SQL Subqueries - Undergraduate Courses | Computer …courses.cs.vt.edu/~cs4604/Fall10/lectures/lecture-04-sq… · · 2010-09-02SQL Subqueries T. M. Murali September 1, 2010 T.

SQL Subqueries

T. M. Murali

September 1, 2010

T. M. Murali September 1, 2010 CS 4604: SQL Subqueries

Linear Notation for Relational Algebra

I Relational algebra expressions can become very long.I Use linear notation to store results of intemediate expressions.

1. A relation name and a parenthesised list of attributes for that relation.Use Answer as the conventional name for the final result.

2. The assignment symbol :=.3. Any expression in relational algebra on the right.

Example of Linear Notation

I Name pairs of students who live at the same address.

I Normal expression:

πS1.Name,S2.Name(σS1.Address = S2.Address(ρS1(Students) × ρS2(Students)))

I Linear notation:

Pairs(P1, N1, A1, P2, N2, A2) := ρS1(Students) × ρS2(Students)

Matched(P1, N1, A1, P2, N2, A2) :=

σA1 = A2(Pairs(P1, N1, A1, P2, N2, A2))

Answer(Name1, Name2) := πN1,N2(Matched(P1, N1, A1, P2, N2, A2)).

Example of Linear Notation

I Name pairs of students who live at the same address.

I Normal expression:

πS1.Name,S2.Name(σS1.Address = S2.Address(ρS1(Students) × ρS2(Students)))

I Linear notation:

Pairs(P1, N1, A1, P2, N2, A2) := ρS1(Students) × ρS2(Students)

Matched(P1, N1, A1, P2, N2, A2) :=

σA1 = A2(Pairs(P1, N1, A1, P2, N2, A2))

Answer(Name1, Name2) := πN1,N2(Matched(P1, N1, A1, P2, N2, A2)).

Interpreting Queries Involving Multiple Relations

SELECT A, B FROM R, S WHERE C;

I Nested loops:

for each tuple t1 in Rfor each tuple t2 in S

if the attributes in t1 and t2 satisfy Coutput the tuples involving attributes A and B.

I Conversion to relational algebra: πA,B(σC (R × S)).

1. Compute R × S .2. Apply the selection operator σC () to R × S .3. Project the resulting tuples to attributes A and B.

I Nested loops:

Motivation for Subqueries

I Find the name of the professor who teaches “CS 4604.”

SELECT Name

FROM Professors, Teach

WHERE (PID = ProfessorPID) AND (Number = ’4604’) AND

(DeptName = ’CS’) ;

I Do we need to take the natural join of two big relations just to get arelation with one tuple?

I Can we rewrite the query without using a join?

I Find the name of the professor who teaches “CS 4604.”SELECT Name

SQL Subquery For Example

SELECT Name

FROM Professors

WHERE PID =

(SELECT ProfessorPID

FROM Teach

WHERE (Number = 4604) AND (DeptName = ’CS’)

I When using =, the subquery must return a single tuple.

SELECT Name

FROM Professors

WHERE PID =

FROM Teach

SELECT Name

FROM Professors

WHERE PID =

FROM Teach

Conditions Involving Relations

I SQL includes a number of operators that apply to a relation andproduce a boolean result.

I These operators are very useful to apply on results of sub-queries.

I Let R be a relation and t be a tuple with the same set of attributes.

I EXISTS R is true if and only if R contains at least one tuple.

I t IN R is true if and only if t equals a tuple in R.I t > ALL R is true if and only if R is unary (has one attribute) and t

is greater than every value in R.I Can use any of the other five comparison operators.I If we use <>, R need not be unary.

I t > ANY R (which is unary) is true if and only if t is greater than atleast one value in R.

I We can use NOT to negate EXISTS, ALL, and ANY.

I t IN R is true if and only if t equals a tuple in R.

I t > ALL R is true if and only if R is unary (has one attribute) and tis greater than every value in R.

I Can use any of the other five comparison operators.I If we use <>, R need not be unary.

is greater than every value in R.

I Can use any of the other five comparison operators.I If we use <>, R need not be unary.

Subqueries Using Conditions

I Find the departments of the courses taken by the student with name’Suri’.

