1. SQL Views
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2. SQL DML (Updating Data) Insert Delete Update
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3. Inserting tuples INSERT INTO Student
VALUES (6, ‘Emily’, ‘324 FL’, NULL);
INSERT INTO Student (sNumber, sName)
VALUES (6, ‘Emily’);
INSERT INTO Professor (pNumber)
SELECT professor.id
FROM Student;
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4. Delete
DELETE FROM Student
WHERE sNumber=‘6’;
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5. SET professor=‘MickyMouse’ WHERE sNumber=‘6’
Update
UPDATE Student
SET professor=‘MickyMouse’
WHERE sNumber=‘6’
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6. Views
NOTE: You can present logical subsets or combinations of the data by creating
views of tables. A view is a virtual table based on a table or another view. A view
contains no data of its own but is like a window through which data from tables can
be viewed or changed. The tables on which a view is based are called base tables.
The view is stored as a SELECT statement in the data dictionary.
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7. Views View is a virtual relation defined by:
Named stored SQL query
Views can be queried like any “base” relation.
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8. Views CREATE VIEW <viewName> as <query>
CREATE VIEW studentProfessor (student, professor) AS
SELECT sName, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
DROP VIEW studentProfessor
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9. Views - Example Student Professor
sNumber
sName
address
professor 1
Dave
320FL 2
Greg 3
Matt
pNumber
pName
address 1 MM
235FL 2 ER
241FL
CREATE VIEW studentProfessor (student, professor) AS
SELECT sName, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
student
professor
Dave MM
Greg
Matt ER
SELECT * from studentProfessor
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10. Views - Example Student Professor
sNumber
sName
address
professor 1
Dave
320FL 2
Greg 3
Matt
pNumber
pName
address 1 MM
235FL 2 ER
241FL
student
professor
Dave MM
Greg
Matt ER
CREATE VIEW studentProfessor (student, professor) AS
SELECT sName, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
advisees
professor 2 MM 1 ER
SELECT professor, count(*) as advisees
FROM studentProfessor
GROUPBY professor;
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11. Querying Views CREATE VIEW studentProfessor (student, professor) AS
SELECT sName, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
student
professor
Dave MM
Greg
Matt ER
SELECT pnumber as professor, count(*)
FROM Student, Professor
WHERE Student.professor =
Professor.pNumber
GROUPBY professor;
SELECT professor, count(*)
FROM studentProfessor
GROUPBY professor;
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12. Views ? Why ? View is a virtual relation ?
Convenience: Queries on base relations might be “complex”
Logical Data Independence: “Base tables” may change, but still queries using views need not change.
Customization: Provide different views of the same data.
Security: Expose only necessary data to users
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13. Updating Views CREATE VIEW MyStudent(num,name)AS SELECT sNumber, sName
Consider views defined with only one relation in the FROM clause such as:
CREATE VIEW MyStudent(num,name)AS
SELECT sNumber, sName
FROM Student;
Question: Are these views updatable? How ?
Answer: Updating these views is done by updating the underlying Student base tables.
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14. Updating Single Relation Views
DELETE FROM MyStudent WHERE name=`Dave';
-- This will delete corresponding row from Student table
DELETE FROM Student WHERE name=`Dave';
This update is valid !
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15. Updating Single Relation Views
INSERT INTO MyStudent VALUES (4, `Mary’);
-- This will be translated to:
INSERT INTO Student (sNumber, sName) VALUES (4, `Mary’);
-- What happens to other values of Student tuple ?
-- What if there is a tuple with sNumber of 4 already ?
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16. Inserting into single relation views
CREATE VIEW MyStudent1(name)
AS SELECT sName FROM Student;
INSERT INTO MyStudent1 VALUES (‘Mary’)
will be translated to:
INSERT INTO Student(sName) VALUES (‘Mary’).
This will return an error as sNumber is primary key, i.e., it must not be null.
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17. Updating Single Relation views
What about views with DISTINCT ? Updatable?
CREATE VIEW MyStudent2(name) AS
SELECT DISTINCT sName
FROM Student;
CREATE VIEW MyStudent3(num) AS
SELECT DISTINCT sNumber
If the SELECT clause specifies DISTINCT, then the view is not updatable.
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18. Updating Single Relation Views
WHERE clause may specify subqueries.
