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MIS710 Module 2aComplex SQL Queries

Arijit Sengupta

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Monotonic and Non-Monotonic Queries

• Monotonic queries: queries for which the size of the results either increase or stay the same as the size of the inputs increase. The result size never decreases

• Non-monotonic queries: queries for which it is possible that the size of the result will DECREASE when the size of the input increases

• Examples of each?

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Examples of monotonic queries

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Examples of non-monotonic queries

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Which of the operations is non-monotonic?

A. Selection

B. Projection

C. Cross Product

D. Union

E. Set Difference

What does this signify?

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Identify – monotonic/non-monotonic?

• Find students who have taken algebra but not calculusA. MonotonicB. Non-monotonic

• Find students who have taken calculus but are not math majorsA. MonotonicB. Non-monotonic

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Answers to prev. slide

• Find students who have taken algebra but not calculus Non-monotonic – you take a student who has taken

algebra but not calculus, add a new registration for calculus, and he is gone from the result

• Find students who have taken calculus but are not math majors Monotonic! Assume Joe has taken calculus and is not

a math major – can you remove him from the result by adding more rows?

• Moral: If something is monotonic, you can solve it with basic SELECT-FROM-WHERE

• If something is non-monotonic – you HAVE to use something else

• Options are: MINUS, NOT IN, NOT EXISTS, Special grouping

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Identify – monotonic/non-monotonic

1. Find the students who have taken some CS courses

2. Find the students who have only taken CS courses

3. Find the students who have taken all the CS courses

4. Find the students who have not taken any CS course

5. Find the students who have taken a non-CS course

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Answers to previous slide

1. Monotonic – if someone has taken some CS course, he stays in the results regardless of what you add/remove

2. Non-monotonic – someone who has only taken CS course will disappear from the result if you add a registration for them for a non-cs course

3. Non-monotonic – if you add a new CS course, then …

4. Non-monotonic – if you add a registration for a CS course…

5. Monotonic! Someone who has taken a non-CS course will continue to stay in the result!

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Thumb rules

• Some clues to identify non-monotonic queriesEvery, AllNo, None, NeverOnly

• Remember – these are just thumb rules and not absolute laws – you need to think about how the queries will behave to determine non-monotonicity

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Lets do the monotonic first!

• Find students who have taken some CS course

SELECT s.*FROM Student s, Reg r, Course cWHERE s.sid = r.sid AND c.cno = r.cno AND c.dept = 'CS'

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The other monotonic query

• Find Students who have taken a Non-CS Course

SELECT s.*FROM Student s, Reg r, Course cWHERE s.sid = r.sid AND c.cno = r.cno AND c.dept != 'CS'

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Solving non-monotonic queries

• We know that MINUS can solve non-monotonic

• Unfortunately representing all queries using MINUS is not easy

• Some are easy:Students who have not taken any CS

courseAll students MINUS students who have

taken some CS course

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Non-monotonic queries using MINUS

• All students MINUS students who have taken some CS course

SELECT s1.*FROM Student s1

MINUS

SELECT s.*FROM Student s, Reg r, Course cWHERE s.sid = r.sid AND c.cno = r.cno AND c.dept = 'CS'

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Nesting Queries - Syntax

SELECT attribute(s)

FROM relation(S)

WHERE attr [not] {in | comparison operator | exists }

( query statement(s) );

List names of students who are taking “MIS415”

select Name

from Student

where SID in

( select SID

from REG

where Cno = ‘MIS415’);

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Sub Queries

List all students enrolled in MIS coursesselect namefrom STUDENTwhere SId in

(select SIdfrom REGwhere cno like ‘MIS%’);

List all students enrolled in MIS coursesselect namefrom STUDENTwhere SId in

(select SIdfrom REGwhere cno like ‘MIS%’);

List all courses taken by Student (Id 1011) select cnamefrom COURSEwhere cnum = any

(select cnofrom REGwhere SId = 1011);

So, IN and ANY essentially are like joins, not that interesting

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Sub Queries - continued

Who received the highest score in MIS 415select SIdfrom REGwhere cno = ‘MIS415’ and

score >=all(select scorefrom REGwhere cno = ‘MIS415’);

Who received the highest score in MIS 415select SIdfrom REGwhere cno = ‘MIS415’ and

score >=all(select scorefrom REGwhere cno = ‘MIS415’);

List all students enrolled in MIS courses.select namefrom STUDENT Swhere exists

(select *from REGwhere SId = S.SId

and cno like ‘MIS%’);

ALL is quite powerfulSince it works on the wholeSet returned by the subquery

Exists is again likeA join!

