Midterm Prep: OQL

• The mindset you should be in is object oriented.

• A number of you based on emails/queries I have been answering are still in the relational mindset– E.g. there is no real concept of a cross product in

OQL (we deal with collections or bags)– You want to leverage the strengths of OQL – use

the functions (appropriately).

Lets look at an exampleselect distinct struct(star1: s1, star2: s2)from s1 in Stars, s2 in Starswhere s1.addr = s2.addr and s1.name < s2.name;

• Note here s1 and s2 are collections – we are going to compare collections.

• Do not think of this as a cross product/join – the underlying OO system.

• I want you to in the exam when asked OQL queries to think object oriented. – Use functions!

Simple Query with Rename

• Give the names of people who are older than 26 years old:

• SELECT SName: p.name FROM p in People WHERE p.age > 26

Watch out for Path expressions

• Find the names of the students of all tutors:– SELECT t.students.name FROM t in Tutors– Illegal use of . Above

• Correct solution– SELECT s.name

FROM t in Tutors, s in t.students• Alternate notation– SELECT s.name

FROM Tutors t, t.students s

Express the query in EnglishSubquery in FROM clause

• What is this query trying to do? SELECT r.name

FROM ( SELECT t FROM Tutors t WHERE t.salary > 300 ) r, r.students sWHERE st IN s AND st.fee > 30

In some implementations the IN is implicit: SELECT t.name

FROM ( SELECT t FROM Tutors t WHERE t.salary > 300 ) r, r.students sWHERE s.fee > 30

• Ans: Find the names of all tutors who have a salary greater than 300 and who have at least one student paying more than 30.

What do these do?• SELECT t.name

FROM t in TAs WHERE t.salary = max ( select ta.salary from ta in TAs )

• Give the names of TAs with the highest salary• SELECT sname, avgFee: AVG(SELECT p.s.fee

FROM partition p)FROM t in Tutors, t.students s GROUP BY sname: s.name

• Give the names of the students and the average fee they pay their Tutors

Using subqueries in the Where clause

• Give the names of people who are not Tas– Assume PEOPLE class and TA class– You can solve this without assuming any connections

between them.• SELECT p.name

FROM p in People WHERE not ( p.name in SELECT t.name FROM t in TAs )

• Views can be used to simplify these things – you are free to use them unless the question explicitly asks you not too.

Rest of the exam• Data Warehousing/Data Mining– Understand the defintions of support and confidence

• Given a transactional dtabase be able to determine these values for given rules

– Understand at a basic level what pivoting, selecting, drill down and roll up imply• Be able to manipulate tables such as the examples in the

notes.– Conceptual questions

• Why is sparse matrix management important in data warehouses?

• What distinguishes them from traditional OLTP?

Object Relational• Brush up on simple declarations as discussed in the notes• Make sure you understand the difference between a

row_type and an ADT• Be able to manipulate simple queries• Be able to instantiate simple functions• Conceptually these are examples of things you may be

asked.– Differences between object relational and OODB and pure

relational– What does the ADT bring to the table.– Role of less than and equals functions and why they are

important

Relational SQL• Be able to solve simple and complicated queries – Pay particular attention to exists, not exists and

mathematical logic operations (pg 207)– Pay attention to later stuff (e.g. recursive operations)

• Conceptual stuff (these are examples)– What is the difference between, relational, object relational

and OQL across different dimensions?• Querying• Performance

• DISCLAIMER: All of these are just examples of things that could be asked on the midterm. You are responsible for everything covered in the notes and in the associated reading material.