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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 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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Delete and Update

Deleting tuples

DELETE FROM Student

WHERE sNumber=6;

Updating tuples

UPDATE Student SET professor=ERWHERE sNumer=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

Convenience: Queries on base relations might be

complex

Logical Data Independence: base tables maychange, but still queries using views need not

change.

Provide different views of the same data.

Security: Expose only necessary data to users

Views can be queried like any base relation.

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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, pName

FROM Student, Professor

WHERE Student.professor =Professor.pNumber;

SELECT * from studentProfessor

student professor

Dave MM

Greg MM

Matt ER

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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;

These views are updatable. Updating theseviews are done by updating the underlying

Student tables.

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Updating Single relation Views

For instance, the following updates are valid:DELETE FROM MyStudent WHERE name=`Dave';

-- This will delete the corresponding rowfrom the Student table

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

Consider the viewCREATE 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) AS

SELECT DISTINCT sNumber FROM Student;

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Updating Single Relation

Views

Note that the WHERE clause may specifysubqueries. Let us consider an extremeexample.

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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Multiple relation views: Delete

Consider a multi-relation view such asCREATE VIEW studentProf(student, professor)

AS SELECT sName, pName

FROM Student, ProfessorWHERE professor=pNumber;

-- Note that the pNumber is key for professor. We

see that sNumber is a key for Student and also for

the view (though sNumber does not appear in theresult). So deleting from the views are possible

by deleting appropriate sNumbers from the Student

table.

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Deleting from multirelation

views

Suppose we drop the key constraint on the

professor table for the same 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 the following slightly modified view

definitionCREATE 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 INSERTINTO 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 akey for the view.