INF1383 -Bancos de Dados

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1 INF1383 -Bancos de Dados Prof. Sérgio Lifschitz DI PUC-Rio Eng. Computação, Sistemas de Informação e Ciência da Computação A LINGUAGEM SQL Última Parte Group By/Having e Visões Alguns slides são baseados ou modificados dos originais dos livros Elmasri and Navathe, Fundamentals of Database Systems Pearson Education, Inc. e Database System Concepts, McGraw Hill Silberschatz, Korth and Sudarshan INF1383 BD1 Sergio Lifschitz Slide 1-34 GROUPING In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation Each subgroup of tuples consists of the set of tuples that have the same value for the grouping attribute(s) The function is applied to each subgroup independently SQL has a GROUP BY-clause for specifying the grouping attributes, which must also appear in the SELECT-clause INF1383 BD1 Sergio Lifschitz Slide

2 Aggregate Functions Group By Find the number of depositors for each branch. select branch_name, count (distinct customer_name) from depositor, account where depositor.account_number = account.account_number group by branch_name Note: Attributes in select clause outside of aggregate functions must appear in group by list INF1383 BD1 Sergio Lifschitz Slide 3-34 GROUPING (cont.) For each department, retrieve the department number, the number of employees in the department, and their average salary. SELECT FROM GROUP BY DNO, COUNT (*), AVG (SALARY) EMPLOYEE DNO The EMPLOYEE tuples are divided into groups--each group having the same value for the grouping attribute DNO The COUNT and AVG functions are applied to each such group of tuples separately The SELECT-clause includes only the grouping attribute and the functions to be applied on each group of tuples A join condition can be used in conjunction with grouping INF1383 BD1 Sergio Lifschitz Slide

3 Aggregate functions: AVG, COUNT, MIN, MAX, SUM,... (user defined functions) Group-by SQL Queries: Aggregation and Grouping Employee Name Dept Salary Joe Toys 45 Nick PCs 50 Jim Toys 35 Jack PCs 40 Find average salary of all employees SELECT Avg(Salary) AS AvgSal FROM Employee AvgSal 42.5 Find the average salary for each department SELECT Dept, Avg(Salary) AS AvgSal FROM Employee GROUP-BY Dept Dept AvgSal Toys 40 PCs 45 INF1383 BD1 Sergio Lifschitz Slide 5-34 GROUPING Example For each project, retrieve the project number, project name, and the number of employees who work on that project. SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME In this case, the grouping and functions are applied after the joining of the two relations INF1383 BD1 Sergio Lifschitz Slide

4 THE HAVING-CLAUSE Sometimes we want to retrieve the values of these functions for only those groups that satisfy certain conditions The HAVING-clause is used for specifying a selection condition on groups rather than on individual tuples! INF1383 BD1 Sergio Lifschitz Slide 7-34 Aggregate Functions Having Clause Find the names of all branches where the average account balance is more than $1,200. select branch_name, avg (balance) from account group by branch_name HAVING avg (balance) > 1200 Note: predicates in the having clause are applied after the formation of groups whereas predicates in the where clause are applied before forming groups INF1383 BD1 Sergio Lifschitz Slide

5 THE HAVING-CLAUSE (cont.) For each project on which more than two employees work, retrieve the project number, project name, and the number of employees who work on that project. SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME HAVING COUNT (*) > 2 INF1383 BD1 Sergio Lifschitz Slide 9-34 Null Values and Aggregates Total all loan amounts select sum (amount ) from loan Above statement ignores null amounts Result is null if there is no non-null amount All aggregate operations except count(*) ignore tuples with null values on the aggregated attributes. INF1383 BD1 Sergio Lifschitz Slide

6 Derived Relations SQL allows a subquery expression to be used in the from clause Find the average account balance of those branches where the average account balance is greater than $1200. select branch_name, avg_balance from (select branch_name, avg (balance) from account group by branch_name ) as branch_avg ( branch_name, avg_balance ) where avg_balance > 1200 Note that we do not need to use the having clause, since we compute the temporary relation branch_avg in the from clause, and the attributes of branch_avg can be used in the where clause. INF1383 BD1 Sergio Lifschitz Slide Test for Absence of Duplicate Tuples The unique construct tests whether a subquery has any duplicate tuples in its result. Find all customers who have at most one account at the Perryridge branch. select T.customer_name from depositor as T where unique ( select R.customer_name from account, depositor as R where T.customer_name = R.customer_name and R.account_number = account.account_number and account.branch_name = Perryridge ) INF1383 BD1 Sergio Lifschitz Slide

