Chapter 6 The Relational Algebra. The SELECT Operation (1/2) The SELECT operation is used to select...

Chapter 6

The Relational Algebra

The SELECT Operation (1/2)

• The SELECT operation is used to select a subset of the tuples from a relation that satisfy a selection condition.

<selection condition>(R)

(DNO=4 AND SALARY>25000) OR (DNO=5 AND SALARY>30000)(EMPLOYEE)

The SELECT Operation (2/2)

• The degree of the relation resulting from a SELECT operation is the same as that of R.

• The number of tuples in the resulting relation is always less than or equal to the number of tuples in R.

• The fraction of tuples selected by a selection condition is referred to as the selectivity of the condition.

• The SELECT operation is commutative.

• We can always combine a cascade of SELECT operations into a single SELECT operation with a conjunctive (AND) condition.

The PROJECT Operation (1/2)

• The PROJECT operation, on the other hand, selects certain columns from the table and discards the other columns.

<attribute list>(R)

SEX, SALARY(EMPLOYEE)

The PROJECT Operation (2/2)

• The number of tuples in a relation resulting from a PROJECT operation is always less than or equal to the number of tuples in R. – If the projection list is a superkey of R—that is, it includes some k

ey of R—the resulting relation has the same number of tuples as R.

• Commutativity does not hold on PROJECT.

Sequences of Operations and the RENAME Operation (1/4)

• In general, we may want to apply several relational algebra operations one after the other.

• Either we can write the operations as a single relational algebra expression by nesting the operations, or we can apply one operation at a time and create intermediate result relations.

• In the latter case, we must name the relations that hold the intermediate results.


FNAME, LNAME, SALARY(DNO= 5(EMPLOYEE)) • DEP5_EMPS DNO=5(EMPLOYEE)

RESULT FNAME, LNAME, SALARY(DEP5_EMPS)


• To rename the attributes in a relation, we simply list the new attribute names in parentheses.

• TEMP DNO=5(EMPLOYEE)

R(FIRSTNAME, LASTNAME, SALARY) FNAME, LNAME, SALARY (TEMP)


• We can also define a RENAME operation—which can rename either the relation name, or the attribute names, or both.

• The general RENAME operation when applied to a relation R of degree n is denoted by S(B1, B2, ..., Bn)(R) or S(R) or (B1, B2, ..., Bn)(R)

Set Theoretic Operations (1/7)

• DEP5_EMPS DNO=5(EMPLOYEE)

RESULT1 SSN(DEP5_EMPS)

RESULT2(SSN) SUPERSSN(DEP5_EMPS)

RESULT RESULT1 RESULT2


• Two relations R(A1, A2, . . ., An) and S(B1, B2, . . ., Bn) are

said to be union compatible if they have the same degree n, and if dom(Ai) = dom(Bi) for 1 i n.

• This means that the two relations have the same number of attributes and that each pair of corresponding attributes have the same domain.


• UNION: The result of this operation, denoted by R S, is a relation that includes all tuples that are either in R or in S or in both R and S. Duplicate tuples are eliminated.

• INTERSECTION: The result of this operation, denoted by R S, is a relation that includes all tuples that are in both R and S.

• SET DIFFERENCE: The result of this operation, denoted by R - S, is a relation that includes all tuples that are in R but not in S.


• Both UNION and INTERSECTION are commutative operations.

• Both union and intersection can be treated as n-ary operations applicable to any number of relations as both are associative operations.


• Next we discuss the CARTESIAN PRODUCT operation—also known as CROSS PRODUCT or CROSS JOIN—denoted by x, which is also a binary set operation, but the relations on which it is applied do not have to be union compatible.

• This operation is used to combine tuples from two relations in a combinatorial fashion.


• In general, the result of R(A1, A2, . . ., An) x S(B1, B2, . . .,

Bm) is a relation Q with n + m attributes Q(A1, A2, . . ., An,

B1, B2, . . ., Bm), in that order.

• If R has nR tuples and S has nS tuples, then R x S will have

nR * nS tuples.

• The operation applied by itself is generally meaningless.


