MadeEasy Subject Test: Databases - Sql

Question

Please help,not able to decode this query

Comments

Seems to be wrongly structured.

 

Say a person is working on any project,then innermost(3rd level) query will return pid for that .

But the upper level query(2nd level) will check Not exists but as we got some pids ,so it will fail,We have and opeator ,so entire second level query will fail,and now 2nd level returned empty set,so 1st level (top most) becomes true,so if employee age is more than 30 and he is working somewhere ,then he will come in output.

2nd case:-

Person is not working on any project ,then second level query will just reduced to only one check i.e(if project==database),as second check always true in this case.Now it will be persons whose age is more than 30 and there is no database project

Please let me know if there is anything wrong in this approach

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finally answer this question

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inner query is giving those pid who pname is "database" and the employee is not working on it and outer query checks if inner query is returns nothing i.e. if there does not exits a project  whose  pname is "database" and the employee is not working on it,then his/her eid is printed provided his/her age is >30 so option (a) is correct

Answer 1

 

    not exist gives true if the inner query is empty