User talk:Tml~enwiki

I have toned down the text to be less dramatic: however, to argue that metadata is NOT critical to EAV systems is like arguing that humans breathe in carbon dioxide and exhale oxygen.
 * Production EAV systems besides my own - e.g., the clinical data systems of Cerner, EpicCare and Oracle (Oracle Clinical (TM)) have a large number of metadata tables, and access to these is restricted to administrator-level users because of the damage that naive users can do if they change the contents of these tables.


 * The book on SQL server 2008 t-SQL programming by Itzik Ben-Gan et al (Microsoft Press) - the authors are *not* fans of EAV models and recommend an alternative approach based on XML - point out (accurately) that EAV models have the drawback that to make them viable a large number of supporting metadata tables are required, as is code infrastructure for user interface generation. To quote(I have pasted the verbatim text, which is available online from Books24X7.com, which the Yale library subscribes to) :

"The so-called open schema solution is quite popular. In this solution, you have the main Products table with common attributes. Then you add an Entity-Attribute-Value (EAV) table, which has three columns: product ID, attribute name, and attribute value. This is a completely dynamic solution, and is quite simple. However, with this solution you cannot easily use declarative constraints. With a single EAV table, the attribute value column is either character or variant data type. In addition, you have to prevent the insertion of meaningless attributes, or binding attributes to products for which the attributes are meaningless. You can create an EAV table for every data type of attributes you need to solve the data type problems. You can add an additional table that holds valid combinations of products and attributes. You can add additional constraints using triggers, middle tier code, or client code. Whatever you do, your solution is not that simple anymore. Your database design does not reflect the business problem anymore; you do not design a database from logical point of view, you design it from a physical point of view. Because of this last fact, I would hardly call this solution relational. Note that I am not saying that if you use such a solution, you have to modify it. In the past, this was probably the best option you had if you needed dynamic schema. However, nowadays I prefer the solution I am going to introduce at the end of this section."

The "additional tables/constraints" refer to metadata.

Hope this helps.

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