MIS710 Module 2a Complex SQL Queries

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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?

• Selection
• Projection
• Cross Product
• Union
• Set Difference
• What does this signify?

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Identify monotonic/non-monotonic?

• Find students who have taken algebra but not
  calculus
• Monotonic
• Non-monotonic
• Find students who have taken calculus but are not
  math majors
• Monotonic
• 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 queries
• Every, All
• No, None, Never
• Only
• 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 c
• WHERE 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 c
• WHERE 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 course
• All 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 c
• WHERE 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
courses select name from STUDENT where SId
in (select SId from REG where cno like
MIS)
List all courses taken by Student (Id 1011)
select cname from COURSE where cnum
any (select cno from REG where 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
415 select SId from REG where cno MIS415
and score gtall (select score from REG whe
re cno MIS415)
ALL is quite powerful Since it works on the
whole Set returned by the subquery
List all students enrolled in MIS
courses. select name from STUDENT
S where exists (select from REG where SId
S.SId and cno like MIS)
Exists is again like A 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 s WHERE
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 s WHERE
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
the subquery 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 As 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 sets
• Solve using MINUS or NOT IN
• Try 2 Rewrite the query to use the tables and
  finding ways to use not exists
• Find students who have taken all CS courses
• Find students such that there is no CS course
  that they have not taken
• Find students such that there doesnt exist any
  CS course for which there doesnt 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 doesnt exist
any CS course for which there doesnt 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 doesnt 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 doesnt 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 querys results!
• Why?
• Logically, students who havent taken any courses
  havent 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.

• 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 gt2.0.

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View Options

• With Check Option enforces the query condition
  for insertion or update
• To enforce the GPA gt2.0 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 loop
• No SQL query will ever take indefinite amount of
  space to get the solution
• Can be used for highly complex problems!