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<p>i. prefix is just a robust way to reference join inherited columns: the 'top' column in the i table. Like table aliases in SQL.</p>
<p>What about this? :</p>
<pre>1> X = data.table(a=1:3,b=1:15, key="a")<br />1> X<br /> a b<br /> 1: 1 1<br /> 2: 1 4<br /> 3: 1 7<br /> 4: 1 10<br /> 5: 1 13<br /> 6: 2 2<br /> 7: 2 5<br /> 8: 2 8<br /> 9: 2 11<br />10: 2 14<br />11: 3 3<br />12: 3 6<br />13: 3 9<br />14: 3 12<br />15: 3 15<br />1> Y = data.table(a=c(1,2,1), top=c(3,4,2))<br />1> Y<br /> a top<br />1: 1 3<br />2: 2 4<br />3: 1 2<br />1> X[Y, head(.SD,i.top)]<br /> a b<br />1: 1 1<br />2: 1 4<br />3: 1 7<br />4: 2 2<br />5: 2 5<br />6: 2 8<br />7: 2 11<br />8: 1 1<br />9: 1 4<br />1> </pre>
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<p>On 24.04.2013 23:43, Eduard Antonyan wrote:</p>
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<div dir="ltr">I assumed they meant create a table :)
<div>that looks cool, what's i.top ? I can get a very similar to yours result by writing:</div>
<div>X[Y][, head(.SD, top[1]), by = a]</div>
<div>and I probably would want the following to produce your result (this might depend a little on what exactly i.top is):</div>
<div>X[Y, head(.SD, i.top), by = a]</div>
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<div class="gmail_extra"><br /><br />
<div class="gmail_quote">On Wed, Apr 24, 2013 at 5:28 PM, Matthew Dowle <span><<a href="mailto:mdowle@mdowle.plus.com">mdowle@mdowle.plus.com</a>></span> wrote:<br />
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<p>That sentence on that linked webpage seems incorect English, since table is a noun not a verb. Should "table" be "join" perhaps?</p>
<p>Anyway, by-without-by is often used with join inherited scope (JIS). For example, translating their example :</p>
<pre>1> X = data.table(a=1:3,b=1:15, key="a")<br />1> X<br /> a b<br /> 1: 1 1<br /> 2: 1 4<br /> 3: 1 7<br /> 4: 1 10<br /> 5: 1 13<br /> 6: 2 2<br /> 7: 2 5<br /> 8: 2 8<br /> 9: 2 11<br />10: 2 14<br />11: 3 3<br />12: 3 6<br />
13: 3 9<br />14: 3 12<br />15: 3 15<br />1> Y = data.table(a=c(1,2), top=c(3,4))<br />1> Y<br /> a top<br />1: 1 3<br />2: 2 4<br />1> X[Y, head(.SD,i.top)]<br /> a b<br />1: 1 1<br />2: 1 4<br />3: 1 7<br />4: 2 2<br />5: 2 5<br />
6: 2 8<br />7: 2 11<br />1> </pre>
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<pre>If there was no by-without-by (analogous to CROSS BY), then how would that be done?</pre>
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<p>On <a>24.04.2013 22</a>:22, Eduard Antonyan wrote:</p>
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<div dir="ltr">By that you mean current behavior? You'd get current behavior by explicitly specifying the appropriate "by" (i.e. "by" equal to the key).
<div>Btw, I'm trying to understand SQL CROSS APPLY vs JOIN using <a href="http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/">http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/</a>, and I can't figure out how by-without-by (or with by-with-by for that matter:) ) helps with e.g. the first example there:</div>
<div>"<span style="font-size: 13px; line-height: 18px;">We table</span><span style="font-size: 13px; line-height: 18px;"> </span><code style="font-size: 13px; line-height: 18px; padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">table1</code><span style="font-size: 13px; line-height: 18px;"> </span><span style="font-size: 13px; line-height: 18px;">and</span><span style="font-size: 13px; line-height: 18px;"> </span><code style="font-size: 13px; line-height: 18px; padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">table2</code><span style="font-size: 13px; line-height: 18px;">.</span><span style="font-size: 13px; line-height: 18px;"> </span><code style="font-size: 13px; line-height: 18px; padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">table1</code><span style="font-size: 13px; line-height: 18px;"> </span><span style="font-size: 13px; line-height: 18px;">has a</span><span style="font-size: 13px; line-height: 18px;"> </span><code style="font-size: 13px; line-height: 18px; padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">column</code><span style="font-size: 13px; line-height: 18px;"> </span><span style="font-size: 13px; line-height: 18px;">called</span><span style="font-size: 13px; line-height: 18px;"> </span><code style="font-size: 13px; line-height: 18px; padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">rowcount</code><span style="font-size: 13px; line-height: 18px;">.</span></div>
<p style="padding: 0px; margin: 0px 0px 15px; line-height: 18px; font-size: 13px;">For each row from <code style="padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">table1</code> we need to select first <code style="padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">rowcount</code> rows from <code style="padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;">table2</code>, ordered by <code style="padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;"><a href="http://table2.id">table2.id</a>"</code></p>
<p style="padding: 0px; margin: 0px 0px 15px; line-height: 18px; font-size: 13px;"><code style="padding: 0px; margin: 0px; font-weight: bold; color: #4488bb;"><br /></code></p>
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<div class="gmail_extra"><br /><br />
<div class="gmail_quote">On Wed, Apr 24, 2013 at 4:01 PM, Matthew Dowle <span><<a href="mailto:mdowle@mdowle.plus.com">mdowle@mdowle.plus.com</a>></span> wrote:<br />
<blockquote class="gmail_quote" style="margin: 0 0 0 .8ex; border-left: 1px #ccc solid; padding-left: 1ex;">But then what would be analogous to CROSS APPLY in SQL?<br />
<div><br /> > I'd agree with Eduard, although it's probably too late to change behavior<br /> > now. Maybe for data.table.2? Eduard's proposal seems more closely<br /> > aligned with SQL behavior as well (SELECT/JOIN, then GROUP, but only if<br /> > requested).<br /> ><br /> > S.<br /> ><br /> >> Date: Mon, 22 Apr 2013 08:17:59 -0700</div>
>> From: <a href="mailto:eduard.antonyan@gmail.com">eduard.antonyan@gmail.com</a><br /> >> To: <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br />
<div>>> Subject: Re: [datatable-help] changing data.table by-without-by<br /> >> syntax to require a "by"<br /> >><br /> >> I think you're missing the point Michael. Just because it's possible to<br /> >> do it<br /> >> the way it's done now, doesn't mean that's the best way, as I've tried<br /> >> to<br /> >> argue in the OP. I don't think you've addressed the issue of unnecessary<br /> >> complexity pointed out in OP.<br /> >><br /> >><br /> >><br /> >> --<br /> >> View this message in context:<br /> >> <a href="http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html">http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html</a><br /> >> Sent from the datatable-help mailing list archive at Nabble.com.<br /> >> _______________________________________________<br /> >> datatable-help mailing list</div>
>> <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br />
<div>>> <a href="https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help">https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help</a><br /> > _______________________________________________<br /> > datatable-help mailing list</div>
> <a href="mailto:datatable-help@lists.r-forge.r-project.org">datatable-help@lists.r-forge.r-project.org</a><br /> > <a href="https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help">https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help</a><br /><br /><br /></blockquote>
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