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    <p>Looks like a many-many relationship for "what users have seen what projects", best implemented with a table with exactly two columns -- a foreign key into the users table and one into the projects table. When a project is created you don't yet add entries to this table (unless you can identify a certain user as the creator who's deemed to have seen the project, in which case you do add that one entry) -- you add an entry (U, P) each time the "user U has seen project P" occurs. When a project is modified you remove all entries in the table regarding that project (again, except maybe the one for the user who is doing the modification, if you know who).</p> <p>Need the SQL for this table's schema and the operations I've outlined in words?</p> <p><strong>Edit</strong>: a commenter requested the SQL, so (assuming InnoDb, in the case of mysql), assuming tables user and project have int primary keys uid and pid respectively:</p> <pre><code>create table Seen (uid int, pid int, foreign key uid references user(uid) on delete cascade, foreign key pid references project(pid) on delete cascade, primary key (uid, pid)) </code></pre> <p>When an existing project with a pid of P is modified, so no user is now deemed to have "seen" it, <code>delete from Seen where pid=P</code> (i.e., remove all entries for P from this table); if the modification is made by user U, so user U is actually deemed to have seen it, you can add <code>and uid!=U</code> to the <code>where</code> clause of the <code>delete</code>.</p> <p>To mark the fact that a certain user U has now seen project P, <code>insert ignore into Seen(uid, pid) values (U, P)</code> (the <code>ignore</code> part means "do nothing if the row was already there", so that recording the "seen" conditions more than once is innocuous, although of course redundant).</p>
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    1. USAlex Martelli
    1. USAlex Martelli
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    1. COI say go for it, more information can never be a bad thing
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      1. USAnthony Forloney
    2. COthanks! SQL would be nice but I think I can handle it :)
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      1. USBundy
 

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