SELECT DeptName

FROM Take

WHERE StudentPID IN

(SELECT PID

FROM Students

WHERE (Name = ’Suri’)

Subqueries Using Conditions

I Find the departments of the courses taken by the student with name’Suri’.

SELECT DeptName

FROM Take

WHERE StudentPID IN

(SELECT PID

FROM Students

WHERE (Name = ’Suri’)

Correlated Subqueries

I Find course names that have been used for two or more courses.

SELECT CourseName

FROM Courses AS First

WHERE CourseName IN

(SELECT CourseName

FROM Courses

WHERE (Number <> First.Number)

OR (DeptName <> First.DeptName));

SELECT CourseName

WHERE CourseName IN

(SELECT CourseName

FROM Courses

(Number <> First.Number)

SELECT CourseName

WHERE CourseName IN

(SELECT CourseName

FROM Courses

Evaluating Correlated Subqueries

SELECT CourseName

WHERE CourseName IN

(SELECT CourseName

FROM Courses

I Evaluate query by looping over tuples of First and for each tuple,evaluate the subquery.

I Scoping rules: an attribute in a subquery belongs to one of the tuplevariables in that subquery’s FROM clause, or to the immediatelysurrounding subquery, and so on.

Evaluating Correlated Subqueries

SELECT CourseName

WHERE CourseName IN

(SELECT CourseName

FROM Courses

I Evaluate query by looping over tuples of First and for each tuple,evaluate the subquery.

I Scoping rules: an attribute in a subquery belongs to one of the tuplevariables in that subquery’s FROM clause, or to the immediatelysurrounding subquery, and so on.

Subqueries in FROM clauses

I Can use a subquery as a relation in a FROM clause.

I We must give such a relation an alias using the AS keyword.

I Let us find different ways of writing the query “Find the names ofProfessors who have taught the student whose name is ’Suri’.”

I The old way:

SELECT Professors.Name

FROM Professors, Take, Teach, Students

WHERE (Professors.PID = Teach.ProfessorPID)

AND (Teach.CourseNumber = Take.CourseNumber)

AND (Teach.DeptName = Take.DeptName)

AND (Take.StudentPID = Student.PID)

AND (Student.Name = ’Suri’);

Subqueries in FROM clauses

I Can use a subquery as a relation in a FROM clause.

I We must give such a relation an alias using the AS keyword.

I Let us find different ways of writing the query “Find the names ofProfessors who have taught the student whose name is ’Suri’.”

I The old way:

SELECT Professors.Name

FROM Professors, Take, Teach, Students

WHERE (Professors.PID = Teach.ProfessorPID)

AND (Teach.CourseNumber = Take.CourseNumber)

AND (Teach.DeptName = Take.DeptName)

AND (Take.StudentPID = Student.PID)

AND (Student.Name = ’Suri’);

Professors who have Taught ’Suri’

I “Find the (names of (Professors who have taught (courses taken bystudent with name ’Suri’))).”

SELECT Name

FROM Professors

WHERE PID IN

FROM Teach

WHERE (Number, DeptName) IN

(SELECT Number, DeptName

FROM Take, Students

WHERE (StudentPID = PID) AND

(Students.Name = ’Suri’)));

I “Find the (names of (Professors who have taught (courses taken bystudent with name ’Suri’))).”

SELECT Name

FROM Professors

WHERE PID IN

FROM Teach

FROM Take, Students

WHERE (StudentPID = PID) AND

(Students.Name = ’Suri’)));

I “Find the (names of (Professors who have taught (courses taken by(student with first name ’Suri’)))).”

SELECT Name

FROM Professors

WHERE PID IN

FROM Teach

FROM Take

WHERE StudentPID IN

(SELECT PID

FROM Students

WHERE Name = ’Suri’)));

I “Find the (names of (Professors who have taught (courses taken by(student with first name ’Suri’)))).”

SELECT Name

FROM Professors

WHERE PID IN

FROM Teach

FROM Take

WHERE StudentPID IN

(SELECT PID

FROM Students

WHERE Name = ’Suri’)));

SQL Subqueries - Undergraduate Courses | Computer …courses.cs.vt.edu/~cs4604/Fall10/lectures/lecture-04-sq… · · 2010-09-02SQL Subqueries T. M. Murali September 1, 2010 T.

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