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Instance Expressions
The relational algebra = calculating with relation instance expressions
- A table (instance) is an instance expression
- The result of an ra operation on instance expressions is an instance expression
- This is just like with variables and expressions in programming languages
Notes:
Working with the relational algebra is like working with variables and expressions in a programming language.
The tables, as they exist at the current moment, are called relation instances.
The relational algebra works with relation instance expressions, or instance expressions for short. Every relation instance is an instance expression. A relation algebra operator applied to one or more instance expressions produces a new instance expression.
When you understand what each algebra operator does, you can also understand the meaning of a complex relation instance expression.
Everything we learn about the relational algebra is the translation from a natural language question to a relational algebra instance expression and back.