Today
Announcements
Finish RDM (Chapter 5), begin relational algebra
Reading
Sections 6.0-6.5
Program 2
Due Friday
Exam
Tuesday Oct 16, in class Closed book Will cover material through Thursday's lecture
Class bufMgr{ ... // Call DB object to allocate a run of new pages and // find a frame in the buffer pool for the first page // and pin it. If buffer is full, ask DB to deallocate // all these pages and return error Status newPage(int& firstPageId, Page*& firstpage,int howmany=1);
// Check if this page is in buffer pool, otherwise // find a frame for this page, read in and pin it. // Also write out the old page if it's dirty before reading // if emptyPage==TRUE, then actually no read is done to bring // the page in. Status pinPage(int PageId_in_a_DB, Page*& page, int emptyPage=0, const char *filename=NULL); ... }
// // in your code // PageId pid; Page *pPageTmp SecondaryIndexHeaderPage *pHeaderPage; MINIBASE_BM->newPage( pid, pPageTmp ); pHeaderPage = (SecondaryIndexHeaderPage*)pPageTmp;
Constraints
Constraints
A key aspect of RDM is the ability to impose constraints on the database state A constraint on a single relation places restrictions on valid relation states
Examples: two students can't have same student ID number
Example of key constraint
Student name cannot be NULL Domain constraints (implicit)
Key Constraints
Superkey of R:
Is a set of attributes SK of R with the following condition: No two tuples in any valid relation state r(R) will have the same value for SK That is, for any distinct tuples t1 and t2 in r(R), t1[SK] t2[SK] This condition must hold in any valid state r(R)
Key of R:
A "minimal" superkey That is, a key is a superkey K such that removal of any attribute from K results in a set of attributes that is not a superkey (does not possess the superkey uniqueness property)
Key Constraints (continued)
Example: Consider the CAR relation schema:
CAR(State, Reg#, SerialNo, Make, Model, Year) CAR has two keys: Key1 = {State, Reg#} Key2 = {SerialNo} Both are also superkeys of CAR {SerialNo, Make} is a superkey but not a key.
In general:
Any key is a superkey (but not vice versa) Any set of attributes that includes a key is a superkey A minimal superkey is also a key
Key Constraints (continued)
If a relation has several candidate keys, one is chosen arbitrarily to be the primary key.
The primary key attributes are underlined.
Example: Consider the CAR relation schema:
CAR(State, Reg#, SerialNo, Make, Model, Year) We chose SerialNo as the primary key
The primary key value is used to uniquely identify each tuple in a relation
Provides the tuple identity
Also used to reference the tuple from another tuple
General rule: Choose as primary key the smallest of the candidate keys (in terms of size) Not always applicable choice is sometimes subjective
CAR table with two candidate keys LicenseNumber chosen as Primary Key
Multiple Relations
Typically, a RDB has many relations
Relational Database Schema
Relational Database Schema:
A set S of relation schemas that belong to the same database. S is the name of the whole database schema S = {R1, R2, ..., Rn} R1, R2, ..., Rn are the names of the individual relation schemas within the database S
COMPANY Database Schema
Referential Integrity Constraints involve Two Relations
Example: DEPT_LOCATION.Dnumber must refer to an existing tuple in DEPARTMENT Operationalized through concept of foreign key
Referential Integrity
Tuples in the referencing relation R1 have attributes FK (called foreign key attributes) that reference the primary key attributes PK of the referenced relation R2.
A tuple t1 in R1 is said to reference a tuple t2 in R2 if t1[FK] = t2[PK].
A referential integrity constraint can be displayed in a relational database schema as a directed arc from R1.FK to R2.
Referential Integrity (or foreign key) Constraint
Statement of the constraint
The value in the foreign key column (or columns) FK of the the referencing relation R1 can be either: (1) a value of an existing primary key value of a corresponding primary key PK in the referenced relation R2, or (2) a null.
