• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 2213
  • Last Modified:

Database Symbols

How do I solve this problem?

6.16) Specify the following queries on the COMPANY relational database schema shown in Figure 5.5 using the relational operators discussed in this chapter. Also show the result of each query as it would apply to the database state in Figure 3.6.

a. Retrieve the names of all employees in department 5 who work more than 10 hours per week on the ProductX project.


The relational operators noted refer to sigma, pi, Intersect, U, and Join.

Figure 5.5 wasn't in the book; Figure 6.5 looked relevant.

The related figures are:
Fig-3.6.png
Fig-6.5.png
0
JCW2
Asked:
JCW2
2 Solutions
 
knightEknightCommented:
We are not allowed to do your homework for you.
0
 
JCW2Author Commented:
No you're not.

I'm looking to understand my problem better (That's what I tried to say with my first sentence).
0
 
JCW2Author Commented:
(There are other possible operators)
0
Industry Leaders: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

 
derekkrommCommented:
can you be more detailed in what you're actually looking for help with here? i can't understand what type of question you're really asking
0
 
ste5anSenior DeveloperCommented:
hi,

This kind of exercises must be precise otherwise they are worthless. Question a) is not solvable, if your given data is complete.

This is quite trival: You don't have the information about workhours per week. You have a PROJECT table which has no reference to weeks and you have the WORKS_ON table, which has a reference to the PROJECT table, but also not reference to weeks. While you can make an educated guess that the hours in WORKS_ON sum up to a weekly work shift, it is not mentioned anywhere. Thus you cannot solve it. You can guess it, but this not the question, as database modeling and data retrieval is about correct answers, not guessed ones.


mfG
--> stefan <--
0
 
virmaiorCommented:
The operators are from set theory in mathematics.  The U means "Union" and in SQL is written by UNION (or UNION ALL) placed between two queries.  This means include all data from both.  (UNION ALL will include duplicates if the entries come from the separate halves).

---

The upside U means intersect and includes only data common to both sets.  In this case A intersect B and B intersect A might yield different data.  In SQL you express this by doing a JOIN and a WHERE

(A intersect B)
SELECT * FROM a LEFT JOIN b ON b.field = a.field WHERE b.field IS NOT NULL

(b intersect a)
SELECT * FROM b LEFT JOIN a ON a.field = b.field WHERE a.field IS NOT NULL

---
JOIN is a way of relating tables in SQL where you can combine them on a common field.  So if you had an order table with an id for the customer, you could retrieve all the orders for a customer and the customer's information in a single query.

----
I am not sure what Sigma and Pi mean in a SQL context.
0
 
JCW2Author Commented:
Tank you for your help.
0

Featured Post

Important Lessons on Recovering from Petya

In their most recent webinar, Skyport Systems explores ways to isolate and protect critical databases to keep the core of your company safe from harm.

Tackle projects and never again get stuck behind a technical roadblock.
Join Now