Principles of Database Systems · Overview - detailed - SQL Fundamental constructs and concepts...

Principles of Database Systems

V. Megalooikonomou

Relational Model – SQL Part I

(based on notes by Silberchatz,Korth, and Sudarshan and notes by C. Faloutsos)

General Overview - rel. model

Formal query languagesrel algebra and calculi

Commercial query languagesSQL (combination of rel algebra and calculus)QBE, (QUEL)

Overview - detailed - SQLFundamental constructs and concepts

check users’ guide for particular implementationDML

select, from, where, renamingset operationsorderingaggregate functionsnested subqueries

other parts: DDL, view definition, embedded SQL, integrity, authorization, etc

DML

General formselect a1, a2, … anfrom r1, r2, … rmwhere P[order by ….][group by …][having …]

Reminder: our Mini-U db

STUDENTSsn Name Address

123 smith main str234 jones forbes ave

TAKESSSN c-id grade

123 cis331 A234 cis331 B

CLASSc-id c-name unitscis331 d.b. 2cis321 o.s. 2

DML – e.g.:

find the ssn(s) of everybody called “smith”

select ssnfrom studentwhere name=“smith”

DML - observation

General formselect a1, a2, … anfrom r1, r2, … rmwhere P

equivalent rel. algebra query?

DML - observation

General formselect a1, a2, … anfrom r1, r2, … rmwhere P

))...21((,...2,1 rmrrPanaa ×××σπ

DML - observation

General formselect distinct a1, a2, … anfrom r1, r2, … rmwhere P

))...21((,...2,1 rmrrPanaa ×××σπ

select clause

select [distinct | all ] namefrom studentwhere address=“main”

where clause

find ssn(s) of all “smith”s on “main”select ssnfrom studentwhere address=“main” and

name = “smith”

where clause

boolean operators (and, or, not, …)comparison operators (<, >, =, …)and more…

What about strings?

find student ssns who live on “main” (stor str or street – i.e., “main st” or “main str” …)

What about strings?

find student ssns who live on “main” (stor str or street) select ssnfrom studentwhere address like “main%”

%: variable-length don’t care_: single-character don’t care

from clause

find names of people taking cis351




TAKESSSN c-id grade

123 cis331 A234 cis331 B

from clause

find names of people taking cis351select namefrom student, takeswhere ???

from clause

find names of people taking cis351select namefrom student, takeswhere student.ssn = takes.ssn and

takes.c-id = “cis351”

renaming - tuple variables

find names of people taking cis351select namefrom ourVeryOwnStudent,

studentTakingClasseswhere ourVeryOwnStudent.ssn =

studentTakingClasses.ssnand studentTakingClasses.c-id = “cis351”

renaming - tuple variables

find names of people taking cis351select namefrom ourVeryOwnStudent as S,

studentTakingClasses as Twhere S.ssn =T.ssn

and T.c-id = “cis351”

renaming - self-join

self -joins: find Tom’s grandparent(s)

PCp-id c-idMary TomPeter MaryJohn Tom

PCp-id c-idMary TomPeter MaryJohn Tom

renaming - self-join

find grandparents of “Tom” (PC(p-id, c-id))

select gp.p-idfrom PC as gp, PCwhere gp.c-id= PC.p-id

and PC.c-id = “Tom”

renaming - theta join

find course names with more units than cis351

select c1.c-namefrom class as c1, class as c2where c1.units > c2.units

and c2.c-id = “cis351”

find course names with more units than cis351select c1.namefrom class as c1, class as c2where c1.units > c2.units

and c2.c-id = “cis351”

])}[2][][1][2

351][1(21|{

nameccnamectunitscunitsc

cisidccCLASScCLASSct

−=−∧>

∧=−∈∃∈∃

Overview - detailed - SQL

DMLselect, from, whereset operationsorderingaggregate functionsnested subqueries


set operations

find ssn of people taking both cis351 and cis331

TAKESSSN c-id grade

123 cis331 A234 cis331 B

set operations


select ssnfrom takeswhere c-id=“cis351” and

c-id=“cis331”?

