2. Introduction to SQL Programming Techniques
Chapter 9
Introduction to SQL Programming Techniques
Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe

3. Chapter Objectives
Specification of more general constraints via assertions
SQL facilities for defining views (virtual tables)
Various techniques for accessing and manipulating a database via programs in general-purpose languages
E.g., Java, C++, etc.

4. Constraints as Assertions
General constraints: constraints that do not fit in the basic SQL categories (presented in chapter 8)
Mechanism: CREAT ASSERTION
Components include:
a constraint name,
followed by CHECK,
followed by a condition

5. Assertions: An Example
“The salary of an employee must not be greater than the salary of the manager of the department that the employee works for’’
CREAT ASSERTION SALARY_CONSTRAINT
CHECK (NOT EXISTS (SELECT *
FROM EMPLOYEE E, EMPLOYEE M, DEPARTMENT D
WHERE E.SALARY > M.SALARY AND
E.DNO=D.NUMBER AND D.MGRSSN=M.SSN))
constraint name,
CHECK,
condition

6. Using General Assertions
Specify a query that violates the condition; include inside a NOT EXISTS clause
Query result must be empty
if the query result is not empty, the assertion has been violated

7. SQL Triggers
Objective: to monitor a database and take initiate action when a condition occurs
A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database.
Triggers are expressed in a syntax similar to assertions and include the following:
Event
Such as an insert, deleted, or update operation
Condition
Action
To be taken when the condition is satisfied

8. Views in SQL
A view is a “virtual” table that is derived from other tables
Allows for limited update operations
Since the table does not physically be stored
Allows full query operations

9. 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 view contents

10. SQL Views: An Example Specify a different WORKS_ON table
CREATE VIEW WORKS_ON_NEW AS
SELECT FNAME, LNAME, PNAME, HOURS
FROM EMPLOYEE, PROJECT, WORKS_ON
WHERE SSN=ESSN AND PNO=PNUMBER
GROUP BY PNAME;

11. SQL Views: An Example

12. Using a Virtual Table
We can specify SQL queries on a newly create table (view):
SELECT FNAME, LNAME
FROM WORKS_ON_NEW
WHERE PNAME=‘Seena’;
View is always up-to-date
Responsibility of the DBMS and not the user
When no longer needed, a view can be dropped:
DROP WORKS_ON_NEW;

13. Updatable Views
Update on a view defined on a single table without any aggregate functions:
Update may map to an update on the underlying base table
Views involving joins:
Often not possible for DBMS to determine which of the updates is intended

14. 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 at the end of the view definition if a view is to be updated

15. Database Programming Objective: Why?
To access a database from an application program
Why?
An interactive interface is convenient but not sufficient
A majority of database operations are made thru application programs (increasingly thru web applications)

16. Database Programming Approaches
Embedded commands:
Database commands are embedded in a general-purpose programming languag
Library of database functions:
Available to the host language for database calls; known as an API
API standards for Application Program Interface

17. Impedance Mismatch
Incompatibilities between a host programming language and the database model, e.g.,
type mismatch and incompatibilities; requires a new binding for each language
set vs. record-at-a-time processing
need special iterators to loop over query results and manipulate individual values

18. Steps in Database Programming
Client program opens a connection to the database server
Client program submits queries to and/or updates the database
When database access is no longer needed, client program closes (terminates) the connection

19. Embedded SQL
Most SQL statements can be embedded in a general-purpose host programming language such as COBOL, C, Java
An embedded SQL statement is distinguished from the host language statements by enclosing it between EXEC SQL or EXEC SQL BEGIN and a matching END-EXEC or EXEC SQL END (or semicolon)
Syntax may vary with language
Shared variables (used in both languages) usually prefixed with a colon (:) in SQL

20. Java Database Connectivity (JDBC)
JDBC provides a standardized library for accessing relational databases
Connecting to the database
Initiating queries
Creating stored (parameterized) queries
Data structure for the query result
JDBC is not embedded SQL
JDBC driver for Oracle needs to be on the client machine
JDBC is located in java.sql package

21. JDBC driver Java application JDBC driver manager Oracle JDBC driver
JDBC API
JDBC driver manager
JDBC driver API
Oracle JDBC driver
Client
Oracle
database
Server

