database modifications cis 4301 lecture notes lecture 19 - 3/30/2006

Database Modifications

CIS 4301

Lecture Notes

Lecture 19 - 3/30/2006

Lecture 19© CIS 4301 - Spring 2006 2

Recall Join Expressions

Cartesian Product: Multiple relations in the FROM clause

Join: Specify join condition in WHERE clause

Remember, when specifying join condition, only tuples which satisfy join condition will be selected

Example R NATURAL JOIN S If a tuple t from R does not match any tuple s from S, all trace of t will disappear from result

Could pose problems – when?


Outer Join Differs from “inner” join Adding to result any tuple of either relation that does not join with at least one tuple of the other relation Recall, those tuples are called dangling tuples

Pad out dangling tuples with NULL’s in those attributes belonging to the other relation before adding to result

Left and Right Outer Join


Example

SELECT *FROM MovieStar LEFT OUTER JOIN MovieExecON MovieStar.Name = MovieExec.name; Dangling tuples of the left (first) relation are padded with NULLs and included in result

SELECT *FROM MovieStar RIGHT OUTER JOIN MovieExecON MovieStar.Name = MovieExec.name; Dangling tuples of the right (second) relation are padded with NULLs and included in result


Example (Older Syntax)

SELECT *

FROM MovieStar, MovieExec

WHERE MovieStar.Name(+) = MovieExec.name;

SELECT *

FROM MovieStar, MovieExec

WHERE MovieStar.Name = MovieExec.name(+);


Where to go for details?

Oracle 9i SQL Reference http://www.cise.ufl.edu/help/database/oracle-docs/server.920/a96540.pdf


Database Modifications: Insert

INSERT INTO R(A1,A2, …, An)

VALUES (v1,v2, …, vn) Example:INSERT INTO Movies(title,year,length,inColor,StudioName,producerC#)

VALUES (‘Star Wars’,1997,191,‘y’,‘Fox’,12345);

May omit list of attributes if we provide all values

Values may be provided explicitly through SFW subquery


Timing of Insertions Add to Studio all movie studios mentioned in Movie who do not appear in Studio.

1 INSERT INTO Studio(name)

2 SELECT studioName

3 FROM Movie

4 WHERE studioName NOT IN

5 (SELECT Name

6 FROM Studio);

Result can be affected by timing of insertion


Database Modifications: Delete

DELETE FROM R WHERE <condition> Delete the fact that R. Moore was a star in the Maltese Falcon

DELETE FROM StarsIn WHERE movietitle =‘The Maltese Falcon’

AND movieYear = 1942AND starName = ‘R. Moore’;


Database Modifications: Update

UPDATE R SET <new value assignment>WHERE <condition>; Find all tuples in R that satisfy WHERE-clause Each tuple will be updated by evaluating the formula(s) in the SET-clause and making assignments

Prepend the tittle ‘Pres.’ in front of every movie executive who is a president of a studio

UPDATE MovieExecSET name = ‘Pres.’ || nameWHERE cert# IN

(SELECT presC# FROM Studio);


SQL Views

Two kinds of relations Physically present in database: tables or base relations

Virtual relations, do not exist physically Virtual relation = view

Can be queried (as if they existed physically) In some cases, can even be modified!

Be clear about which type of relation you mean

View defined by an expression (like a query)

CREATE VIEW <view-name> AS <view-definition>

SFW query


Example

Create a view of the Movie relation that includes only movies made by ParamountCREATE VIEW Paramount_Movies AS

SELECT title, yearFROM MovieWHERE studioName = ‘Paramount’;

Use it in query:SELECT titleFROM Paramount_MoviesWHERE year = 1970;


Facts About Views Can be queried just like any other table Allow data to been seen differently by different users

Often used as a security mechanism Cannot always be updated

Updateable views: translate the modification of the view into equivalent modifications on the corresponding base table(s)

Complex rules about which views are updateable


Modifying Views

Suppose we try to insert (through view Paramount_Movies) the movie (‘Star Trek’, 1979) into the base relation Movie

Problem, studioName is not part of the view not updateable

Would leave studioName in Movie without value (NULL)

New view definition:CREATE VIEW Paramount_Movies AS

SELECT title, year, studioNameFROM MovieWHERE studioName = ‘Paramount’;


More View Examples

CREATE VIEW MovieProd ASSELECT title, nameFROM Movie, Movie ExecWHERE producerC#= cert#;

What happens when we try:INSERT INTO MovieProd VALUES

(‘Greatest …’, ‘Cecile B. DeMille’); Cannot update the two relations Movie and MovieExec correctly

MovieProd is not updateable!


Interpreting Queries Involving Views

In order to process query, it must first be represented by its expression tree in relational algebra

To evaluate, leaves must be base tables In case query involves views, some of the leaves are virtual relations

Represent virtual relations by their definitions, which must also be in terms of expression trees

To form the query over base tables, substitute for each leaf in tree that is a view, the root of a copy of the tree that defines that view


ExampleView:CREATE VIEW ParamountMovie AS

SELECT title, year

FROM Movie

WHERE studioName = ‘Paramount’;

Query:SELECT title

FROM ParamountMovie

WHERE year = 1979;

title, year

studioName = ‘Paramount’

Movie

title

year = 1979

ParamountMovie

base table virtual table


Expressing the Query in Terms of Base Tables

title, year

studioName = ‘Paramount’

Movie

title

year = 1979 simplifiedtitle

year = 1979 AND studioName

= ‘Paramount’

Movie

replace virtual table byits view definition


Indexes

Index I on attribute A of relation R Data structure that makes it efficient to find those tuples that have a fixed value for attribute A

E.g., “Find all MovieStars where gender = ‘M’”

Prevents database from having to scan all tuples of a relation

Useful when relation is large Different types of indexes for different types of lookup

Can include multiple attributes in index Determining which attributes to index on is not always easy and requires experience

Trade-off between speed and overhead for space and maintenance


Sample Indexes

CREATE INDEX Movie_index ON Movie(producerC#) TABLESPACE cis4301_sp06_ind;

DROP INDEX Movie_index;

CREATE TABLE Test (a INT PRIMARY KEY USING INDEX TABLESPACE cis4301_sp06_ind, b VARCHAR(20) UNIQUE USING INDEX TABLESPACE cis4301_sp06_ind); Drawback: using the implicit declaration, we cannot name index!


Selection of Indexes Why do we not index ALL attributes and their combinations?

Two factors Speed vs. overhead


Intro to Database Tuning

Assume we have the schemaStarsIn(movieTitle,movieYear,starName)

Assume we have following query mixQ1:

SELECT starNameFROM StarsInWHERE starName = s;

Q2:SELECT starNameFROM StarsInWHERE movieTitle = t AND movieYear = y;

Q3:INSERT INTO StarsIn VALUES (t,y,s);

Which indexes should we create?

database modifications cis 4301 lecture notes lecture 19 - 3/30/2006

Documents