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SQL: Updates (DML) and Views (DDL)

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SQL DML (Updating the Data)• Insert• Delete• Update

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Inserting tuplesINSERT INTO Student VALUES

(6, ‘Emily’, ‘324 FL’, NULL);

INSERT INTO Student (sNumber, sName) VALUES (6, ‘Emily’);

INSERT INTO Professor (pNumber)SELECT professorFROM Student;

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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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Views

NOTE: You can present logical subsets or combinations of the data by creatingviews of tables. A view is a virtual table based on a table or another view. A viewcontains no data of its own but is like a window through which data from tables canbe 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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Views

View is a virtual relation defined by:

Named stored SQL query

Views can be queried like any “base” relation.

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Views

CREATE VIEW <viewName> as <query>

CREATE VIEW studentProfessor (student, professor) ASSELECT sName, pNameFROM Student, ProfessorWHERE Student.professor = Professor.pNumber;

DROP VIEW studentProfessor

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Views - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

CREATE VIEW studentProfessor (student, professor) AS

SELECT sName, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

SELECT * from studentProfessor

student professor

Dave MM

Greg MM

Matt ER

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Views - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

CREATE VIEW studentProfessor (student, professor) AS

SELECT sName, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

SELECT professor, count(*) FROM studentProfessorGROUPBY professor;

student professor

Dave MM

Greg MM

Matt ER

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Querying ViewsCREATE VIEW studentProfessor (student, professor) AS

SELECT sName, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

SELECT pnumber as professor, count(*) FROM Student, ProfessorWHERE Student.professor = Professor.pNumberGROUPBY professor;

student professor

Dave MM

Greg MM

Matt ER

SELECT professor, count(*) FROM studentProfessorGROUPBY professor;

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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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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 are updatable?

Answer: Updating these views are done by updating the underlying Student tables.

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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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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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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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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) ASSELECT DISTINCT sNumber FROM Student;

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Updating Single Relation Views

WHERE clause may specify subqueries.

CREATE VIEW MyStudent3 (num, name) ASSELECT sNumber, sName FROM StudentWHERE 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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Multiple relation views: Delete

Consider a multi-relation view such asCREATE VIEW studentProf (studentname, profname)AS SELECT sName, pNameFROM Student, ProfessorWHERE SName = PName;

-- Ambigious what base table to update ? -- Side effects of other tuples disappear out of the view.

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Multiple relation viewsConsider a multi-relation view such as

CREATE VIEW studentProf (studentname, profname)AS SELECT sName, pNameFROM Student, ProfessorWHERE SName = PName;

sNumber sName address professor

1 MM 320FL 1

2 MM 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

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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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Views - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

CREATE VIEW studentProfessor (student, professor) AS

SELECT sNumber, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

student professor

1 MM

2 MM

3 ER

1. pNumber is key in Professor2. sNumber is key of Student3. sNumber is key of view

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Deleting from multi-relation views

Try the following update statements:

DELETE FROM studentProfessor WHERE professor='MM';

-- What will be deleted ?

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Views - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

CREATE VIEW studentProfessor (student, professor) AS

SELECT sNumber, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

student professor

1 MM

2 MM

3 ER

DELETE FROM studentProfessor WHERE professor='MM';-- This will actually delete the two rows in the student table.

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Views - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 235FL

2 ER 241FL

Professor

CREATE VIEW studentProfessor (student, professor) AS

SELECT sNumber, pNameFROM Student, ProfessorWHERE Student.professor =

Professor.pNumber;

Suppose we drop key constraint on the professor table for this view.

Now delete will fail because there is no table whose key is the key of the view.

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Inserting into multi-relation views

Consider view definition:CREATE VIEW studentProf(student, professor) AS SELECT sNumber, pName FROM Student, ProfessorWHERE 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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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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Views - Summary

Views are useful – Virtual relations

Querying through views is always possible

Updating through views has limitations