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sql - Relational Algebra - Select with no predicate

问题描述:

I know about relational algebra, but with regard to the select part of it, what should I say if there isn't any specific condition.

For example I have a Hotels Table and I just want to output that result.

Should I just use σ(Hotels) or do I need to put something like σALL(Hotels)???

I need your help!

网友答案:

Use Reference(R) if you dont have any condition

Reference Refers to an existing relation.

Algebra: R

SQL: SELECT * FROM table

Examples: SELECT * FROM Hotels

Returns every tuple in the Member table that you need. (No particular order is requested or implied.)

If you need to apply any condition only then use

σ<selection condition>(R)

Ref: http://bohr.wlu.ca/cp363/notes/theory/relational_algebra.php

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