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Announcements

• Read 6.7 – 6.10 for Friday• Homework 6, due Friday 10/29• Research paper

– List of sources - due 10/29

• Department Seminar – The Role of Experimentation in

Computer Science by Marvin Zelkowitz today 3:00 pm in MH 040

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SQL Database Manipulation Language

Lecture 16

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DELETE OperatorDELETEFROM tablenameWHERE predicate;

• Used for deleting existing records from database• Can delete zero, one, many, or all records• Operation may not work if referential integrity

would be lost• Can use a sub-query to target records to be

deleted• If you delete all records from a table, its

structure still remains, and you can insert into it later

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SELECT StatementSELECT [DISTINCT] col-name [AS newname], [,col-name..]…FROM table-name [alias] [,table-name]…[WHERE predicate][GROUP BY col-name [,col-name]…[HAVING predicate]or[ORDER BY col-name [,col-name]…];

• Powerful command – equivalent to relational algebra’s SELECT, PROJECT, JOIN and more…

• Can be applied to one or more tables or views• Can display one or more columns (renaming if desired)• Predicate is optional, and may include usual operators and

connectives• Can put results in order by one or more columns• Can also group together records with the same value for column(s)• Can also use predefined functions

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Example – Simple Retrieval with Condition

• Get names, IDs, and number of credits of all Math majors

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Example – Use of Asterisk Notation for “all columns”

• Get all information about CSC Faculty

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Example – Retrieval without Condition

• Get the course number of courses in which students are enrolled

• Eliminate duplicates

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Example – Use of “ORDERED BY” and “AS”

• Get names and IDs of all Faculty members, arranged in alphabetical order by name. Call the resulting columns FacultyName and FacultyNumber

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Example – Use of Multiple Conditions

• Get names of all math majors who have more than 30 credits

standard comparison operators: =, <>, <, <=, >, >=standard logical operators: AND, OR, and NOT

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Example – Natural Join

• Find IDS and names of all students taking ART103A

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Example – Natural Join with Ordering

• Find stuId and grade of all students taking any course taught by the Faculty member whose facId is F110. Arrange in order by stuId.

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Example – Use of Aliases

• Get a list of all courses that meet in the same room, with their schedules and room numbers

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Example – Natural Join of Three Tables

• Find course numbers and the names and majors of all students enrolled in the courses taught by Faculty member F110

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Example – Join without Equality Condition

• Find all combinations of students and Faculty where the student’s major is different from the Faculty member’s department

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Examples – Using a subquery with Equality

• Find the numbers of all the courses taught by Byrne of the math department

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Example – Subquery Using ‘IN’

• Find the names and IDS of all Faculty members who teach a class in Room H221

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Example – Nest Subqueries

• Get an alphabetical list of names and IDs of all students in any class taught by F110

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Example – Using EXISTS

• Find the names of all students enrolled in CSC201A

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Example – Query Using NOT EXIST

• Find the names of all students who are not enrolled in CSC201A

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Example – Query Using UNION

• Get IDs of all Faculty who are assigned to the history department or who teach in Room H221

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Example – Using Functions

• Find the total number of students enrolled in ART103A

COUNT returns the number of values in the columnSUM returns the sum of the values in the columnAVG returns the mean of the values in the columnMAX returns the largest value in the columnMIN returns the smallest value in the column

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Example – Using Functions

• Find the number of departments that have Faculty in them.

• Find the average number of credits student have.

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Examples – Using Functions

• Find the student with the largest number of credits.

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Examples – Using Functions

• Find the ID of the student(s) with the highest grade in any course

• Find names and IDs of students who have less than the average number of credits

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Example – Using an Expression and a String Constant

• Assuming each course is three credits list, for each student, the number of courses he or she has completed

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Example – Use of GROUP BY

• For each course, show the number of students enrolled

GROUP BY allows us to put together all records with a single value in the specified field

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Example – Use of HAVING

• Find all courses in which fewer than three students are enrolled

HAVING is used to determine which groups have a quality, just as WHERE is used with tuples to determine which records have some quality.