CREATE VIEW MyStudent4 (num, name) AS
SELECT sNumber, sName
FROM Student
WHERE sName IN
(SELECT pName FROM Professor);
-- Insert into this view will insert into Student table
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19. Updating Single Relation Views
WHERE clause may specify subqueries.
CREATE VIEW MyStudent4 (num, name) AS
SELECT sNumber, sName
FROM Student
WHERE sNumber NOT IN
(SELECT sNumber FROM Student);
-- This view will always have 0 tuples.
-- Insert into this view will still insert into Student table, even though that tuple does not appear in the view.
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20. Multiple Relation Views: Delete
CREATE VIEW studentProf (studentname, profname)
AS SELECT sName, pName
FROM Student, Professor
WHERE sName = pName;
Student
Professor
sNumber
sName
address
professor 1 MM
320FL 2 3
Matt
pNumber
pName
address 1 MM
235FL 2 ER
241FL
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21. Multiple Relation Views: Delete
Updating this multi-relation view?
CREATE VIEW studentProf (studentname, profname)
AS SELECT sName, pName
FROM Student, Professor
WHERE SName = PName;
-- Ambigious what base table to update !
-- Side effects as other tuples may disappear out of the view !
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22. Multi-Relation View
Deletes can be done against multi-relation views if there is a table with same key as the view.
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23. Views - Example Student Professor pNumber is key in Professor
sNumber
sName
address
professor 1
Dave
320FL 2
Greg 3
Matt
pNumber
pName
address 1 MM
235FL 2 ER
241FL
student
profname 1 MM 2 3 ER
CREATE VIEW studentProfessor (student, profname) AS
SELECT sNumber, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
pNumber is key in Professor
sNumber is key of Student
sNumber is key of view
What is key in
studentProfessor table?
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24. Multi-Relation View
Reminder : Deletes can be done against multi-relation views if there is a table such that the view and table have same key.
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25. Deleting from multi-relation views
Try following update statements:
DELETE FROM studentProfessor WHERE profname ='MM';
-- What will be deleted ?
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26. Views - Example Student Professor
sNumber
sName
address
professor 1
Dave
320FL 2
Greg 3
Matt
pNumber
pName
address 1 MM
235FL 2 ER
241FL
student
profname 1 MM 2 3 ER
CREATE VIEW studentProfessor (student, professor) AS
SELECT sNumber, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
DELETE FROM studentProfessor WHERE profname='MM';
-- This will actually delete the two rows in Student table.
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27. Views - Example Student Professor
sNumber
sName
address
professor 1
Dave
320FL 2
Greg 3
Matt
pNumber
pName
address 1 MM
235FL ER
241FL
CREATE VIEW studentProfessor (student, professor) AS
SELECT sNumber, pName
FROM Student, Professor
WHERE Student.professor = Professor.pNumber;
Now delete will fail because there is no table whose key is the key of the view.
Suppose we drop key constraint
on professor table for this view.
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28. Inserting into multi-relation views
Consider view definition:
CREATE VIEW studentProf(student, professor)
AS SELECT sNumber, pName FROM Student, Professor
WHERE professor=pNumber;
INSERT INTO Studentprof(student) VALUES (4);
-- the above insert will succeed; put into Student table
INSERT INTO Studentprof VALUES (4, 'ER');
-- THIS ABOVE INSERT WILL FAIL AS IT TRIES TO INSERT INTO Professor TABLE AS WELL.
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29. Inserting into multi-relation views
Insert will succeed only if
The insert translates to insert into only one table.
The key for the table to be inserted will also be a key for the view.
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30. Controlling Updates on Views
CREATE VIEW studentProf (student, professor, salary) AS
SELECT sName, pName, salary
FROM Student s, Professor p
WHERE s.advisor = pName;
Application can use TRIGGER to create desired effect of updates on view:
CREATE OR REPLACE TRIGGER DeleteMyView
INSTEAD OF DELETE ON studentProf FOR EACH ROW
DECLARE name varchar (10);
BEGIN
name := :old.student;
DELETE FROM Student where sName=name;
END; .
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31. Materialized Views Materialized View : Cache content of view Why ?
Issues :
Type of “connectivity” to base data
Keeping the view content up-to-date, also called view maintenance
Utilization of INSTEAD OF TRIGGERS for maintenance
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32. Views - Summary Views are useful – Virtual relations
Querying through views is always possible
Updating through views has limitations – consider use of INSTEAD OF TRIGGERS
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