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    <p>Good question. Note the following (admittedly buried) in <code>?data.table</code> :</p> <blockquote> <p>When <code>i</code> is a <code>data.table</code>, <code>x</code> must have a key. <code>i</code> is joined to <code>x</code> using the key and the rows in <code>x</code> that match are returned. An equi-join is performed between each column in <code>i</code> to each column in <code>x</code>'s key. The match is a binary search in compiled C in O(log n) time. If <code>i</code> has less columns than <code>x</code>'s key then many rows of <code>x</code> may match to each row of <code>i</code>. If <code>i</code> has more columns than <code>x</code>'s key, the columns of <code>i</code> not involved in the join are included in the result. <strong>If <code>i</code> also has a key</strong>, it is <code>i</code>'s key columns that are used to match to <code>x</code>'s key columns and a binary merge of the two tables is carried out.</p> </blockquote> <p>So, the key here is that <code>i</code> doesn't have to be keyed. Only <code>x</code> must be keyed.</p> <pre><code>X2 &lt;- data.table(id = 11:15, y_id = c(14,14,11,12,12), key="id") id y_id [1,] 11 14 [2,] 12 14 [3,] 13 11 [4,] 14 12 [5,] 15 12 Y2 &lt;- data.table(id = 11:15, b = letters[1:5], key="id") id b [1,] 11 a [2,] 12 b [3,] 13 c [4,] 14 d [5,] 15 e Y2[J(X2$y_id)] # binary search for each item of (unsorted and unkeyed) i id b [1,] 14 d [2,] 14 d [3,] 11 a [4,] 12 b [5,] 12 b </code></pre> <p>or,</p> <pre><code>Y2[SJ(X2$y_id)] # binary merge of keyed i, see ?SJ id b [1,] 11 a [2,] 12 b [3,] 12 b [4,] 14 d [5,] 14 d identical(Y2[J(X2$y_id)], Y2[X2$y_id]) [1] FALSE </code></pre>
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    1. POTranslating SQL joins on foreign keys to R data.table syntax
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    1. USMatt Dowle
    1. USMatt Dowle
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    1. POTranslating SQL joins on foreign keys to R data.table syntax
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    1. COthanks. but `identical(Y2[J(X2$y_id)], Y2[X2$y_id]) == TRUE`, is there a difference?
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      1. USDavid LeBauer
    2. CO@David Have changed the example data to make it clearer. The previous example data had key values the same as the row numbers i.e. `1:5`.
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      1. USMatt Dowle
 

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