set operations


select ssnfrom takeswhere c-id=“cis351” and

c-id=“cis331”

set operationsfind ssn of people taking both cis351 and cis331

(select ssn from takes where c-id=“cis351” )intersect(select ssn from takes where c-id=“cis331” )

other ops: union , except



other parts: DDL, embedded SQL, auth etc

Ordering

find student records, sorted in name orderselect *from studentwhere

Ordering

find student records, sorted in name orderselect *from studentorder by name asc

asc is the default

Ordering

find student records, sorted in name order; break ties by reverse ssnselect *from studentorder by name, ssn desc

Aggregate functions

find avg grade, across all studentsselect ??from takes

SSN c-id grade123 cis331 4234 cis331 3

Aggregate functions

find avg grade, across all studentsselect avg(grade)from takes

result: a single numberWhich other functions?


Aggregate functions

A: sum, count, min, max (std)

Aggregate functionsfind total number of enrollments

select count(*)from takes


Aggregate functions

find total number of students in cis331select count(*)from takeswhere c-id=“cis331”


Aggregate functions

find total number of students in each courseselect count(*)from takeswhere ???


Aggregate functions

find total number of students in each courseselect c-id, count(*)from takesgroup by c-id


c-id countcis331 2

Aggregate functions

find total number of students in each courseselect c-id, count(*)from takesgroup by c-idorder by c-id


c-id countcis331 2

Aggregate functions

find total number of students in each course, and sort by count, decreasingselect c-id, count(*) as popfrom takesgroup by c-idorder by pop desc


c-id popcis331 2

Aggregate functions- ‘having’

find students with GPA > 3.0



find students with GPA > 3.0select ???, avg(grade)from takesgroup by ??? SSN c-id grade

123 cis331 4234 cis331 3


find students with GPA > 3.0select ssn, avg(grade)from takesgroup by ssn???


SSN avg(grade)123 4234 3


find students with GPA > 3.0select ssn, avg(grade)from takesgroup by ssnhaving avg(grade)>3.0

‘having’ <-> ‘where’ for groups




find students and GPA,for students with > 5 courses

select ssn, avg(grade)from takesgroup by ssnhaving count(*) > 5




DMLselect, from, where, renamingset operationsorderingaggregate functionsnested subqueries


DML

General formselect a1, a2, … anfrom r1, r2, … rmwhere P[order by ….][group by …][having …]

Reminder: our Mini-U db




TAKESSSN c-id grade

123 cis331 A234 cis331 B

DML - nested subqueries

find names of students of cis351select namefrom studentwhere ...

“ssn in the set of people that take cis351”


find names of students of cis351select namefrom studentwhere ………...

select ssnfrom takeswhere c-id =“cis351”


find names of students of cis351select namefrom studentwhere ssn in (

select ssnfrom takeswhere c-id =“cis351”)


‘in’ compares a value with a set of values‘in’ can be combined with other boolean opsit is redundant (but user friendly!):select namefrom student …..where c-id = “cis351” ….


‘in’ compares a value with a set of values‘in’ can be combined with other boolean opsit is redundant (but user friendly!):select namefrom student, takeswhere c-id = “cis351” and

student.ssn=takes.ssn


find names of students taking cis351 and living on “main str”

select namefrom studentwhere address=“main str” and ssn in

(select ssn from takes where c-id =“cis351”)


‘in’ compares a value with a set of valuesother operators like ‘in’ ??


find student record with highest ssnselect *from studentwhere ssn

is greater than every other ssn


find student record with highest ssnselect *from studentwhere ssn greater than every

select ssn from student


find student record with highest ssnselect *from studentwhere ssn > all (

select ssn from student)almost correct


find student record with highest ssnselect *from studentwhere ssn >= all (

select ssn from student)


find student record with highest ssn -without nested subqueries?

select S1.ssn, S1.name, S1.addressfrom student as S1, student as S2where S1.ssn > S2.ssn

is not the answer (what does it give?)




