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    <p>Yes, well, half your problems are due to not having a resolved Data Model, and that will cause problems in SQL everywhere; if you fix that, the SQL will be much easier. The other half is not being able to use SQL effectively.</p> <ol> <li><p>You have not posted info re the other tables, so I can't give you a reasonably accurate DM, I am going just from what you have provided, and of course made huge assumptions about the other related tables, which are most probably incorrect. If you provide the info, I can provide an accurate DM.</p> <ul> <li>Is a Category Independent or "owned" by only one Member ? <ul> <li>Modelled: "No."</li> </ul></li> <li>On what basis is a Category "shared" ? <ul> <li>Modelled: sharing is the same as ownership; a shared Category is simply one that is owned by more than one Member</li> </ul></li> <li>Then, what is the difference between an "owned" Category and a "shared" Category ?</li> <li>We do not know what the Categories are for (Subjects ?). In which case why are Categories, not Subjects, owned/shared by Members ?</li> <li>Your Category, if it is owned, really should not be called Category, rather something like CategoryOwned.</li> <li>I cannot see how CategoryViewed shows all the Views; it allows only one (the last ?) view per Member. Where is the ViewDate ? <ul> <li>Modelled: As a log of Views. If you want only one (the last) view per Member, remove ViewDate from the PK. </li> </ul></li> </ul> <p><a href="http://www.softwaregems.com.au/Documents/Student%20Resolutions/DavidRyder%20DM.pdf" rel="nofollow"><strong>Interim Data Model</strong></a></p></li> <li><p>It is not a good idea to perform Unions, etc, unless you absolutely have to. Right now the data is small; when the database is large, you will certainly feel the effects of the work tables being created; filled; destroyed. So far from your requirement, there is no need for Unions, Distincts, worktables, etc. But the DM does present problems.</p></li> <li><p>SQL. I cannot see how your SQL (or any of the other answers) retrieves your requirement of "the last time that category was viewed [by any Member]". Additionally, you should always try to get your SQL correct (returning the correct set of data) for each set, before Unioning, etc: you have not done that; that is why you have duplicate rows. Using DISTINCT is a stupid way to fix the problem; you are correct in seeking understanding instead.</p></li> <li><p>I won't attempt to provide the SQL for your model. Here is the SQL for the Interim Data Model provided. Obviously, this is much, much cheaper than Unions: <pre><code>SELECT [Member] = Member.Name, [Category = Category.Name, [LastView] = ( SELECT MAX(ViewDate) FROM CategoryView WHERE CategoryId = cm.CategoryId ) FROM CategoryMember cm, Member m, Category c WHERE cm.MemberId = '$MemberId' AND m.MemberId = cm.MemberId AND c.CategoryId = cm.CategoryId</pre></code></p></li> </ol>
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