Union Operation in RDBMS - Fundamental Relational Algebra Concept

 

            Union Operation - RDBMS

        Fundamental Relational Algebra Concept

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* Scenario where a Union Operation over a rdbms table could be used : -

Consider a query to find the names of all the bank customers who have either an account or a loan or both in a bank .

 

* To find the names of the customer who have an account and also a deposit account in the bank , the search query would consider the search to take over "depositor" relation table and the "borrower" relation table .

 

* In order to find out the names of all the customers with a loan in the bank , the query that could be used for the operation would be :

 

{figure-01}

The upper relational equation is a form of projection which allows us to produce the relevant relation that returns the argument relation specified within the parenthesis . So as we are interested in finding out the names of the customers who have a loan account , the

relevant result would be fetched from the projection relation as mentioned in the above statement .

 

* Similarly , if we want to know the names of all the customers with an account in the bank , then it can be expressed in the following projection equation :

 

{figure-02}

 

 

* Therefore , in order to find the answer to the raised question in our scenario that is discussed in the opening statement , one needs to do a "Union" operation over the two sets which is : we need all the customer names that appear in either both or two of the relations which can be found out by using the binary operation Union upon both the queries . The relevant relational equation can be represented in the given manner :

 

    {figure-03}

 

* The resulting relation for the query would result in a relation with the relevant tuples from both the tables

 

* The resulting relations are sets from which duplicate values are eliminated .

 

In Hindsight :

In our example , we took the union of two sets , both of which consisted of the attribute "customer_names" values . In general , one must ensure that unions are taken between compatible relations which would generate the appropriate results without duplicates.

 

* Points to note when using a Union Operation over relational sets :

 

If r is a set and s is set , and one needs to find { r U s } , If r is a set and s is set , and one needs to find { r U s } , then the conditions that should satisfy and hold for both relations

1)        The relations r and s must be of the same type and they must have the same number of attributes

 

2)   The domains of the ith attribute of r and ith attribute of s must be the same for all values of i . One may note that , both r and s are either database relations or temporary relations that are the result of relational algebra expressions as given in figure 1 and figure 2 of the article .