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copied!<p>As this is a many-to-many relation, you migth might want to have a look at using an associative table.</p> <p>Using your example, it might look something like this:</p> <p>Your tables that can <strong>have</strong> comments:</p> <pre><code>+----------+------------+ | student | student_id | +----------+------------+ | Steve | 12 | | Larry | 57 | | Sunshine | 88 | +----------+------------+ +--------+---------+ | dept | dept_id | +--------+---------+ | Math | 2 | | P.E. | 5 | | Drama | 12 | +--------+---------+ </code></pre> <p>Then you need to keep track of the actual comments:</p> <pre><code>+-----------------------+------------+ | comment | comment_id | +-----------------------+------------+ | I love Math! | 3 | | Larry is my hero... | 5 | | Sunshine <3 Me! | 6 | +-----------------------+------------+ </code></pre> <p>Now, you need an association between these tables, this is where your associative table comes into play. You now associate what student or dept has what comments, like this:</p> <pre><code>+------------+------------+ | student_id | comment_id | +------------+------------+ | 57 | 5 | | 57 | 6 | +------------+------------+ +---------+------------+ | dept_id | comment_id | +---------+------------+ | 2 | 3 | +---------+------------+ </code></pre> <p>This is both effective and elegant. Give it a shot!</p> <p><em>(And to save you another question perhaps)</em></p> <p>You could of course use just one association table if you are concerned about having so many association tables, but I would advice against it since it is not as neat and removes some possibilities for referential integrity checks that you can have with the first solution:</p> <pre><code>+-----------+------------+---------+ | entity_id | comment_id | entity | +-----------+------------+---------+ | 57 | 5 | student | | 57 | 6 | student | | 2 | 3 | dept | +-----------+------------+---------+ </code></pre> <p>(Which in turn should prompt you to add a lookup table for those entities... but let's not go there)</p>

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