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PODifficulty with SQL UNION involving queries with different number of columns, and other woes
 

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  1. PODifficulty with SQL UNION involving queries with different number of columns, and other woes
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    <p>I'm part of a Toastmasters club, and just got put in charge of scheduling which members serve which roles in the weekly meetings. I'm writing a small app to help manage this, with the idea that a database can help me determine fairly which member is most overdue to serve in each role.</p> <p>To simplify matters, let's say that my database has two tables... <code>MEMBER</code> and <code>ROLE_PERFORMED</code>:</p> <pre><code>MEMBER ------ ID int NAME varchar ROLE_PERFORMED -------------- MEMBER_ID int (half of primary key, and also foreign key) DATE date (other half of primary key) ROLE int (0-11, mapped to an enumeration at the application layer) </code></pre> <p>My goal is to write a SQL query for each role type, which will give me:</p> <ul> <li><p><code>MEMBER.NAME</code>, for all rows in the <code>MEMBER</code> table</p></li> <li><p>A second column, containing the <em>most recent</em> <code>ROLE_PERFORMED.DATE</code> value matching that member and role... or else a <code>NULL</code> (or some other placeholder) if the member has <em>never</em> served in that role</p></li> </ul> <p>I could then order by the date/placeholder column, and assign roles in order of who has gone the longest time without performing that role. Something like this:</p> <pre><code>ROLE NAME MOST_RECENTLY_PERFORMED -------------------------------- 1 John &lt;null&gt; 1 Joe 2010-02-25 1 Bob 2010-09-14 </code></pre> <p>The approach I'm currently taking is to try a <code>UNION</code>... between one <code>SELECT</code> that grabs those members who have performed the role, and a second SELECT grabbing members who have never performed the role. Something like this:</p> <pre><code>SELECT m.name, r.date FROM member m, role_performed r WHERE m.id = r.member_id and r.role = 1 UNION SELECT m.name, (NULL?????) FROM member m, role_performed r WHERE (?????) </code></pre> <p>There are three problems with this, however, for which I'm hoping someone may have solutions:</p> <ul> <li><p>The first <code>SELECT</code> grabs <em>all</em> dates for which the member performed the role, not just the most recent.</p></li> <li><p>The second <code>SELECT</code> causes the <code>UNION</code> to fail, because I can't find an acceptable placeholder value to use for the second date column... and <code>UNION</code>'s require each query to return the same number of columns.</p></li> <li><p>The second <code>WHERE</code> clause is simply beyond my limited SQL skills. How do you find all <code>MEMBER</code> rows, which do <em>not</em> have a <code>ROLE_PERFORMED</code> rows matching that member and that role? Bear in mind that there <em>can</em> still be <code>ROLE_PERFORMED</code> rows matching that member and some <em>other</em> role.</p></li> </ul> <p>Like I said, I'm not the greatest SQL guru in the world... so I would welcome altogether different approaches if I'm making this more complicated than it needs to be. Heck, I would welcome a better question title to help meaningfully present what I'm asking! Thanks a lot in advance.</p>
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