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My problem is that I would like to do a left join with dplyr like this:

x <- left.join(TableA, TableB)

How can I do to bring just a specific field from TableB? Imagine TableB has 3 fields x,y,z and I just want x in TableA

pogibas
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Suanbit
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  • What field do you want to match the two tables on? Use use `select()` to keep only the columns for joining and whatever columns you want to merge in. It would be easier to help with a [reproducible example](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) – MrFlick Sep 06 '17 at 14:30
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    You can use `select` to keep only those vars in `TableB` first: `x <- TableA %>% left_join(select(TableB, id, x), by = "id")` – austensen Sep 06 '17 at 14:31
  • but in the example that you said "id" is the common var in each table to join the two tables, What I want to do is... imagine TableA containds the vars "id" and "euros", TableB has "id" - "city" - "country". What I want to do is bring city(which is on tableB) to tableA but only that field not "country " for exameple. – Suanbit Sep 07 '17 at 16:55

3 Answers3

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To join both tables as desired, you have to select field x and an id-field from TableB for the join. You can do this with the select() function. Let's assume for the join that your id-field in TableB is y.

x <- TableA %>% 
  left_join(select(TableB, x, y), by = c("id" = "y"))
Søren Schaffstein
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You subset TableB first. So you do something like:

TableB <- TableB[,'x,']

left_join(TableA,TableB)

The obvious disadvantage of this method is that we are bound to join with column x. Suppose you want to join with column y but want only x in the end result, then you can do the following:

select(left_join(TableA,TableB, by = 'y'), -c(y,z))
Adhishwar
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x <- TableA %>% left_join(select(TableB, id), by.x = "id")

Sandeep
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  • That joins all fields in tableB to tableA by "id". I want to join just one var from tableB to tableA :) Thanks! – Suanbit Sep 07 '17 at 16:56