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    <p><strong>NULL values</strong></p> <blockquote> <p>It also means item will have 5 ids including its own. This all cause null values (apparently a big no no, which I can understand) because if place and price are optional and are not used on one entry to the items table I will have a null value there</p> </blockquote> <p>Personally I think this is one situation where <code>NULL</code> values are perfect, and I certainly wouldn't have any doubts about putting this into my database design.</p> <p>One way I've seen others achieve the same thing without <code>NULL</code> values is to create a record in the optional tables (place and price in your example) with an ID of 0 that signifies there's no related record - but this just makes 10 times more work for the developer of the application to filter these records out - it's FAR easier to do a join and if you don't get any records back, there are no related records in the optional table.</p> <p>Just remember to do a <code>LEFT</code> or <code>RIGHT OUTER</code> join if you want to return <code>Item</code>s regardless of whether they have a <code>Place</code> or <code>Price</code> associated (you'll get <code>NULL</code> values in the optional table columns for <code>Item</code>s that don't have associated records) and an <code>INNER</code> join if you <strong>only</strong> want the <code>Item</code>s that <strong>do</strong> have an associated optional record.</p> <p><strong>Composite Keys</strong></p> <p>A composite key is a key in a table that's made up of more than one column. If each of your <code>Person</code>, <code>Item</code>, <code>Place</code> and <code>Price</code> all have an ID (even if it's just an auto-incrementing number) you won't need a composite key - just a primary key column in each table, and a foreign key column in the <code>Item</code> table for each related table - e.g. <code>item_id</code>, <code>person_id</code>, <code>place_id</code>, <code>price_id</code>. You state that <code>Item</code> has its own ID, so you shouldn't need a composite key - just a primary key on the <code>item_id</code> column.</p>
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    1. USAndy Shellam
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    1. COHow would you make sure the correct ids get filled in on INSERT? For example when I insert I don't get the correct id's for the other tables would I need to do this with mysql statements?
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      1. USCool Hand Luke UK
 

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