I have a table storing words, correspondingly there is another table storing the explanations of the words. It it very common that a word might have many explanations. So the primary key of the explanation table absolutely cannot be merely the word id, but the both the word id and the explanation, composite key, can decide a unique explanation record.
The explanation table can take the composite key as its primary key, but is it always like this in practice? Personally I doubt, as I have seen many examples from my real experience that in such case like the explanation table, usually a new id field will be added as the primary key.
Question: I would like to know which solution is better? Why I always see an extra ID is used as a primary key instead of a composite key?