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Example – Use of LIKE

• Get details of all MTH courses

% The percent character stands for any sequence of characters of any length >= 0_ The underscore character stands for any single character.

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Example – Use of NULL

• Find the stuId and classNumber of all students whose grades in that course are missing

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Example – Inserting multiple records

• Create and fill a new table that shows each course and the number of students enrolled in it

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Example – Updating with a Query

• Change the room to B220 for all courses taught by Tanaka

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Example – Delete with a subquery

• Erase all enrollment records for Owen McCarthy

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Active Databases-Constraints

• DBMS monitors database to prevent illegal states, using constraints and triggers

• Constraints– can be specified when table is created, or later– IMMEDIATE MODE: constraint checked when

each INSERT, DELETE, UPDATE is performed– DEFERRED MODE: postpones constraint

checking to end of transaction – write SET CONSTRAINT name DEFERRED

– Can use DISABLE CONSTRAINT name, and later ENABLE CONSTRAINT name

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Triggers• More flexible than constraints• Must have three parts:

– event, some change made to the database– condition, a logical predicate (can be empty)– action, a procedure done when the event occurs and the condition

is true, also called firing the trigger• Can be fired before or after insert, update, delete• Trigger can access values it needs as :OLD. and :NEW.

– prefix :OLD refers to values in a tuple deleted or to the values replaced in an update

– prefix :NEW refers to the values in a tuple just inserted or to the new values in an update.

• Can specify whether trigger fires just once for each triggering statement, or for each row that is changed by the statement

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Trigger SyntaxCREATE OR REPLACE TRIGGER trigger_name[BEFORE/AFTER] [INSERT/UPDATE/DELETE] ON

table_name[FOR EACH ROW] [WHEN condition]BEGIN

trigger bodyEND;• Can disable triggers using ALTER TRIGGER name

DISABLE;• Later write ALTER TRIGGER name ENABLE;• Can drop triggers using DROP TRIGGER name;

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Trigger for Student Enrolling in a Class

CREATE TRIGGER ADDENROLLAFTER INSERT ON RevEnrollFOR EACH ROWBEGIN

UPDATE RevClassSET currentEnroll = currentEnroll + 1WHERE RevClass.classNumber = :NEW.classNumber;

END;

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Trigger for Student Dropping a Class

CREATE TRIGGER DROPENROLLAFTER DELETE ON RevEnrollFOR EACH ROWBEGIN

UPDATE RevClassSET currentEnroll = currentEnroll – 1WHERE RevClass.classNumber = :OLD.classNumber;

END;

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Trigger for Student Changing ClassesCREATE TRIGGER SWITCHENROLLAFTER UPDATE OF classNumber ON RevEnrollFOR EACH ROWBEGIN

UPDATE RevClassSET currentEnroll = currentEnroll + 1WHERE RevClass.classNumber = :NEW.classNumber;UPDATE RevClassSET currentEnroll = currentEnroll – 1WHERE RevClass.classNumber = :OLD.classNumber;

END;

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Trigger for Checking for Over-enrollment Before Enrolling StudentCREATE TRIGGER ENROLL_REQUESTBEFORE INSERT OR UPDATE OF classNumber ON RevEnrollFOR EACH ROWDECLARE

numStu number;maxStu number;

BEGINset maxEnroll into maxStufrom RevClasswhere RevClass.classNumber = :NEW.classNumber;

set currentEnroll + 1 into numStufrom RevClasswhere RevClass.classNumber = :NEW.classNumber;

if numStu > maxStu

RequestClosedCoursePermission(:NEW.stuId, :NEW.classNumber, RevClass.currentEnroll, RevClass.maxEnroll);end if;

END;

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Example Trigger

• Prevent students from enrolling in two classes that meet at the same time

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Ending Transactions

• COMMIT makes permanent changes in the current transaction

• ROLLBACK undoes changes made by the current transaction