Held: Wednesday, April 12, 2006
Summary:
Today we begin our exploration of declarative database languages
by considering one of the first papers on relational databases.
Related Pages:
Notes:
- Neither grading nor answer keys are available. Sorry.
- Are there questions on the exam?
- The reading for Friday is the preliminary paper on SEQUEL.
Overview:
- Context.
- Codd's Contributions.
- Why Read This Paper?.
- Thinking About Relations.
- Operations on Relations.
What do we know about the state of computing (and databases) at the time this was written?
- Most of the data was on disk (or tape!), so access was slow.
- DBMS's were rare.
- Client programmers knew and relied on the structure of data in their code.
- Two primary data models: Hierarchical and Network
- Hierarchical: Each portion of the database (file) is a homogeneous collection of records. Records may contain other records.
- Network: Records can be shared by other records.
- ...
What are the primary contributions of this paper? Here are some.
- A clear attempt to separate what you want to do with data from how the data are represented.
- A model of data that persists to today.
- The use of formal systems to explain the model.
Why do we care about separating access from representation?
- Easier to modify the representation.
- Easier to write programs.
- Easier to modify the representation on the fly, as you observe (or are warned of) different patterns of access.
Up to you to answer this question.
Codd treats relations in a variety of ways in the paper.
- At one level, a relation is simply a set of tuples.
- We can also think of each relation as a predicate (a function that returns true or false). The relation, when applied to a tuple, returns true only when the tuple belongs to the set.
- We can also think of each relation as a function from n-m of the values in the tuple to tuples of the remaining m values.
- Of course, this is a kind of multiple-valued function. Consider
- If we supply 0 as the A parameter, we can get back 1 or 2.
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Check with Bobby