1. SQL Views and Updates
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2. SQL DML (Updating the 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
FROM Student;
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4. Delete and Update Deleting tuples DELETE FROM Student
WHERE sNumber=‘6’;
Updating tuples
UPDATE Student SET professor=‘ER’
WHERE sNumber=‘6’
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5. 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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6. Views View is a virtual relation defined by:
Named stored SQL query
Views can be queried like any “base” relation.
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7. 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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8. 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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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
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 professor, count(*)
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. Updating Views
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?
Answer: Updating these views are done by updating the underlying Student tables.
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13. Updating Single Relation Views
DELETE FROM MyStudent WHERE name=`Dave';
-- This will delete the corresponding row from the Student table
DELETE FROM Student WHERE name=`Dave';
The update is valid !
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14. Updating Single Relation Views
INSERT INTO MyStudent VALUES (4, `Mary’);
-- This will be translated to:
INSERT INTO Student (sNumber, sName) VALUES (4, `Mary’);
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15. 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 must not be null.
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16. Updating Single Relation views
If the SELECT clause specifies DISTINCT, then the view is not updatable.
For instance, the following view is not updatable.
CREATE VIEW MyStudent2(num) AS
SELECT DISTINCT sNumber
FROM Student;
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17. Updating Single Relation Views
WHERE clause may specify subqueries.
CREATE VIEW MyStudent3 (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, though that tuple does not appear in the view.
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18. Multiple relation views: Delete
Consider a multi-relation view such as
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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19. Multiple relation views
Consider a multi-relation view such as
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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20. Multi-Relation View
Deletes can be done against multi-relation views if there is a table such that the view and the table have the same key.
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21. 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
professor 1 MM 2 3 ER
CREATE VIEW studentProfessor (student, professor) 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
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22. Deleting from multi-relation views
Try the following update statements:
DELETE FROM studentProfessor WHERE professor='MM';
-- What will be deleted ?
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23. 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 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 professor='MM';
-- This will actually delete the two rows in the student table.
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24. 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 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 the professor table for this view.
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25. 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 VALUES (4, 'ER');
-- THIS ABOVE INSERT WILL FAIL AS IT TRIES TO INSERT INTO Professor TABLE AS WELL.
INSERT INTO Studentprof(student) VALUES (4);
-- THIS ABOVE INSERT WILL SUCCEED.
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26. 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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27. Views Conclusion Views are useful – Virtual relations
Querying through views is always possible
Updating through views has limitations
DBA can set up INSTEAD OF triggers
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