In case (2), the FK in R1 should not be a part of its own primary key.
Referential Integrity Constraints for COMPANY database
Other Types of Constraints
Semantic Integrity Constraints:
based on application semantics and cannot be expressed by the model per se Example: "the max. no. of hours per employee for all projects he or she works on is 56 hrs per week"
A constraint specification language may have to be used to express these SQL-99 allows triggers and ASSERTIONS to express for some of these
Update Operations on Relations Must not Violate Constraints
INSERT a tuple. DELETE a tuple. MODIFY a tuple. Several update operations may have to be grouped together (a transaction) Updates may propagate to cause other updates automatically. This may be necessary to maintain integrity constraints.
Update Operations on Relations
In case of integrity violation, several actions can be taken:
Cancel the operation that causes the violation (RESTRICT or REJECT option) Perform the operation but inform the user of the violation Trigger additional updates so the violation is corrected (CASCADE option, SET NULL option) Execute a user-specified error-correction routine
Possible violations for each operation
INSERT may violate any of the constraints:
Domain constraint: if one of the attribute values provided for the new tuple is not of the specified attribute domain Key constraint: if the value of a key attribute in the new tuple already exists in another tuple in the relation Referential integrity: if a foreign key value in the new tuple references a primary key value that does not exist in the referenced relation Entity integrity: if the primary key value is null in the new tuple
Possible violations for each operation
DELETE may violate only referential integrity:
If the primary key value of the tuple being deleted is referenced from other tuples in the database Can be remedied by several actions: RESTRICT, CASCADE, SET NULL (see Chapter 8 for more details)
RESTRICT option: reject the deletion CASCADE option: propagate the new primary key value into the foreign keys of the referencing tuples SET NULL option: set the foreign keys of the referencing tuples to NULL
One of the above options must be specified during database design for each foreign key constraint
Possible violations for each operation
UPDATE may violate domain constraint and NOT NULL constraint on an attribute being modified Any of the other constraints may also be violated, depending on the attribute being updated:
Updating the primary key (PK): Similar to a DELETE followed by an INSERT Need to specify similar options to DELETE Updating a foreign key (FK): May violate referential integrity Updating an ordinary attribute (neither PK nor FK): Can only violate domain constraints
In-Class Exercise
(Taken from Exercise 5.15) Consider the following relations for a database that keeps track of student enrollment in courses and the books adopted for each course: STUDENT(SSN, Name, Major, Bdate) COURSE(Course#, Cname, Dept) ENROLL(SSN, Course#, Quarter, Grade) BOOK_ADOPTION(Course#, Quarter, Book_ISBN) TEXT(Book_ISBN, Book_Title, Publisher, Author) Draw a relational schema diagram specifying the foreign keys for this schema.
Relational Algebra Chapter 6
Relational Algebra
Relational Algebra: a formal query language associated with the relational model example query: "retrieve the last name and department location of all employees that more than 35,000" "select" operator TMP1 Salary > 30000 (EMPLOYEE) "join" operator
TMP1 (Dno == Dnumber) DEPT_LOCATION RESULT Lname, DLocation (TMP2)
"projection" operator
TMP2
Relational Algebra queries are composed of a set of operators
Inputs and outputs of queries are relations
thus the algebra is "closed"
Query is evaluated using states of the input relations and produces a state of the output relation.
unary operator input relation input relation binary operator Output relation input relation Output relation
Relational Algebra expression
Output from one operator can input to another
EMP.
