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POmysql-php querying a list to build a table
 

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    <p>I've had the same problem with sql for a while and as I sit down to write the same 'brute-force' hack I always use, I figure there MUST be a more efficient way to do what I want to do.</p> <p>I have tables similar to this:</p> <pre><code>grades(gradeID, taskID, grade, username, date) tasks(taskID, task...) assignment(assignmentID, title...) assignment_tasks(assignmentID, taskID) assignment_students(assignmentID, username) students(username, forename...) </code></pre> <p>the last 5 tables are pretty static, set up once and mostly left alone.</p> <p>for the grades table, a new record is created every time a new grade is entered for a task.</p> <p>I want to produce a summary table for an assignment, which might consist of say, 5 tasks and each student may have any number of grades for each task, with the most recent one being the one I want to display in the summary.</p> <p>What I'd normally do is query a list of students, and a list of assignment tasks, then build a giant nested for loop looping through each task for each student, querying the most recent grade for each, so assuming 30 students, 5 tasks, that's 152 queries, which has always struck me as waaay too many.</p> <p>I figure (hope) I have a horribly embarrassing gap in my sql knowledge and there's a much cleverer way to do it.</p> <p>Edit: Thanks for the answer - I'm still working on building the actual database so I can test it, but I suspect that the answer below doesn't cover the following issues:</p> <p>if a student hasn't attempted a task/assignment, there won't be any entries for them in the grades table but they still need to show in the summary table with a default grade for each task ("u"). I think this bit makes it harder.</p> <p>Edit again: I have the embryo database now and it works in that I get a list of most recent grades with gaps where there's no grade. Transposing that list is now the subject of <a href="https://stackoverflow.com/questions/1000716/php-list-into-a-table">another question</a>!</p>
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