Testbook Test Series: Databases - Relational Algebra

Question

 

The number of rows returned by sql query on given relation:

SELECT * from emp where eno NOT IN (SELECT manager from emp)

Query intends to find the number of employees who are not MANAGER so emp 2,3,4 are ruled out. But since it includes comparison with NULL so it will be executed like this for emp 1,

1 NOT IN {2,3,4,NULL}

I thought it in opposite way,

1 IN {2,3,4,NULL} will return false. So NOT IN will give true and number of rows returned will be 1.

Answer given is 0. Please clarify.

Comments

UNKNOWN doesn't mean FALSE either. Why in the question it is considered FALSE then?

Yes, I know. But the final answer is UNKNOWN means not returning any row. that's why zero is the answer.

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If you are not getting please comment.

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@Shaik Masthan @kumar.dilip Thanks for the clarification. This line did it for me.

 if the row satisfies true ===> should be in output

else " Not in output "