TMP1 TMP2 Dept. Loc
RESLT
Relational Algebra Overview
Relational Algebra consists of several groups of operations
Unary Relational Operations SELECT (symbol: (sigma)) PROJECT (symbol: (pi)) RENAME (symbol: (rho)) Relational Algebra Operations From Set Theory UNION ( ), INTERSECTION ( ), DIFFERENCE (or MINUS, ) CARTESIAN PRODUCT ( x ) Binary Relational Operations JOIN (several variations of JOIN exist) DIVISION Additional Relational Operations OUTER JOINS, OUTER UNION AGGREGATE FUNCTIONS (These compute summary of information: for example, SUM, COUNT, AVG, MIN, MAX)
SELECT
Unary Relational Operations: SELECT
The SELECT operation (denoted by (sigma)) is used to select a subset of the tuples from a relation based on a selection condition. The selection condition acts as a filter Keeps only those tuples that satisfy the qualifying condition Tuples satisfying the condition are selected whereas the other tuples are discarded (filtered out) Examples:
Select the EMPLOYEE tuples whose department number is 4:
DNO = 4 (EMPLOYEE)
Select the employee tuples whose salary is greater than $30,000:
SALARY > 30,000 (EMPLOYEE)
Unary Relational Operations: SELECT
In general, the select operation is denoted by <selection condition>(R) where
the symbol (sigma) is used to denote the select operator the selection condition is a Boolean (conditional) expression specified on the attributes of relation R tuples that make the condition true are selected
appear in the result of the operation
tuples that make the condition false are filtered out
discarded from the result of the operation
Unary Relational Operations: SELECT (contd.)
SELECT Operation Properties
The SELECT operation <selection condition>(R) produces a relation S that has the same schema (same attributes) as R SELECT is commutative:
<condition1>( < condition2> (R)) = <condition2> ( < condition1> (R)) Because of commutativity property, a cascade (sequence) of SELECT operations may be applied in any order: <cond1>(<cond2> (<cond3> (R)) = <cond2> (<cond3> (<cond1> ( R))) A cascade of SELECT operations may be replaced by a single selection with a conjunction of all the conditions: <cond1>(< cond2> (<cond3>(R)) = <cond1> AND < cond2> AND < cond3>(R))) The number of tuples in the result of a SELECT is less than (or equal to) the number of tuples in the input relation R
PROJECT
Unary Relational Operations: PROJECT PROJECT Operation is denoted by (pi) This operation keeps certain columns (attributes) from a relation and discards the other columns.
PROJECT creates a vertical partitioning The list of specified columns (attributes) is kept in each tuple The other attributes in each tuple are discarded
Example: To list each employee's first and last name and salary, the following is used:
LNAME, FNAME,SALARY(EMPLOYEE)
Unary Relational Operations: PROJECT (cont.) The general form of the project operation is: <attribute list>(R)
(pi) is the symbol used to represent the project operation <attribute list> is the desired list of attributes from relation R.
The project operation removes any duplicate tuples
This is because the result of the project operation must be a set of tuples Mathematical sets do not allow duplicate elements.
Unary Relational Operations: PROJECT (contd.) PROJECT Operation Properties
The number of tuples in the result of projection <list>(R) is always less or equal to the number of tuples in R If the list of attributes includes a key of R, then the number of tuples in the result of PROJECT is equal to the number of tuples in R PROJECT is not commutative <list1> ( <list2> (R) ) = <list1> (R) as long as <list2> contains the attributes in <list1>
Set Theory Operators: UNION, INTERSECTION & MINUS
Relational Algebra Operations from Set Theory: UNION UNION Operation
Binary operation, denoted by The result of 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 The two operand relations R and S must be "type compatible" (or UNION compatible) R and S must have same number of attributes Each pair of corresponding attributes must be type compatible (have same or compatible domains)
Relational Algebra Operations from Set Theory: UNION
Example:
To retrieve the social security numbers of all employees who either work in department 5 (RESULT1 below) or directly supervise an employee who works in department 5 (RESULT2 below) We can use the UNION operation as follows:
DEP5_EMPS DNO=5 (EMPLOYEE) RESULT1 SSN(DEP5_EMPS) RESULT2(SSN) SUPERSSN(DEP5_EMPS) RESULT RESULT1 RESULT2
The union operation produces the tuples that are in either RESULT1 or RESULT2 or both
Example of the result of a UNION operation UNION Example
Relational Algebra Operations from Set Theory
Type Compatibility of operands is required for the binary set operation UNION , (also for INTERSECTION , and SET DIFFERENCE , see next slides) R1(A1, A2, ..., An) and R2(B1, B2, ..., Bn) are type compatible if:
they have the same number of attributes, and the domains of corresponding attributes are type compatible (i.e. dom(Ai)=dom(Bi) for i=1, 2, ..., n).
