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POSQL SELECT statement with complicated 'if/else' requirement
 

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  1. POSQL SELECT statement with complicated 'if/else' requirement
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    <p>I am using a MySQL database, here's my situation: </p> <p>I need a select query to be able to get a list of projects that can be completed using N supplies, where N is an array of supplies. This list of projects must include all projects that can be completed using any or all of N supplies, but can not include any projects that require supplies not listed in N. <em>(e.g. in the make sketch project from the tables below paper has no substitute; however, pencil can be substituted by pen. If the query searches for projects that can be completed using pencil, pen, and pencil sharpener then 'make sketch' should not return as a project that can be completed, even though it uses some of the supplies listed)</em></p> <p>Additionally, some of the supplies required by certain projects can be substituted by other supplies; however, just because one project can use a substitute supply item does not mean that another project would work with that same substitute. <em>(e.g. in the sharpen pencil project below pen cannot be a substitute for pencil, however, for make drawing it can)</em></p> <p>These are my tables:</p> <pre><code>Projects +----+---------------------+ | id | name | +----+---------------------+ | 1 | make sketch | | 2 | sharpen pencil | | 3 | make paper airplane | +----+---------------------+ Supplies +----+------------------+ | id | name | +----+------------------+ | 1 | paper | | 2 | pencil | | 3 | pen | | 4 | pencil sharpener | +----+------------------+ ProjectSupplies +----+-----------+------------+ | id | projectid | supplyid | +----+-----------+------------+ | 1 | 1 | 1 | | 2 | 1 | 2 | | 3 | 2 | 2 | | 4 | 2 | 4 | | 5 | 3 | 1 | +----+-----------+------------+ SubstituteSupplies +-------------------+------------+ | projectsuppliesid | supplyid | +-------------------+------------+ | 2 | 3 | +-------------------+------------+ </code></pre> <p>The data isn't exhaustive by any means but you should get the point.</p> <p>This is the query I came up with <em>previous to updating the database</em>(see update below), however, it breaks the rules because the query result includes projects that require paper simply because it <code>COUNT</code>'s both the supplyid and the substitute as two separate requirements rather than as simply fulfilling the same supply requirement.</p> <pre><code>SELECT projects.name FROM supplies INNER JOIN projectsupplies ON supplyid = supplies.id OR substitute = supplies.id INNER JOIN projects ON projects.id = projectid WHERE supplies.id IN (2,3,4) GROUP BY projects.name HAVING COUNT(*) &lt;= 3 ORDER BY projects.id </code></pre> <p><strong>Is there a way to turn this:</strong></p> <pre><code>INNER JOIN projectsupplies ON supplyid = supplies.id OR substitute = supplies.id </code></pre> <p><strong>into essentially this:</strong></p> <pre><code>INNER JOIN projectsupplies ON (supplies.id = supplyid) ? (supplies.id = supplyid) : (supplies.id = substitute) </code></pre> <p><strong>or something similar to that using an if statement or whatever in order to make the query result correct?</strong></p> <p><strong>One problem that I had been experiencing is that the above query will return 'make sketch' as a valid project even though, as specified in the query, there is no paper.</strong></p> <p>The end goal is to be able to accomplish this on a large scale with many projects and many supplies.</p> <p><strong>UPDATE:</strong> I found an issue in the design of my database that made it impossible to allow a supply to have more than one substitute. I corrected the problem to allow for many substitutes, and updated the tables above as necessary, so now the <code>SELECT</code> query above is no longer applicable. I still, however, need to accomplish the same goal that is mentioned at the top of this post</p>
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