- So far we have considered how to structure data using the relational model.
- We need a language to answer questions — queries — about the data.
- SQL is that language.
- Before we get to SQL, it is important to learn Relational Algebra.
- is the basis of SQL semantics,
- And of query optimization techniques which are essential to making SQL practical.
- An is a set of operations on values of
some type, which yield values of the same type.
- E.g. +, −, ×, and ÷ are operations on numbers which yield numbers.
- An algebra for relations (tables).
- Set-oriented: a relation is a set.
- This was a revolutionary idea in 1970, because all database languages at
the time were record-at-a-time.