• FEMALE_EMPS SEX=’F’(EMPLOYEE)

EMPNAMES FNAME, LNAME, SSN(FEMALE_EMPS)

EMP_DEPENDENTS EMPNAMES x DEPENDENTACTUAL_DEPENDENTS SSN=ESSN(EMP_DEPENDENTS)

RESULT FNAME, LNAME, DEPENDENT_NAME(ACTUAL_DEPENDENTS)

The JOIN Operation (1/7)

• DEPT_MGR DEPARTMENT MGRSSN=SSN EMPLOYEE

RESULT DNAME, LNAME, FNAME(DEPT_MGR)


• The result of the JOIN is a relation Q with n + m attributes Q(A1, A2, . . ., An, B1, B2, . . ., Bm) in that order; Q has one t

uple for each combination of tuples—one from R and one from S—whenever the combination satisfies the join condition.


• A general join condition is of the form: <condition> AND <condition> AND . . . AND <condition> where each condition is of the form Ai Bj, Ai is an attribut

e of R, Bj is an attribute of S, Ai and Bj have the same doma

in, and (theta) is one of the comparison operators {=, <,

, >, , }.

• A JOIN operation with such a general join condition is called a THETA JOIN.


• The most common JOIN involves join conditions with equality comparisons only.

• Such a JOIN, where the only comparison operator used is =, is called an EQUIJOIN.


• A new operation called NATURAL JOIN—denoted by *—was created to get rid of the second (superfluous) attribute in an EQUIJOIN condition.

• In general, NATURAL JOIN is performed by equating all attribute pairs that have the same name in the two relations.


• PROJ_DEPT PROJECT * (DNAME, DNUM,MGRSSN,MGRSTARTDATE) (DEPARTMENT)

• DEPT_LOCS DEPARTMENT * DEPT_LOCATIONS


• In general, if R has nR tuples and S has nS tuples, the result

of a JOIN operation R <join condition>S will have between zero

and nR * nS tuples.

• The expected size of the join result divided by the maximum size nR * nS leads to a ratio called join selectivity, which

is a property of each join condition.

A Complete Set of Relational Algebra Operations

• It has been shown that the set of relational algebra operations {, , , -, x} is a complete set; that is, any of the other relational algebra operations can be expressed as a sequence of operations from this set.

The DIVISION Operation (1/3)

• Retrieve the names of employees who work on all the projects that ‘John Smith’ works on.

• SMITH FNAME=’John’ AND LNAME=’Smith’(EMPLOYEE)

SMITH_PNOS PNO(WORKS_ON ESSN=SSN SMITH)

SSN_PNOS ESSN,PNO (WORKS_ON)

SSNS(SSN) SSN_PNOS ÷ SMITH_PNOSRESULT FNAME, LNAME(SSNS * EMPLOYEE)


• In general, the DIVISION operation is applied to two relations R(Z) ÷ S(X), where X Z.

• Let Y = Z - X (and hence Z = X Y); that is, let Y be the set of attributes of R that are not attributes of S.

• The result of DIVISION is a relation T(Y) that includes a tuple t if tuples tR appear in R with tR[Y] = t, and with tR[X]

= tS for every tuple tS in S.


• The DIVISION operator can be expressed as a sequence of , x, and - operations as follows:– T1 Y(R)

T2 Y((S x T1) - R)

T T1 - T2

Additional Relational Operations

Aggregate Functions and Grouping (1/2)

• We can define an AGGREGATE FUNCTION operation, using the symbol ℱ (pronounced "script F"), to specify these types of requests as follows: <grouping attributes> ℱ <function list> (R) where <grouping attributes> is a list of attributes of the relation specified in R, and <function list> is a list of (<function> <attribute>) pairs.

• In each such pair, <function> is one of the allowed functions—such as SUM, AVERAGE, MAXIMUM, MINIMUM, COUNT—and <attribute> is an attribute of the relation specified by R.

• The resulting relation has the grouping attributes plus one attribute for each element in the function list.

Aggregate Functions and Grouping (2/2)

R(DNO, NO_OF_EMPLOYEES, AVERAGE_SAL) (DNO ℱ COUNT SSN, AVERAGE SALARY (EMPLOYEE))

• DNO ℱ COUNT SSN, AVERAGE SALARY(EMPLOYEE)

• ℱ COUNT SSN, AVERAGE SALARY(EMPLOYEE)

Recursive Closure Operations

• Another type of operation that, in general, cannot be specified in the basic relational algebra is recursive closure.

• This operation is applied to a recursive relationship between tuples of the same type.

OUTER JOIN and OUTER UNION Operations (1/5)

• A set of operations, called OUTER JOINs, can be used when we want to keep all the tuples in R, or those in S, or those in both relations in the result of the JOIN, whether or not they have matching tuples in the other relation.


