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Relational Algebra and Joins or Select

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Have a test on SQL, only teacher wants to know what type of relational algebra is a select, where, from--I have no idea and can not understand the teacher.

We are using Concepts of Database Management by Pratt

No I understand quieries, but not relational algebra

Any Help would be great
Mar 9 '07 #1
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ADezii
Expert 5K+
P: 8,619
Have a test on SQL, only teacher wants to know what type of relational algebra is a select, where, from--I have no idea and can not understand the teacher.

We are using Concepts of Database Management by Pratt

No I understand quieries, but not relational algebra

Any Help would be great
In order to implement a Database Management System (DBMS), there must exist a set of rules which state how the database system will behave. For instance, somewhere in the DBMS must be a set of statements which indicate than when someone inserts data into a row of a relation, it has the effect which the user expects. One way to specify this is to use words to write an `essay' as to how the DBMS will operate, but words tend to be imprecise and open to interpretation. Instead, relational databases are more usually defined using Relational Algebra.

Relational Algebra is :

__1. The formal description of how a relational database operates
__2. An interface to the data stored in the database itself
__3. The mathematics which underpin SQL operations

Operators in relational algebra are not necessarily the same as SQL operators, even if they have the same name. For example, the SELECT statement exists in SQL, and also exists in relational algebra. These two uses of SELECT are not the same. The DBMS must take whatever SQL statements the user types in and translate them into relational algebra operations before applying them to the database.
Mar 9 '07 #2

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