1. CS 370 Database Systems Lecture 13 Introduction to SQL

2. SQL Aggregate Functions

3. Aggregate Functions Operates on a column of a relation, and return a value avg: average value min: minimum value max: maximum value sum: sum of values count: number of values

4. Aggregate Functions(cont.) Find the average account balance at the Perryridge branch. select avg(balance) from account where branch-name=“Perryridge” account select balance from account where branch-name =“Perryridge” Avg() 120,000 balance

5. Aggregate Functions(cont.) Find the numbers of tuples in the customer relation. select count(*) from customer –remember * stands for all attributes –compare to: select count(customer-city) from customer Find the number of depositors in the bank select count (distinct customer-name) from depositor –distinct is redundant if you know customer-name is a key

6. Modes of Interaction between Users and DBMS

7. DBMS user Ad hoc query result Stored procedure query result Embedded SQL C/Java program result SQL

8. Back to SQL … Set and Nested Queries

9. Null values It is possible for tuples to have a null value, denoted by null, for some of their attributes; null signifies an unknown value or that a value does not exist. The result of any arithmetic expression involving null is null. Roughly speaking, all comparisons involving null return false. More precisely, –Any comparison with null returns unknown –(true or unknown) = true, (false or unknown) = unknown (unknown or unknown) = unknown, (true and unknown) = unknown, (false and unknown) = false (unknown and unknown) = unknown –Result of where clause predicate is treated as false if it evaluates to unknown

10. Null Values (cont.) Find all loan numbers which appear in the loan relation with null values for amount. select loan-number from loan where amount is null Total of all loan amounts select sum(amount) from loan All aggregate operations except count(*) ignore tuples with null values on the aggregated attributes.

11. SQL - Nested Subqueries Every SQL statement returns a relation/set in the result; remember a relation could be null or merely contain a single atomic value You can replace a value or set of values with a SQL statement (ie., a subquery) select *select * from loanfrom loan where amount > 1200where amount > select avg(amount) from loan Illegal if the subquery returns the wrong type for the comparison

12. Check for each borrower if he/she is also a depositor Returns the set of depositors Example Nested Query Find all customers who have both an account and a loan in the bank. select distinct customer-name from borrower where customer-name in (select customer-name from depositor)

13. Example Query Find all customers who have a loan at the bank but do not have an account at the bank. select distinct customer-name from borrower where customer-name not in (select customer-name from depositor)

14. Returns borrowers with loan at Perryridge Returns depositor information Set Membership Example Important link: (Perryridge, D Lee) Find all customers who have both an account and a loan at the Perryridge branch. select distinct customer-name from borrower, loan where borrower.loan-number = loan.loan-number and branch-name=“Perryridge” and (branch-name, customer-name) in (select branch-name, customer-name from depositor, account where depositor.account-number = account.account-number)

15. Views Provide a mechanism to hide certain data from the view of certain users. To create a view we use the command: create view view-name as where: – is any legal SQL query –the name of the view is represented by view-name

16. 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.name-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”

17. Delete all account records at the Perryridge branch delete from account where branch-name = “Perryridge” Conceptually, delete is done in two steps: –find the tuples you want to delete: select * from account where branch-name = “Perryridge” –delete the tuples you found. Modification of the Database - Deletion

18. Delete all accounts at every branch located in Needham. delete from depositor where account-number in (select account-number from branch, account where branch-city = “Needham” and branch.branch-name = account.branch-name) delete from account where branch-name in (select branch-name from branch where branch-city = “Needham”) The first delete is needed to enforce “every depositor must have at least one account”. Note that the two deletes can’t be reordered. Modification of the Database - Deletion

19. Example Query Delete the records of all accounts with balances below the average at the bank delete from account where balance < (select avg (balance) from account) –Problem: as we delete tuples from deposit, the average balance changes –solution used in SQL: First, compute and save the average balance of the bank in a variable Next, delete all accounts with balance less than the average

20. Modification of the database - Insertion Add a new tuple to account insert into account values (“Perryridge”, A-9732, 1200) To reorder attributes, specify attribute names explicitly: insert into account (branch-name, balance, account-number) values (“Perryridge”, 1200, A-9732) Add a new tuple to account with balance set to null insert into account values ( “Perryridge”, “A-777”, null)

