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POTranslating SQL joins on foreign keys to R data.table syntax
 

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  1. POTranslating SQL joins on foreign keys to R data.table syntax
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    <p>The <a href="http://cran.r-project.org/web/packages/data.table/index.html"><code>data.table</code></a> package provides many of the same table handling methods as SQL. If a table has a key, that key consists of one or more columns. But a table can't have more than one key, because it can't be sorted in two different ways at the same time.</p> <p>In this example, <code>X</code> and <code>Y</code> are <code>data.table</code>s with a single key column "id"; <code>Y</code> also has a non-key column "x_id".</p> <pre><code> X &lt;- data.table(id = 1:5, a=4:8,key="id") Y &lt;- data.table(id = c(1,1, 3,5,7), x_id=c(1,4:1), key="id") </code></pre> <p>The following syntax would join the tables on their keys:</p> <pre><code> X[Y] </code></pre> <p>How can I translate the following SQL syntax to data.table code?</p> <pre><code> select * from X join Y on X.id = Y.x_id; </code></pre> <p>The closest that I have gotten is:</p> <pre><code>Y[X,list(id, x_id),by = x_id,nomatch=0] </code></pre> <p>However, this does not do the same inner join as the SQL statement.</p> <hr> <p>Here is a more clear example in which the foreign key is y_id, and we want the join to look up values of Y2 where <code>X2$y_id = Y2$id</code>. </p> <pre><code> X2 &lt;- data.table(id = 1:5, y_id = c(1,1,2,2,2), key="id") Y2 &lt;- data.table(id = 1:5, b = letters[1:5], key="id") </code></pre> <p>I would like to produce the table:</p> <pre><code> id y_id b 1 1 "a" 2 1 "a" 3 2 "b" 4 2 "b" 5 2 "b" </code></pre> <p>similar to what is done by the following kludge:</p> <pre><code>&gt; merge(data.frame(X2), data.frame(Y2), by.x = "y_id", by.y = "id") y_id id b 1 1 1 a 2 1 2 a 3 2 3 b 4 2 4 b 5 2 5 b </code></pre> <p>However, when I do this:</p> <pre><code> X2[Y2, 1:2,by = y_id] </code></pre> <p>I do not get the desired result:</p> <pre><code> y_id V1 [1,] 1 1 [2,] 1 2 [3,] 2 1 [4,] 2 2 </code></pre>
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    1. USMatt Dowle
    1. USDavid LeBauer
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    1. COYou can temporarily change `X2` keys and set them to `"y_id"`; then perform a normal join `Y2[X2]` (or `X2[Y2]` depending on the direction) and then restore the previous key of `X2`.
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      1. USdigEmAll
    2. CO@digEmAll that is useful, I thought that was what the `by` argument does... but can this be applied to a multiple (>2) table join where the target (left) table has >1 foreign key?
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      1. USDavid LeBauer
    3. COI agree with @digEmAll: `setkey(X2, y_id)` followed by `X2[Y2, nomatch=0]` is all you need for your example. And this should work with several keys as well. However, I'm not really familiar with the foreign key syntax in SQL, so if you are struggling with more keys, could you extend your example?
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      1. USChristoph_J
 

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