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Data dictionary

Can someone explain to me how a data dictionary works? Its relation to ER diagrams and data normalisation. An example would also be great!
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A data dictionary is just a bunch of key / value pairs. So you have some sort of unique identifier that is a key and it links to some sort of value which might be a single value or a compound data structure containing a bunch of different values. In an ER type diagram a single entry in a dictionary might relate to row of a table in a database but that is down to implementation. Normalisation would only occur if one of the values in the dictionary was linked to a different table as per the normalisation rules.

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