22. Six steps in using JDBC Load the driver Establish a connection
Create a statement object
Execute a query
Process the results
Close the connection

23. JDBC usage steps - details
Step 1 : Load the driver
try {
Class.forName("oracle.jdbc.driver.OracleDriver"); }
catch(Exception e)
System.out.println("JDBC ORACLE driver failed to load");

24. JDBC usage steps - details
Step 2 : Establish a connection
try {
con = }
catch(Exception e)
System.out.println("Failed to connect to the database");

25. JDBC usage steps - details
Step 3 : Create a statement
Statement statement = con.createStatement();
Step 4 : Execute a query
String query = “SELECT FNAME, SALARY FROM EMPLOYEE”;
ResultSet rs = Statement.executeQuery(query);
(For SELECT)
String query = “UPDATE EMPLOYEE SET SALARY = WHERE FNAME = “John”;
Statement.executeUpdate(query);
(For UPDATE, INSERT, DELETE)

26. JDBC usage steps - details
Step 5 : Process the result
While (rs.next()) {
System.out.println(rs.getString(1) + “ “ + rs.getInt(2)); }
First column has index 1, not 0
ResultSet provides various getXXX methods that take a column index, and return the data

27. JDBC usage steps - details
Step 6 : Close the connection
con.close();
As opening a connection is expensive, postpone this step if additional database operations are expected

28. Online resources JDBC tutorial JDBC API reference
JDBC API reference

29. Pro*C Pro*C program C code Executable SQLLIB Embedded SQL
Allows you to embed SQL in C code
Pre-compiler
C compiler
Pro*C program
C code
Executable
Embedded SQL statements
SQL statements replaced
By library calls
SQLLIB
Oracle runtime library

30. Pro*C++ syntax
SQL statements can be placed anywhere between a C++ block
All SQL statements need to start with EXEC SQL and be terminated by a semicolon {
int a;
/* ... */
EXEC SQL SELECT salary INTO :a
FROM Employee
WHERE SSN= ;
cout<<"The salary is :“<<a; }
Be sure to include sqlca.h

31. Connecting to the database
Legal way
char *username = "SCOTT";
char *password = "TIGER"; …
EXEC SQL CONNECT :username IDENTIFIED BY :password;
Illegal way
EXEC SQL CONNECT SCOTT IDENTIFIED BY TIGER;
EXEC SQL CONNECT 'SCOTT' IDENTIFIED BY 'TIGER';

32. Dynamic sql Used to dynamically create entire SQL statements
char *s = "INSERT INTO emp VALUES(1234, 'jon', 3)";
EXEC SQL PREPARE q FROM :s;
EXEC SQL EXECUTE q;
Alternately,
EXEC SQL EXECUTE IMMEDIATE :s;

33. Host variables
Key to the communication between the host program and the database
They should be declared in a Declare section
EXEC SQL BEGIN DECLARE SECTION;
/* Declare all host variables inside this section:
*/ char *uid = "scott/tiger";
...
EXEC SQL END DECLARE SECTION;

34. Transactions
Sequence of SQL statements that are treated as a single unit of work
Transaction ends when you issue
EXEC SQL COMMIT;

35. Cursors Used for fetching multiple rows of data First, declare it.
EXEC SQL DECLARE emp_cursor CURSOR FOR
SELECT ename, empno, sal
FROM emp
WHERE deptno = :dept_number;
Then, open it.
EXEC SQL OPEN emp_cursor;
Fetch the records (move the cursor to the next tuple).
for (;;) {
EXEC SQL FETCH emp_cursor INTO :emp_name1, :emp_no1, :salary1; }

36. Cursors (continued) Finally, close it. EXEC SQL CLOSE emp_cursor;
Error handling
Every executable SQL statement returns a status code to the SQLCA variable sqlcode
A zero status code means that Oracle executed the statement without detecting an error or exception
A positive status code means that Oracle executed the statement but detected an exception
A negative status code means that Oracle did not execute the SQL statement because of an error.

37. Online reference Pro*C/C++ Precompiler Programmer's Guide