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    <p>I think that something has been lost in translation over here, but I did my best to make an <strong>ER</strong> diagram of this.</p> <p>In general, there are two approaches. The first one is to propagate keys and the second one is to have an auto-increment integer as a <strong>PK</strong> for each table.</p> <p>The second approach is often driven by ORM tools which use a <strong>DB</strong> as object-persistence storage, while the first one (using key propagation) is more common for hand-crafted <strong>DB</strong> design.</p> <p>In general, the model with key propagation offers better performance for "random queries", mostly because you can "skip tables" in joins. For example, in the model with key propagation you can join the <code>Purchase</code> table directly to the <code>Item</code> table to report purchases by <code>ItemName</code>. In the other model you would have to join <code>Price</code> and <code>ItemPriceCategory</code> tables too -- just to get to the <code>ItemID</code>.</p> <p>Basically, the model with key propagation is essentially relational -- while the other one is object-driven. ORM tools either prefer or enforce the model with separate ID (second case), but offer other advantages for development.</p> <p>Your example seems to be trying to use some kind of a combination of these two -- not necessarily bad, it would help if you could talk to original designer. </p> <hr> <p><strong>With key propagation</strong></p> <p><img src="https://i.stack.imgur.com/5p7Mg.png" alt="enter image description here"></p> <hr> <p><strong>Independent keys for each table</strong> <img src="https://i.stack.imgur.com/1IBRB.png" alt="enter image description here"></p>
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    1. POMySQL - autoincrement + compound primary key - performance & integrity
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    1. USDamir Sudarevic
    1. USDamir Sudarevic
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    1. COThanks for taking the time to make the diagrams. I am working with the designers of the original database to improve it. The idea is indeed to get a best of both worlds approach, we want to have good data integrity AND use the tables in an ORM way, specifically an AR setup. Having the autoinc ID is very useful in AR as it allows knowing immediately the status of the object, but we don't want to solely trust the PHP to keep data integrity.
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