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    <p>A relational database should handle this "adjacency list" model just fine.</p> <p>First of all, don't use 0 in the <code>parent_comment_id</code> of the "root" comment, use NULL. Then you can build a FOREIGN KEY from <code>parent_comment_id</code> to <code>comment_id</code> that would prevent you from mistakenly attaching a reply to a non-existent comment.</p> <blockquote> <p>it will require me to go through the whole comments table just to find whether a comment has replies or not</p> </blockquote> <p>Assuming you have indexed the <code>parent_comment_id</code> (which <a href="https://stackoverflow.com/a/1721893/533120">InnoDB did automatically</a> if you created the FK above), finding the first level of replies to the given comment will require an index <strong>range scan</strong>. To understand index range scans and why they are efficient, you first need to understand the <a href="http://use-the-index-luke.com/sql/anatomy" rel="nofollow noreferrer">Anatomy of an SQL Index</a>.</p> <p>Finding the second level would require another range scan etc. Unfortunately, MySQL doesn't support the recursive query that would allow you to do all that in a single database round-trip, but it should still be fairly efficient.</p> <p>If you have preformed <strong>measurements</strong> and concluded that's a problem, there are other strategies for representing hierarchies (with different trade-offs), such as "nested sets" and "closures". Take a look at <a href="http://www.slideshare.net/billkarwin/models-for-hierarchical-data" rel="nofollow noreferrer">this presentation by Bill Karwin</a>.</p>
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    1. POmysql one-to-many table vs. key-value database list
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    1. USBranko Dimitrijevic
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    1. COgreat answer regarding mysql's part of the question. I am only using one level of parenthood so I only need to create the foreign key for the index and i should be fine. I will use mysql for it, but I still believe that key-store databases are more suited for lists since you are looking for 1 key to find a list of primary keys, maybe because i do not understand the underlying algorithms behind mysql. Thanks.
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