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# Relational Algebra expression am I correct?

Posted on 2011-09-11
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Provide an expression in relational algebra that shows the following query:
Find the names, genders and social security numbers of all employees who have been managers in the last 2 years.

* employee (emp_id,f_name,l_name,dob,ssn,gender,hire_date)
* department (dept_id,name)
* emp_in_dept (emp_id,dept_id,begin_date,end_date)
* mgr_of_dept (dept_id,emp_id,begin_date,end_date)
* salary(emp_id,salary,begin_date,end_date)
* job_title(emp_id,title,begin_date,end_date)

I started with this but I am not sure how to complete the comparing part.

PROJECT f_name, l_name, gender, ssn, (
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Question by:Rowley4
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Expert Comment

ID: 36519350
Rowley4, is this a school assignment? Not a big deal if it is, we just need to ensure we are "teaching" you correctly if it is. :)

With that in mind, please post what you have tried so far. It appears it got cut off. With your code as a starting point, we can help guide you.
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Author Comment

ID: 36519364
I have started the code, but I am unsure how to finish it. This is what I have so far.

PROJECT f_name, l_name, gender, ssn, (

I am jut not sure how to write the comparing part when I have no actual dates. I thought it would be > 2009? But not sure how to put that in.
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Author Comment

ID: 36519375
This is separate from my other question.
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Expert Comment

ID: 36519407
What you have there isn't close to a SQL statement, so not sure what to put next either. so this homework. I will see what I can mock up to help explain things for you like Patrick did in your other question.
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Author Comment

ID: 36519410
I am using Relational Algebra for this one rather than just SQL.
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Accepted Solution

Kevin Cross earned 500 total points
ID: 36519461
See if this helps get you going: http://db.grussell.org/section010.html#_Toc67114473
Something like this:
PROJECT f_name, l_name, gender, ssn (SELECT begin_date >= '20090101' (employee JOIN employee.emp_id = mgr_of_dept.emp_id mgr_of_dept))

Assumptions: employee managed a department; could have a manager in title only, which you could capture by making the relationship to job_title. Hopefully this helps you get going. You can combine the PROJECT and SELECT for what you need.
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Author Comment

ID: 36519466
Yes!! Perfect.. This was a great.
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Author Closing Comment

ID: 36519467
This is exactly what I needed. Thank you so much!
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