Databases
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    <p>I would propose to have a hybrid between <a href="https://en.wikipedia.org/wiki/Database_normalization" rel="nofollow">normalized</a> data and <a href="http://en.wikipedia.org/wiki/Denormalization" rel="nofollow">denormalized</a> data .<br> So using the normalized structure provided by <em>eggyal</em> i would do the following denormalized structure :</p> <pre><code>CREATE TABLE Items_Tags_Denormalized ( Item_ID BIGINT UNSIGNED REFERENCES Items (Item_ID), Tags BLOB, PRIMARY KEY (Item_ID) ) ENGINE=InnoDB; </code></pre> <p>In column <code>Tags</code> you would have all the tags (<code>Tag_Title</code>) for the corresponding <code>Item_ID</code>.<br> Now you have 2 ways to achieve this: </p> <ul> <li><p>create a cron that runs periodically which will build this table <code>Items_Tags_Denormalized</code> using <code>GROUP_CONCAT</code> or whatever suits you (advantage: doesn't put additional load when you insert or delete in <code>Items_Tags</code> table; disadvantage: the denormalized table will not always be up to date (depending on how often do you run the cron))</p></li> <li><p>create <a href="http://dev.mysql.com/doc/refman/5.0/en/trigger-syntax.html" rel="nofollow">triggers</a> for <code>Items_Tags</code> table on insert and delete in order to keep up to date the <code>Items_Tags_Denormalized</code> table (advantage: the denormalized table will always be up to date;disadvantage: additional load when you insert or delete in <code>Items_Tags</code> table)</p></li> </ul> <p>Choose whatever solution suits your needs best considering the advantages and disadvantages.</p> <p>So in the end you will have the <code>Items_Tags_Denormalized</code> table from which you will <strong>only read without doing additional operations</strong>.</p>
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    1. POTagging query with group_concat
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    1. COWhy not add the denormalised `Tags` field to the `Items` table (which is how SO does it)?
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    2. COit is crucial to have separate models : one normalized and one denormalized , your solution is ok but from design pov i would suggest to keep it separate for various reasons : you need to rebuild the table , you need to add more columns etc plus if you add denormalized tags column in items the performance of the table will drop : bigger size = slower queries
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