The resulting relation for R1R2 (also for R1R2, or R1R2, see next slides) has the same attribute names as the first operand relation R1 (by convention)
Relational Algebra Operations from Set Theory: INTERSECTION
INTERSECTION is denoted by The result of the operation R S, is a relation that includes all tuples that are in both R and S
The attribute names in the result will be the same as the attribute names in R
The two operand relations R and S must be "type compatible"
Relational Algebra Operations from Set Theory: SET DIFFERENCE (cont.) SET DIFFERENCE (also called MINUS or EXCEPT) is denoted by The result of R S, is a relation that includes all tuples that are in R but not in S The attribute names in the result will be the same as the attribute names in R
The two operand relations R and S must be "type compatible"
Example to illustrate the result of UNION, INTERSECT, and DIFFERENCE
Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT
CARTESIAN (or CROSS) PRODUCT Operation
This operation is used to combine tuples from two relations in a combinatorial fashion. Denoted by R(A1, A2, . . ., An) x S(B1, B2, . . ., Bm) Result is a relation Q with degree n + m attributes: Q(A1, A2, . . ., An, B1, B2, . . ., Bm), in that order. The resulting relation state has one tuple for each combination of tuples--one from R and one from S. Hence, if R has nR tuples (denoted as |R| = nR ), and S has nS tuples, then R x S will have nR * nS tuples. The two operands do NOT have to be "type compatible"
Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT (cont.) Generally, CROSS PRODUCT is not a meaningful operation
Can become meaningful when followed by other operations
Example (not meaningful):
FEMALE_EMPS SEX='F'(EMPLOYEE) EMPNAMES FNAME, LNAME, SSN (FEMALE_EMPS) EMP_DEPENDENTS EMPNAMES x DEPENDENT
EMP_DEPENDENTS will contain every combination of EMPNAMES and DEPENDENT
whether or not they are actually related
Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT (cont.) To keep only combinations where the DEPENDENT is related to the EMPLOYEE, we add a SELECT operation as follows Example (meaningful):
FEMALE_EMPS SEX='F'(EMPLOYEE) EMPNAMES FNAME, LNAME, SSN (FEMALE_EMPS) EMP_DEPENDENTS EMPNAMES x DEPENDENT ACTUAL_DEPS SSN=ESSN(EMP_DEPENDENTS) RESULT FNAME, LNAME, DEPENDENT_NAME (ACTUAL_DEPS)
RESULT will now contain the name of female employees and their dependents
Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT
CARTESIAN (or CROSS) PRODUCT Operation
This operation is used to combine tuples from two relations in a combinatorial fashion. Denoted by R(A1, A2, . . ., An) x S(B1, B2, . . ., Bm) Result is a relation Q with degree n + m attributes: Q(A1, A2, . . ., An, B1, B2, . . ., Bm), in that order. The resulting relation state has one tuple for each combination of tuples--one from R and one from S. Hence, if R has nR tuples (denoted as |R| = nR ), and S has nS tuples, then R x S will have nR * nS tuples. The two operands do NOT have to be "type compatible"
Binary Relational Operations: JOIN
JOIN Operation (denoted by )
The sequence of CARTESIAN PRODECT followed by SELECT is used quite commonly to identify and select related tuples from two relations A special operation, called JOIN combines this sequence into a single operation This operation is very important for any relational database with more than a single relation, because it allows us combine related tuples from various relations The general form of a join operation on two relations R(A1, A2, . . ., An) and S(B1, B2, . . ., Bm) is: ...