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    <p>If you are constrained to use an SQL database and you need to do efficient type-aware (not merely string) queries on the attributes (like list all items with category shirt having size between 4 and 8), then the structure of the database is the hard part. The Java will follow from that. If I understand correctly, you will need something like this:</p> <pre><code>categories: id : integer (pk) name : varchar attributes: id : integer (pk) of_category : integer (fk -&gt; categories.id) name : varchar type : char(1) // 'N' for number, 'S' for string, etc. items: id : integer (pk) of_category : integer (fk -&gt; categories.id) number_values: value : number of_item : integer (pk, fk -&gt; items.id) of_attribute : integer (pk, fk -&gt; attributes.id) string_values: value : varchar of_item : integer (pk, fk -&gt; items.id) of_attribute : integer (pk, fk -&gt; attributes.id) ... additional table for each attribute type </code></pre> <p>Now for the example query:</p> <pre><code>select * from categories c, items i, attributes a, number_values v where c.name = 'shirt' and a.of_category = c.id and a.name = 'size' and a.id = v.of_attribute and i.id = v.of_item and v.value between 4 and 8 </code></pre> <p>Hairy multiple joins are the price to be paid for runtime-defined attrbutes. </p> <p>Once you have the tables right, modeling them as Java maps is straightforward. There is redundancy in the structure above: for example the character "type" field in the attribute rows. Consider triggers to make consistency checks.</p>
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    1. POHow to store a java object with variable number of attributes
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    1. COThanks! I chose dharam's as the correct answer only because he submitted it first. I wish I could have chosen both because I asked a two part question and your answer was spot on. Could you elaborate what you mean by "consider triggers to make consistency checks"?
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    2. COAh, well. It took a few minutes to work out the normal form of the database tables. I'm glad to be helpful. Triggers are an SQL feature that allow code to be run when INSERT, UPDATE, and DELETE operations occur (and some other cases). You can do consistency checks in this code. For example, you can check that a value in the number table actually refers to an attribute with type field 'N'. All in the name of data integrity. Trigger code is typically implemented in stored procedures. Google will show the way.
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