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Database Application Development
Chapter 6
1
Overview
SQL in application code
 Embedded SQL
 Cursors
 Dynamic SQL
 JDBC
 Stored procedures

2
SQL in Application Code


SQL commands can be called from within a
host language (e.g., C++ or Java) program.
 SQL statements can refer to host variables
(including special variables used to return status).
 Must include statement to connect to right database.
Two main integration approaches:
 Embed SQL in the host language (e.g., Pro*C,
Embedded SQL, SQLJ)
 Create special API (Call Level Interface) to call SQL
commands (eg: JDBC, ODBC, PHP …)
3
SQL in Application Code (Contd.)
Impedance mismatch issues:
 Type mismatch
• Data type casting (declare variables)

Set-oriented
• SQL relations are (multi-) sets of records, with no a priori
bound on the number of records.
• Usually no such data structure exists in procedural
programming languages such as C++.
• SQL supports a mechanism called a cursor to handle this.
4
Embedded SQL
5
Embedded SQL

Approach: Embed SQL in host language.
 Given host language with embedded SQL
 A preprocessor converts SQL statements into
special function calls.
 Then regular compiler used to compile the host
language+function class into executable.

Final executable works for one DBMS only
(not portable)
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Embedded SQL: Main Constructs

Connect to DB
EXEC SQL CONNECT

Declare variables that can be used by both
SQL and host language
EXEC SQL BEGIN DECLARE SECTION
…
EXEC SQL END DECLARE SECTION

Executing SQL statements
EXEC SQL …
7
Embedding SQL in C: Oracle
#include
#include
#include
#include
<stdio.h>
<stdlib.h>
<string.h>
<sqlca.h>
EXEC SQL BEGIN DECLARE SECTION;
VARCHAR userid[20];
VARCHAR passwd[20];
int value;
EXEC SQL END DECLARE SECTION;
void sql_error (char *msg) {
printf (“%s”, msg); exit (1);
}
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Embedding SQL in C: Oracle
int main () {
strcpy (userid.arr,
userid.len = strlen
strcpy (passwd.arr,
passwd.len = strlen
“me”);
(userid.arr);
“no-me”);
(passwd.arr);
EXEC SQL WHENEVER SQLERROR DO sql_error (“Oracle Error\n”);
EXEC SQL CONNECT :userid IDENTIFIED BY :passwd;
EXEC SQL CREATE TABLE Test (a int);
EXEC SQL INSERT INTO Test VALUES (1);
EXEC SQL SELECT MAX (a) INTO :value from R;
printf (“Max value=%d\n”,value);
}
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Cursors

Can declare a cursor on a query statement which
generates a relation.

Can open a cursor, repeatedly fetch a tuple, move
the cursor, until all tuples have been retrieved.

Control order: ORDER BY, in queries that are accessed
through a cursor

Can also modify/delete tuple pointed to by cursor.

Must close cursor at end.
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Cursor that gets names of sailors who’ve
reserved a red boat, in alphabetical order
EXEC SQL DECLARE sinfo CURSOR FOR
SELECT S.sname
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid
AND B.color=‘red’
ORDER BY S.sname
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Cursors
EXEC SQL DECLARE myCursor CURSOR FOR SELECT bid
from Reservations;
EXEC SQL OPEN myCursor;
EXEC SQL WHENEVER NOT FOUND DO break;
while (1) {
EXEC SQL FETCH myCursor INTO :num;
…
}
EXEC SQL CLOSE myCursor;
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Embedding SQL in C: An Example
char SQLSTATE[6];
EXEC SQL BEGIN DECLARE SECTION
char c_sname[20]; short c_minrating; float c_age;
EXEC SQL END DECLARE SECTION
c_minrating = random();
EXEC SQL DECLARE sinfo CURSOR FOR
SELECT S.sname, S.age
FROM Sailors S
WHERE S.rating > :c_minrating
ORDER BY S.sname;
do {
EXEC SQL FETCH sinfo INTO :c_sname, :c_age;
printf(“%s is %d years old\n”, c_sname, c_age);
} while (SQLSTATE != ‘02000’);
-- empty cursor
EXEC SQL CLOSE sinfo;
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Compiling

Create files with extension .pc, such as test.pc

Preprocessor = proc
 Available at $ORACLE_HOME/bin

SET your library path as
 setenv LD_LIBRARY_PATH
${LD_LIBRARY_PATH}:${ORACLE_HOME}/lib

Make them as
make -f $ORACLE_HOME/precomp/demo/proc/demo_proc.mk build
EXE=test OBJS=test.o
(or)
$ORACLE_HOME/bin/proc iname=test.pc code=cpp parse=none
g++ -I$ORACLE_HOME/precomp/public test.c
-lclntsh -lm
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Dynamic SQL

SQL queries are not always known at
compile time

Example: spreadsheet, graphical DBMS
frontend, web access.

Allow construction of SQL statements
(query strings) on-the-fly
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Dynamic SQL Example
char c_sqlstring[] =
{“DELETE FROM Sailors WHERE rating>5”};
-- parse, compile and bind to variable:
EXEC SQL PREPARE readytogo FROM
:c_sqlstring;
EXEC SQL EXECUTE readytogo;
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Embedding vs Database APIs
1.
Embedding: Modify compiler
(see discussion thus far)
2. API : Provide library with
standard database call interface
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Database APIs
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Database APIs

Special standardized interface to libraries of functions
provided explicitly for SQL statements:
 No preprocessor, instead host language compiler compiles code.