21. Modification of the Database - Insertion Create a $200 savings account for all loan customers of the Perryridge branch. Let the loan number serve as the account number for the new savings account. insert into account select branch-name, loan-number, 200 from loan where branch-name=“Perryridge” insert into depositor select customer-name, loan-number from loan, borrower where branch-name=“Perryridge” and loan.account-number = borrower.account-number

22. Modification of the database - Updates Increase all accounts with balance over $10,000 by 6%, all other accounts receive 5%. –Write two update statements: update account set balance = balance *1.06 where balance >10000 update account set balance = balance *1.05 where balance  10000 –the order is important –can be done better using the case statement

23. Case Statement for Conditional Updates Same query as before: Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%. update account set balance = case when balance <= 10000 then balance *1.05 else balance * 1.06 end

24. 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 (“Perryridge”, “L-307”,null) into the loan relation. Updates on more complex views are difficult or impossible to translate, and hence are disallowed.

25. View Update Problems Why can’t views be updated? create view works-on-view as select employee-name, project-name, hours from employee, project, works-on where employee.id=works-on.eid and project.id=works-on.pid employee projectworks-on works-on-view AI AI incorrect! 789 correct

26. Find the tuple relating “Dik Lee” to the “database” project updateworks-on setpid = ( select id from project where name=`AI’ ) whereeid = ( select id from employee where name = `Dik Lee’ ) and pid = ( select id from project where name = ‘database’ ) Replace old pid with the pid of “AI” Correct Update Procedure The correct update procedure is application dependent and must be written by the database application programmer A trigger can invoke the procedure automatically upon updates

27. Rules for Updatable Views Rules for legal view updates: A view built on a single defining table is updatable if the view contains the primary key of the defining table Views defined on multiple tables are in general not updatable Views involving aggregate functions on the defining table are not updatable

28. The SQL syntax for CREATE TABLE is CREATE TABLE "table_name" ("column 1" "data_type_for_column_1", "column 2" "data_type_for_column_2",... ) So, if we are to create the customer table specified as above, we would type in CREATE TABLE customer (First_Name char(50), Last_Name char(50), Address char(50), City char(50), Country char(25), Birth_Date date) Review: Data Definition Language

29. sidsnameratingage 1Frodo722 2Bilbo239 3Sam827 Sailors sidbidday 11029/12 21029/13 Reserves bidbnamecolor 101Ninared 102Pintablue 103Santa Maria red Boats CREATE TABLE Sailors (sid INTEGER, sname CHAR(20), rating INTEGER, age REAL, PRIMARY KEY sid) CREATE TABLE Boats (bid INTEGER, bname CHAR (20), color CHAR(10) PRIMARY KEY bid) CREATE TABLE Reserves (sid INTEGER, bid INTEGER, day DATE, PRIMARY KEY (sid, bid, day), FOREIGN KEY sid REFERENCES Sailors, FOREIGN KEY bid REFERENCES Boats) NOT NULL, Review: Data Definition Language

30. CS370 Spring 2007 A foreign key constraint is an Integrity Constraint: –a condition that must be true for any instance of the database; –Specified when schema is defined. –Checked when relations are modified. Primary/foreign key constraints; but databases support more general constraints as well. –e.g. domain constraints like: Rating must be between 1 and 10 ALTER TABLE SAILORS ADD CONSTRAINT RATING CHECK (RATING >= 1 AND RATING < 10) Or even more complex (and potentially nonsensical): ALTER TABLE SAILORS ADD CONSTRAINT RATING CHECK (RATING*AGE/4 <= SID) Integrity Constraints (ICs)

31. CS370 Spring 2007 Integrity Constraints can help prevent data consistency errors …but they have drawbacks: –Expensive –Can’t always return a meaningful error back to the application. e.g: What if you saw this error when you enrolled in a course online? “A violation of the constraint imposed by a unique index or a unique constraint occurred”. –Can be inconvenient e.g. What if the ‘Sailing Class’ application wants to register new (unrated) sailors with rating 0? So they aren’t widely used –Software developers often prefer to keep the integrity logic in applications instead