=TRUE)

# test it more:
d1 <- data.table(a=rep(1:2,each=3), b=1:6, key="a,b")
d2 <- data.table(a=0:1, bb=10:11, key="a")
d3 <- data.table(a=0:1, key="a")
d4 <- data.table(a=0:1, b=0:1, key="a,b")

merge(d1, d2)
merge(d2, d1)
merge(d1, d2, all=TRUE)
merge(d2, d1, all=TRUE)

merge(d3, d1)
merge(d1, d3)
merge(d1, d3, all=TRUE)
merge(d3, d1, all=TRUE)

merge(d1, d4)
merge(d1, d4, by="a", suffixes=c(".d1", ".d4"))
merge(d4, d1)
merge(d1, d4, all=TRUE)
merge(d4, d1, all=TRUE)

# new feature, no need to set keys anymore
set.seed(1L)
d1 <- data.table(a=sample(rep(1:3,each=2)), z=1:6)
d2 <- data.table(a=2:0, z=10:12)
merge(d1, d2, by="a")
merge(d1, d2, by="a", all=TRUE)

# new feature, using by.x and by.y arguments
setnames(d2, "a", "b")
merge(d1, d2, by.x="a", by.y="b")
merge(d1, d2, by.x="a", by.y="b", all=TRUE)
merge(d2, d1, by.x="b", by.y="a")