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Database Modifications CIS 4301 Lecture Notes Lecture 19 - 3/30/2006 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? © CIS 4301 - Spring 2006 Lecture 19 2 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 © CIS 4301 - Spring 2006 Lecture 19 3 Example SELECT * FROM MovieStar LEFT OUTER JOIN MovieExec ON 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 MovieExec ON MovieStar.Name = MovieExec.name; Dangling tuples of the right (second) relation are padded with NULLs and included in result © CIS 4301 - Spring 2006 Lecture 19 4 Example (Older Syntax) SELECT * FROM MovieStar, MovieExec WHERE MovieStar.Name(+) = MovieExec.name; SELECT * FROM MovieStar, MovieExec WHERE MovieStar.Name = MovieExec.name(+); © CIS 4301 - Spring 2006 Lecture 19 5 Where to go for details? Oracle 9i SQL Reference http://www.cise.ufl.edu/help/database/oracle -docs/server.920/a96540.pdf © CIS 4301 - Spring 2006 Lecture 19 6 Database Modifications: Insert INSERT INTO R(A1,A2, …, An) VALUES (v1,v2, …, vn) Example: INSERT INTO Movies(title,year,length,inColor,Studio Name,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 © CIS 4301 - Spring 2006 Lecture 19 7 Timing of Insertions Add to Studio all movie studios mentioned in Movie who do not appear in Studio. 1 INSERT INTO Studio(name) 2 3 4 5 6 SELECT studioName FROM Movie WHERE studioName NOT IN (SELECT Name FROM Studio); Result can be affected by timing of insertion © CIS 4301 - Spring 2006 Lecture 19 8 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 = 1942 AND starName = ‘R. Moore’; © CIS 4301 - Spring 2006 Lecture 19 9 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 MovieExec SET name = ‘Pres.’ || name WHERE cert# IN (SELECT presC# FROM Studio); © CIS 4301 - Spring 2006 Lecture 19 10 SQL Views Two kinds of relations Virtual relation = view Physically present in database: tables or base relations Virtual relations, do not exist physically 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 © CIS 4301 - Spring 2006 Lecture 19 11 Example Create a view of the Movie relation that includes only movies made by Paramount CREATE VIEW Paramount_Movies AS SELECT title, year FROM Movie WHERE studioName = ‘Paramount’; Use it in query: SELECT title FROM Paramount_Movies WHERE year = 1970; © CIS 4301 - Spring 2006 Lecture 19 12 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 © CIS 4301 - Spring 2006 Lecture 19 13 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, studioName FROM Movie WHERE studioName = ‘Paramount’; © CIS 4301 - Spring 2006 Lecture 19 14 More View Examples CREATE VIEW MovieProd AS SELECT title, name FROM Movie, Movie Exec WHERE 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! © CIS 4301 - Spring 2006 Lecture 19 15 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 © CIS 4301 - Spring 2006 Lecture 19 16 Example View: CREATE VIEW ParamountMovie AS SELECT title, year FROM Movie WHERE studioName = ‘Paramount’; Query: SELECT title FROM ParamountMovie WHERE year = 1979; title, year studioName = ‘Paramount’ title year = 1979 Movie ParamountMovie base table © CIS 4301 - Spring 2006 virtual table Lecture 19 17 Expressing the Query in Terms of Base Tables title year = 1979 simplified title year = 1979 AND title, year studioName = ‘Paramount’ Movie studioName = ‘Paramount’ Movie replace virtual table by its view definition © CIS 4301 - Spring 2006 Lecture 19 18 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 Different types of indexes for different types of lookup Useful when relation is large 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 © CIS 4301 - Spring 2006 Lecture 19 19 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! © CIS 4301 - Spring 2006 Lecture 19 20 Selection of Indexes Why do we not index ALL attributes and their combinations? Two factors Speed vs. overhead © CIS 4301 - Spring 2006 Lecture 19 21 Intro to Database Tuning Assume we have the schema StarsIn(movieTitle,movieYear,starName) Assume we have following query mix Q1: SELECT starName FROM StarsIn WHERE starName = s; Q2: SELECT starName FROM StarsIn WHERE movieTitle = t AND movieYear = y; Q3: INSERT INTO StarsIn VALUES (t,y,s); Which indexes should we create? © CIS 4301 - Spring 2006 Lecture 19 22