Formal Relational Query Languages
3 important questions on Formal Relational Query Languages
What is Relational Algebra and why do we use it for SQL and working with databases in in general?
Relational Algebra.
- A procedural query language, containing a collection of operations.
- These operations help to create a new relation from one/two existing relations:
- Input:
- It always takes one or two relations as input.
- Output:
- It always produces a new relation.
We can divide relational algebra's fundamental operations into unary and binary operations. What is the difference between the two and which operations fall under which category?
Unary Operations:
- Operations that work on a single relation.
- Fundamental Unary Operations:
- Selection (σ).
- Projection (π).
- Renaming (ρ).
- Operations that work on two relations.
- Fundamental Binary Operations:
- Union (∪).
- Difference (−).
- Cartesian Product (×).
- Join (⨝).
Consider the following query in Tuple Relational Calculus.
What does ∈ mean?
- It means "is an element of."
- Used tot state that an object (in this case a tuple) belongs to a collection.
- E.g., t = {1, 2, 3, 4}.
- We can say: 2 ∈ t.
- 2 is an element of t.
- In {t | t ∈ instructor... }:
- Means: find all tuples ( t ) that are members of the set of tuples that makes up the instructor relation.
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