S1 S2

S1. ssn S2.ssn ….123 123 …234 123 …123 234234 234

S1 x S2

S1.ssn>S2.ssn



select S1.ssn, S1.name, S1.addressfrom student as S1, student as S2where S1.ssn > S2.ssn

gives all but the smallest ssn -aha!


find student record with highest ssn -without nested subqueries?

select S1.ssn, S1.name, S1.addressfrom student as S1, student as S2where S1.ssn < S2.ssn

gives all but the highest - therefore….

DML - nested subqueriesfind student record with highest ssn - without

nested subqueries?(select * from student) except(select S1.ssn, S1.name, S1.addressfrom student as S1, student as S2where S1.ssn < S2.ssn)


(select * from student) except(select S1.ssn, S1.name, S1.addressfrom student as S1, student as S2where S1.ssn < S2.ssn)

select *from studentwhere ssn >= all (select ssn from student)


Drill: Even more readable than select * from studentwhere ssn >= all (select ssn fromstudent)


Drill: Even more readable than select * from studentwhere ssn >= all (select ssn fromstudent)

select * from studentwhere ssn in(select max(ssn) from student)


Drill: find the ssn of the student with the highest GPA




grade

234 cis331 B

TAKESSSN c-id

123 cis331 A


Drill: find the ssn and GPA of the student with the highest GPA

select ssn, avg(grade) from takeswhere



select ssn, avg(grade) from takesgroup by ssnhaving avg( grade) …...

greater than every other GPA on file



select ssn, avg(grade) from takesgroup by ssnhaving avg( grade) >= all

( select avg( grade )from student group by ssn )

} all GPAs


‘in’ and ‘>= all’ compares a value with a set of valuesother operators like these?


<all(), <>all() ...‘<>all’ is identical to ‘not in’>some(), >= some () ...‘= some()’ is identical to ‘in’exists


Drill for ‘exists’: find all courses that nobody enrolled in

select c-id from class ….with no tuples in ‘takes’


TAKESSSN c-id grade

123 cis331 A234 cis331 B


Drill for ‘exists’: find all courses that nobody enrolled in

select c-id from class where not exists

(select * from takeswhere class.c-id = takes.c-id)

DML - derived relations

find the ssn with the highest GPA

select ssn, avg(grade) from takesgroup by ssnhaving avg( grade) >= all

( select avg( grade )from student group by ssn )


find the ssn with the highest GPAQuery would be easier, if we had a table

like: helpfulTable (ssn, gpa):

then what?

HelpfulTableSsn Gpa123 3.5678 3.3


select ssn, gpafrom helpfulTablewhere gpa in (select max(gpa)

from helpfulTable)

HelpfulTableSsn Gpa123 3.5678 3.3


find the ssn with the highest GPA -Query for helpfulTable (ssn, gpa)?


find the ssn with the highest GPAQuery for helpfulTable (ssn, gpa)?

select ssn, avg(grade)from takesgroup by ssn


find the ssn with the highest GPA

helpfulTable(ssn,gpa)

select ssn, avg(grade)from takesgroup by ssn

select ssn, gpafrom helpfulTablewhere gpa = (select max(gpa)

from helpfulTable)


find the ssn with the highest GPAselect ssn, gpafrom (select ssn, avg(grade)

from takesgroup by ssn)as helpfulTable(ssn, gpa)

where gpa in (select max(gpa) from helpfulTable)

Views

find the ssn with the highest GPA -we can create a permanent, virtual table:

create view helpfulTable(ssn, gpa) asselect ssn, avg(grade)from takesgroup by ssn

Views

views are recorded in the schema, for ever (i.e., until ‘drop view…’)typically, they take little disk space, because they are computed on the fly(but: materialized views…)

Overview of a DBMSDBAcasual

user

DML parser

buffer mgr

trans. mgr

DMLprecomp.

DDL parser

catalog

create view..


DMLselect, from, where, renamingset operationsorderingaggregate functionsnested subqueries



DMLother parts:

modificationsjoinsDDLembedded SQLauthorization

Principles of Database Systems · Overview - detailed - SQL Fundamental constructs and concepts...

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