SQL: Updates (DML) and Views (DDL) Murali Mani SQL DML (Updating - - PDF document

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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=‘ER’ WHERE sNumer=‘6’

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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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Views

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.

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

CREATE VIEW <viewName> as <query> DROP VIEW <viewName>

CREATE VIEW studentProfessor (student, professor) AS SELECT sName, pName FROM Student, Professor WHERE Student.professor = Professor.pNumber; DROP VIEW studentProfessor

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Views - Example

2 320FL Matt 3 1 320FL Greg 2 1 320FL Dave 1 professor address sName sNumber

Student

241FL ER 2 235FL MM 1 address pName pNumber

Professor

CREATE VIEW studentProfessor (student, professor) AS SELECT sName, pName FROM Student, Professor WHERE Student.professor = Professor.pNumber; SELECT * from studentProfessor

ER Matt MM Greg MM Dave professor student

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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 these views 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 row

from 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 view

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) AS SELECT DISTINCT sNumber FROM Student;

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Updating Single Relation Views

Note that the WHERE clause may specify

• subqueries. Let us consider an extreme

example.

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 as

CREATE VIEW studentProf(student, professor) AS SELECT sName, pName FROM Student, Professor WHERE 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 the result). So deleting from the views are possible by deleting appropriate sNumbers from the Student table.

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Deleting from multi-relation views

Try the following update statements:

DELETE FROM Studentprof WHERE professor='MM';

• - This will actually delete the two rows in

the student table.

• - Therefore 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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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 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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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.