7 Example Query Find all customers who have at least two accounts at the Perryridge branch. select distinct T.customer_name from depositor as T where not unique ( select R.customer_name from account, depositor as R where T.customer_name = R.customer_name and R.account_number = account.account_number and account.branch_name = Perryridge ) INF1383 BD1 Sergio Lifschitz Slide Summary of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory. The clauses are specified in the following order: SELECT <attribute list> FROM <table list> [WHERE <condition>] [GROUP BY <grouping attribute(s)>] [HAVING <group condition>] [ORDER BY <attribute list>] INF1383 BD1 Sergio Lifschitz Slide

8 Summary of SQL Queries (cont.) The SELECT-clause lists the attributes or functions to be retrieved The FROM-clause specifies all relations (or aliases) needed in the query but not those needed in nested queries The WHERE-clause specifies the conditions for selection and join of tuples from the relations specified in the FROM-clause GROUP BY specifies grouping attributes HAVING specifies a condition for selection of groups ORDER BY specifies an order for displaying the result of a query A query is evaluated by first applying the WHERE-clause, then GROUP BY and HAVING, and finally the SELECT-clause INF1383 BD1 Sergio Lifschitz Slide Views In some cases, it is not desirable for all users to see the entire logical model (i.e, all the actual relations stored in the database.) Consider a person who needs to know a customer s loan number but has no need to see the loan amount. This person should see a relation described, in SQL, by (select customer_name, loan_number from borrower, loan where borrower.loan_number = loan.loan_number ) A view provides a mechanism to hide certain data from the view of certain users. Any relation that is not of the conceptual model but is made visible to a user as a virtual relation is called a view. INF1383 BD1 Sergio Lifschitz Slide

9 View Definition A view is defined using the create view statement which has the form create view v as < query expression > where <query expression> is any legal SQL expression. The view name is represented by v. Once a view is defined, the view name can be used to refer to the virtual relation that the view generates. View definition is not the same as creating a new relation by evaluating the query expression Rather, a view definition causes the saving of an expression; the expression is substituted into queries using the view. INF1383 BD1 Sergio Lifschitz Slide Views in SQL A view is a virtual table that is derived from other tables Allows for limited update operations (since the table may not physically be stored) Allows full query operations A convenience for expressing certain operations INF1383 BD1 Sergio Lifschitz Slide

10 Specification of Views SQL command: CREATE VIEW a table (view) name a possible list of attribute names (for example, when arithmetic operations are specified or when we want the names to be different from the attributes in the base relations) a query to specify the table contents INF1383 BD1 Sergio Lifschitz Slide Example Queries A view consisting of branches and their customers create view all_customer as (select branch_name, customer_name from depositor, account where depositor.account_number = account.account_number ) union (select branch_name, customer_name from borrower, loan where borrower.loan_number = loan.loan_number ) Find all customers of the Perryridge branch select customer_name from all_customer where branch_name = Perryridge INF1383 BD1 Sergio Lifschitz Slide

11 Views Defined Using Other Views One view may be used in the expression defining another view A view relation v 1 is said to depend directly on a view relation v 2 if v 2 is used in the expression defining v 1 A view relation v 1 is said to depend on view relation v 2 if either v 1 depends directly to v 2 or there is a path of dependencies from v 1 to v 2 A view relation v is said to be recursive if it depends on itself. INF1383 BD1 Sergio Lifschitz Slide SQL Views: Another Example Specify a different WORKS_ON table CREATE TABLE WORKS_ON_NEW AS SELECT FNAME, LNAME, PNAME, HOURS FROM EMPLOYEE, PROJECT, WORKS_ON WHERE SSN=ESSN AND PNO=PNUMBER GROUP BY PNAME; INF1383 BD1 Sergio Lifschitz Slide

