Data Access Layer is typically a key layer in any application as most applications almost always have some need to access data from databases. SQL is supported by default by all relational databases, but it is also a fact that every database vendorshave their own custom functions or extensions added on to SQL as a value added feature. This gives rise to an interesting problem of how should a data access layer be modeled.
There are multiple schools of thought on Data Access Layer design. The frequently used models are – (a) Entity or Query Modeling, or (b) Object Relational Modeling. Both these approaches have their own pros and cons and these are beyond the scope of this manual. It should suffice to know that this is a critical question that an application design should answer.
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