Left Join Question

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От Ryan Wells
Тема Left Join Question
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Msg-id 52F2AD70C422474B857FC9E3F7B62539028DB1E8@exchange.DOCS.COM
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Ответы Re: Left Join Question  (ries van Twisk <pg@rvt.dds.nl>)
Re: Left Join Question  (Mark Roberts <mailing_lists@pandapocket.com>)
Re: Left Join Question  (Richard Huxton <dev@archonet.com>)
Список pgsql-sql
<div class="Section1"><p class="MsoNormal">While looking through our data layer code today, I ran across this query:<p
class="MsoNormal"> <pclass="MsoNormal">SELECT <p class="MsoNormal">tasks.*,<p class="MsoNormal">clients.FirstName,<p
class="MsoNormal">clients.LastName,<pclass="MsoNormal">clients.MiddleInitial,<p class="MsoNormal">iteminfo.CreatedBy,<p
class="MsoNormal">iteminfo.StationId,<pclass="MsoNormal">iteminfo.CreatedDate,<p
class="MsoNormal">changelog.LastModified,<pclass="MsoNormal">changelog.LastModifiedBy,<p
class="MsoNormal">changelog.LastModifiedAt,<pclass="MsoNormal">ticklers.Due,<p class="MsoNormal">ticklers.Reminder<p
class="MsoNormal"> <pclass="MsoNormal">FROM tasks <p class="MsoNormal"> LEFT JOIN clients ON tasks.ClientId =
clients.ClientId<p class="MsoNormal">  LEFT JOIN iteminfo ON tasks.Id = iteminfo.ItemId <p class="MsoNormal">   LEFT
JOINchangelog ON tasks.Id = changelog.ItemId <p class="MsoNormal">    LEFT JOIN ticklers ON tasks.Id =
ticklers.RelatedId<p class="MsoNormal"> <p class="MsoNormal">WHERE tasks.Id = '123456';<p class="MsoNormal"> <p
class="MsoNormal">(I'vecleaned it up so it's  easier to read.)<p class="MsoNormal"> <p class="MsoNormal">The basic data
structureis that we have a todo list that contains a list of tasks which may or may not be associated with clients,
items,log entries, or ticklers (scheduled reminders).  <p class="MsoNormal"> <p class="MsoNormal">The query works as
intended: it returns a result-set with all the necessary data to display in the todo list.   The performance is not a
majorconcern, although it can be slow for large lists.<p class="MsoNormal"> <p class="MsoNormal">Since it works, my
questionis really more about principles:  Given that each of the tables in question will contain tens of thousands of
rows,is a nested join really the best way to approach this?<p class="MsoNormal"> <p class="MsoNormal">Thanks!<p
class="MsoNormal">Ryan</div>

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