• For example, suppose that we want a list of all employee names and also the name of the departments they manage if they happen to manage a department; we can apply an operation LEFT OUTER JOIN, denoted by , to retrieve the result as follows: – TEMP (EMPLOYEE SSN=MGRSSN DEPARTMENT)

RESULT FNAME, MINIT, LNAME, DNAME(TEMP)


• A similar operation, RIGHT OUTER JOIN, keeps every tuple in the second or right relation.

• A third operation, FULL OUTER JOIN, keeps all tuples in both the left and the right relations when no matching tuples are found, padding them with null values as needed.


• The OUTER UNION operation was developed to take the union of tuples from two relations if the relations are not union compatible.

• This operation will take the UNION of tuples in two relations that are partially compatible, meaning that only some of their attributes are union compatible.


• For example, an OUTER UNION can be applied to two relations whose schemas are STUDENT(Name, SSN, Department, Advisor) and FACULTY(Name, SSN, Department, Rank).

• The resulting relation schema is R(Name, SSN, Department, Advisor, Rank), and all the tuples from both relations are included in the result.

• Student tuples will have a null for the Rank attribute, whereas faculty tuples will have a null for the Advisor attribute.

• A tuple that exists in both will have values for all its attributes.

Examples of Queries in Relational Algebra (1/7)

• Retrieve the name and address of all employees who work for the ‘Research’ department.

• RESEARCH_DEPT DNAME=’Research’(DEPARTMENT)

RESEARCH_EMPS (RESEARCH_DEPT DNUMBER=DNOEMPLOYEE)

RESULT FNAME, LNAME, ADDRESS(RESEARCH_EMPS)


• For every project located in ‘Stafford’, list the project number, the controlling department number, and the department manager’s last name, address, and birthdate.

• STAFFORD_PROJS PLOCATION=’Stafford’(PROJECT)

CONTR_DEPT (STAFFORD_PROJS DNUM=DNUMBER DEPARTMENT)

PROJ_DEPT_MGR (CONTR_DEPT MGRSSN=SSN EMPLOYEE)

RESULT PNUMBER, DNUM, LNAME, ADDRESS, BDATE(PROJ_DEPT_MGR)


• Find the names of employees who work on all the projects controlled by department number 5.

• DEPT5_PROJS(PNO) PNUMBER(DNUM= 5(PROJECT))

EMP_PRJO(SSN, PNO) ESSN, PNO(WORKS_ON)

RESULT_EMP_SSNS EMP_PRJO ÷ DEPT5_PROJS RESULT LNAME, FNAME(RESULT_EMP_SSNS * EMPLOYEE)


• Make a list of project numbers for projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manager of the department that controls the project.

• SMITHS(ESSN) SSN(LNAME=’Smith’(EMPLOYEE))

SMITH_WORKER_PROJ PNO(WORKS_ON * SMITHS)

MGRS LNAME, DNUMBER(EMPLOYEE SSN=MGRSSN DEPARTMENT)

SMITH_MANAGED_DEPTS (DNUM) DNUMBER(LNAME= ’Smith’(MGRS))

SMITH_MGR_PROJS(PNO) PNUMBER(SMITH_MANAGED_DEPTS * PRO

JECT) RESULT (SMITH_WORKER_PROJS SMITH_MGR_PROJS)


• List the names of all employees with two or more dependents.

• T1(SSN, NO_OF_DEPTS) ESSN ℱ COUNT DEPENDENT_NAME(DEPENDENT)

T2 NO_OF_DEPS 2(T1)

RESULT LNAME, FNAME(T2 * EMPLOYEE)


• Retrieve the names of employees who have no dependents. • ALL_EMPS SSN(EMPLOYEE)

EMPS_WITH_DEPS(SSN) ESSN(DEPENDENT)

EMPS_WITHOUT_DEPS (ALL_EMPS - EMPS_WITH_DEPS) RESULT LNAME, FNAME(EMPS_WITHOUT_DEPS * EMPLOYEE)


• List the names of managers who have at least one dependent.

• MGRS(SSN) MGRSSN(DEPARTMENT)

EMPS_WITH_DEPS(SSN) ESSN(DEPENDENT)

MGRS_WITH_DEPS (MGRS EMPS_WITH_DEPS) RESULT LNAME, FNAME(MGRS_WITH_DEPS * EMPLOYEE)

Chapter 6 The Relational Algebra. The SELECT Operation (1/2) The SELECT operation is used to select...

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