Join two data tables and use only one column from second dt

Question

Let\'s say I have two data tables (dt1 and dt2), and I want to get dt3 using data tables. A,B,C,E,F,G,H are column names. dt1 key is column A, and dt2 key is column E. Data

Answer 1

In order to perform a left join to df1 and add H column from df2, you can combine binary join with the update by reference operator (:=)

setkey(setDT(dt1), A) 
dt1[dt2, H := i.H]

See here and here for detailed explanation on how it works

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With the devel version (v >= 1.9.5) we could make it even shorter by specifying the key within setDT (as pointed by @Arun)

setDT(dt1, key = "A")[dt2, H := i.H]

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Edit 24/7/2015

You can now run a binary join using the new on parameter without setting keys

setDT(dt1)[dt2, H := i.H, on = c(A = "E")]