12 Using a Virtual Table We can specify SQL queries on a newly create view: SELECT FNAME, LNAME FROM WORKS_ON_NEW WHERE PNAME= Seena ; When no longer needed, a view can be dropped: DROP WORKS_ON_NEW; INF1383 BD1 Sergio Lifschitz Slide Efficient View Implementation Query modification: present the view query in terms of a query on the underlying base tables disadvantage: inefficient for views defined via complex queries (especially if additional queries are to be applied to the view within a short time period) INF1383 BD1 Sergio Lifschitz Slide

13 Efficient View Implementation View materialization: involves physically creating and keeping a temporary table assumption: other queries on the view will follow concerns: maintaining correspondence between the base table and the view when the base table is updated strategy: incremental update INF1383 BD1 Sergio Lifschitz Slide Update of a View Create a view of all loan data in the loan relation, hiding the amount attribute create view branch_loan as select branch_name, loan_number from loan Add a new tuple to branch_loan insert into branch_loan values ( Perryridge, L-307 ) This insertion must be represented by the insertion of the tuple ( L-307, Perryridge, null ) into the loan relation INF1383 BD1 Sergio Lifschitz Slide

14 View Update Update on a single view without aggregate operations: update may map to an update on the underlying base table Views involving joins: an update may map to an update on the underlying base relations not always possible INF1383 BD1 Sergio Lifschitz Slide Updates Through Views (Cont.) Some updates through views are impossible to translate into updates on the database relations create view v as select branch_name from account insert into v values ( L-99, Downtown, 23 ) Others cannot be translated uniquely insert into all_customer values ( Perryridge, John ) Have to choose loan or account, and create a new loan/account number! Most SQL implementations allow updates only on simple views (without aggregates) defined on a single relation INF1383 BD1 Sergio Lifschitz Slide

15 Un-updatable Views Views defined using groups and aggregate functions are not updateable Views defined on multiple tables using joins are generally not updateable WITH CHECK OPTION: must be added to the definition of a view if the view is to be updated to allow check for updatability and to plan for an execution strategy INF1383 BD1 Sergio Lifschitz Slide Views another example Create permanent or temporary tables holding result of a query Syntax: CREATE VIEW <TABLE> AS <query> Once defined, views can be used in queries like any other relation Their content is automatically updated when database changes CREATE VIEW Berto-movies (movie, actor) AS SELECT title, actor FROM movie WHERE director = Bertolucci SELECT movie FROM Berto-movies WHERE actor = Winger INF1383 BD1 Sergio Lifschitz Slide

16 Typical use: find objects that always satisfy property X SQL s way of saying this: find the actors for which there is no movie by Bertolucci in which they do not act OR equivalently: find the actors not among the actors for which there is some movie by Bertolucci in which they do not act Views can simplify nested queries Example find actors playing in every movie by Berto : SELECT Actor FROM Movie WHERE Actor NOT IN (SELECT m1.actor FROM Movie m1, Movie m2, WHERE m2.director= Berto AND m1.actor NOT IN (SELECT Actor FROM Movie WHERE Title=m2.Title)) The shaded query finds actors for which there is some movie by Berto in which they do not act The top lines complement the shaded part INF1383 BD1 Sergio Lifschitz Slide Same query using views CREATE VIEW Berto-Movies AS SELECT title FROM Movie WHERE director = Bertoucci ; CREATE VIEW Not-All-Berto AS SELECT m.actor FROM Movies m, Berto-Movies WHERE Berto-Movies.title NOT IN (SELECT title FROM Movies WHERE actor = m.actor); CREATE VIEW Answer AS SELECT actor FROM Movies WHERE actor NOT IN (SELECT * FROM Not-All-Berto) INF1383 BD1 Sergio Lifschitz Slide

17 With Clause The with clause provides a way of defining a temporary view whose definition is available only to the query in which the with clause occurs. Find all accounts with the maximum balance with max_balance (value) as select max (balance) from account select account_number from account, max_balance where account.balance = max_balance.value INF1383 BD1 Sergio Lifschitz Slide Complex Query using With Clause Find all branches where the total account deposit is greater than the average of the total account deposits at all branches. with branch_total (branch_name, value) as select branch_name, sum (balance) from account group by branch_name with branch_total_avg (value) as select avg (value) from branch_total select branch_name from branch_total, branch_total_avg where branch_total.value >= branch_total_avg.value INF1383 BD1 Sergio Lifschitz Slide