UGC NET CSE | December 2014 | Part 2 | Question: 17

Question

Which of the following is true ?

I. Implementation of self-join is possible in SQL with table alias.

II. Outer-join operation is basic operation in relational algebra.

III. Natural join and outer join operations are equivalent.

• A. $I$ and $II$ are correct.
• B. $II$ and $III$ are correct. 
• C. Only $III$ is correct.
• D. Only $I$ is correct.

Answer 1

ans is D only 1 is correct self join is implemented in a single table by alias names

rest 2 are incorrect

Answer 2

Answer A)

I) yes aliasing is required for self join. In self join we are joining same table 2 times. Aliasing gives these two tables separate name , to make clear what is happening.

II) Yes outer join is a basic operation

III) No, In natural join and outer join both table constructed in separate ways

Comments

answer will be (D)

 

aliasing is required in self join

 

 

outer join is not a basic operation in RA. all the basic operations are:

select,project,rename,cartesian product, set difference,union

 

no, natural join is cartesian product b/w two tables followed by selection condition but outer join is cartesian product followed by some condition and also all the elements of left table should be included in result