After merging, the column used to merge lost
<p>If I run below codes</p>

<pre><code>dt&lt;-data.table(col1=c(0,1,2),col2=c("a","b","c"),col3=c("aa","ab","cc"))
setkey(dt,col1)
dt1&lt;-data.table(a=c(1,2))
dt[dt1]
</code></pre>

<p>I got below results</p>

<pre><code>   col1 col2 col3
1:    1    b   ab
2:    2    c   cc
</code></pre>

<p>However, the expected result is </p>

<pre><code>   a col1 col2 col3
1:      0    a   aa
2: 1    1    b   ab
3: 2    2    c   cc
</code></pre>

<p>How to get the expected result?</p>