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Non-monotonic queries using NOT IN and NOT Exists

• Students who have not taken CS courses

Using NOT IN

SELECT s.*FROM Student sWHERE s.sid not IN ( SELECT r.sid FROM Reg r, Course c WHERE r.cno = c.cno AND c.dept = ‘CS’)

Using NOT EXISTS

SELECT s.*FROM Student sWHERE NOT EXISTS ( SELECT * FROM Reg r, Course c WHERE r.cno = c.cno AND c.dept = ‘CS’ AND r.sid = s.sid)

Notice how the alias ‘s’ from outside thesubquery is used in the subquery!These are called “correlated” subqueries.

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The “default” results

• Notice that sometimes queries using not exists will give you results that appear “by default”

• For example, students who have not taken ANY course will show up in the result of the last query

• Okay for this query?

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Writing non-monotonic queries

• See if you can rewrite the statement below in English with double negatives.

• Remember – the new sentence MUST mean the same!• Find students who have taken ALL CS courses

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Try another one

• Find students who have got A’s in All courses they took

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Tackling non-monotonic queries

• Try 1: See if you can write it as a difference between two setsSolve using MINUS or NOT IN

• Try 2: Rewrite the query to use the tables and finding ways to use not existsFind students who have taken all CS coursesFind students such that there is no CS course

that they have not takenFind students such that there doesn’t exist

any CS course for which there doesn’t exist a registration for that student in that course!

Yes, a lot of double negatives!

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Step by Step

• Find students who have taken all CS courses

Find students

such that there doesn’t exist

any CS course

for which there doesn’t exist

a registration

for that student

in that course!

SELECT s.*FROM Student s

WHERE not exists (

SELECT c.* FROM Course c WHERE c.dept = ‘CS’

AND NOT EXISTS (

SELECT r.* FROM Reg r WHERE

r.sid = s.sid

AND r.cno = c.cno))

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Should we do one more?

• Find students who have ONLY taken CS courses

• Find students such that every course they have taken is a CS course

• Find students such that every registration they have corresponds to a CS course

• Find students such that there doesn’t exist any registration record for that student corresponding to a non-CS course

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Translating this query

Find students

such that

there doesn’t exist

any registration record

for that student

corresponding to a

non-CS course

SELECT s.*

FROM Student s

WHERE not Exists ( SELECT r.*

FROM Reg r, Course c

WHERE r.sid = s.sid

AND r.cno = c.cno

AND c.cno != ‘CS’)

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Default result here

• You will notice that students who have not taken any courses will show up in this query’s results!

• Why?• Logically, students who haven’t taken

any courses haven’t taken any non-cs courses

• To get rid of this default behavior, you may want to perform a join on the outermost query, so that you retrieve students who have taken some course

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Query with the default removed

SELECT s.*

FROM Student s, Reg R1

WHERE s.sid = R1.sid

AND not Exists ( SELECT r.*

FROM Reg r, Course c

WHERE r.sid = s.sid

AND r.cno = c.cno

AND c.cno != ‘CS’)

Now this query will only retrieve students who have taken at least one course, none of which is CS.

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Relational Views

• Relations derived from other relations.• Views have no stored tuples.• Are useful to provide multiple user views.

View 1 View 2 View N

BaseRelation 1

BaseRelation 2

•What level in the three layer model do views belong?•Which kind of independence do they support?

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View Creation

Create View view-name [ ( attr [ , attr ] ...) ]

AS subquery

[ with check option ] ;

DROP VIEW view-name;

Create a view containing the student ID, Name, Age and GPA for those who are qualified to take 300 level courses, i.e., GPA >=2.0.

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View Options

• With Check Option enforces the query condition for insertion or update

To enforce the GPA >=2.0 condition on all new student tuples inserted into the view

• A view may be derived from multiple base relations

Create a view that includes student IDs, student names and their instructors’ names for all CIS 300 students.

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View Retrieval

Queries on views are the same as that on base relations.

Queries on views are expanded into queries on their base relations.

select Name, Instructor-Name

from CIS300-Student

where Name = Instructor-Name;

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View: Update

Update on a view actually changes its base relation(s)!

update Qualified-Student

set GPA = GPA-0.1

where StudentID = ‘s3’;

insert into Qualified-Student

values ( ‘s9’, ‘Lisa’, 4.0 )

insert into Qualified-Student

values ( ‘s10’, ‘Peter’, 1.7 )

Why are some views not updateable?

What type of views are updateable?

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Summary

• SQL is a low-complexity, declarative query language

• The good thing about being declarative is that internally the query can be changed automatically for optimization

• Good thing about being low-complexity? No SQL query ever goes into an infinite loopNo SQL query will ever take indefinite amount of

space to get the solution

• Can be used for highly complex problems!