Pass SQL strings from PL language
Presents result sets in language-friendly way

Examples : Sun’s JDBC: Java API
MS ODBC – Open DB Connection

Supposedly DBMS-neutral




“driver” traps calls & translates them into DBMS-specific code
Database can be across a network
Same executable works on different DBMSs without recompiling
Independent both at source code and at executable level
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JDBC Architecture: 4 Components
 Application
• initiates and terminates connections, submits SQL
statements
 Driver manager
• load JDBC driver at run-time
 Driver
• Registers with manager
• Connects to data source, transmits requests and
returns/translates results and error codes into DBMS
specific calls
 Data source
• processes SQL statements
22
JDBC Classes and Interfaces
Steps to submit a database query:
 Load JDBC driver
 Connect to data source
 Execute SQL statements
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JDBC Driver Management

All drivers managed by DriverManager class

Options for Loading JDBC driver:
 In Java code (dynamic loading of class in java):
Class.forName(“oracle/jdbc.driver.Oracledriver”);
 When starting Java application:
-Djdbc.drivers=oracle/jdbc.driver
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Connections in JDBC
We interact with data source through sessions.
Each connection identifies a logical session.

JDBC URL:
jdbc:<subprotocol>:<otherParameters>
Example:
String url=“jdbc:oracle:www.bookstore.com:3083”;
Connection con;
try{
con =
DriverManager.getConnection(url,usedId,password);
} catch SQLException except { …}
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Connection Class Interface
public int getTransactionIsolation()
void setTransactionIsolation(int level)
Sets isolation level for current connection.
 public boolean getAutoCommit() and
void setAutoCommit(boolean b)
If autocommit is set, then each SQL statement is
considered its own transaction. Otherwise, a
transaction is committed using commit(), or
aborted using rollback().
 public boolean isClosed()
Checks whether connection is still open.

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Executing SQL Statements

Three ways of executing SQL statements:
 Statement (static or dynamic SQL statements)
 PreparedStatement (semi-static SQL statements)
 CallableStatement (stored procedures)

PreparedStatement class:
Precompiled & parameterized SQL statements:
 Structure of query is fixed
 Values of parameters are determined at run-time
28
Executing SQL Statements (Contd.)
String sql=“INSERT INTO Sailors VALUES(?,?,?,?)”;
PreparedStatment pstmt=con.prepareStatement(sql);
pstmt.clearParameters();
pstmt.setInt(1,sid);
pstmt.setString(2,sname);
pstmt.setInt(3, rating);
pstmt.setFloat(4,age);
// since no rows are returned, use executeUpdate()
int numRows = pstmt.executeUpdate();
Where numRows is # of rows modified.
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ResultSets


PreparedStatement.executeUpdate only returns
number of affected records
PreparedStatement.executeQuery returns data,
encapsulated in a ResultSet object (a cursor)
ResultSet rs=pstmt.executeQuery(sql);
// rs is now a cursor
While (rs.next()) {
// process the data
}
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ResultSets
A ResultSet is a very powerful cursor:
 previous(): moves one row back
 absolute(int num): moves to the row with the
specified number
 relative (int num): moves forward or
backward
 first() and last()
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Matching Java and SQL Data Types
SQL Type
BIT
CHAR
VARCHAR
DOUBLE
FLOAT
INTEGER
REAL
DATE
TIME
TIMESTAMP
Java class
Boolean
String
String
Double
Double
Integer
Double
java.sql.Date
java.sql.Time
java.sql.TimeStamp
ResultSet get method
getBoolean()
getString()
getString()
getDouble()
getDouble()
getInt()
getFloat()
getDate()
getTime()
getTimestamp()
32
A (Semi-)Complete Example
Connection con = // connect
DriverManager.getConnection ( url, ”login", ”pass“ );
Statement stmt = con.createStatement(); // set up stmt
String query = "SELECT name, rating FROM Sailors";
ResultSet rs = stmt.executeQuery(query);
try { // handle exceptions
// loop through result tuples
while (rs.next()) {
String s = rs.getString(“name");
Int n = rs.getFloat(“rating");
System.out.println(s + " " + n);
}
} catch(SQLException ex) {
System.out.println(ex.getMessage ()
+ ex.getSQLState () + ex.getErrorCode ());
}
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Stored Procedures
41
Stored Procedures

What is a stored procedure:
 Program executed through single SQL statement
 Executed in process space of the server

Advantages:
 Can encapsulate application logic while staying
“close” to the data
 Reuse of application logic by different users
 Avoid tuple-at-a-time return of records through
cursors
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Defining a stored procedure
CREATE PROCEDURE <procedureName> [(<paramList>)]
<localDeclarations>
<procedureBody>;
A parameter in the paramList is specified as:
<name> <mode> <type>
<mode> is one of {IN, OUT, INOUT}
eg: val1 IN int
You can drop procedure by
DROP PROCEDURE <procedureName>
In PL/SQL, you can replace procedure by
CREATE OR REPLACE PROCEDURE <procedureName> …
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PL/SQL Engine
47
Stored Procedures: Examples
Stored procedure do not have to be in SQL:
CREATE PROCEDURE TopSailors(
IN num INTEGER)
LANGUAGE JAVA
EXTERNAL NAME “file:///c:/storedProcs/rank.jar”
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Example: Procedure in PSM
CREATE PROCEDURE testProcedure
BEGIN
INSERT INTO Student VALUES (5, ‘Joe’);
END;
Oracle PL/SQL:
CREATE PROCEDURE testProcedure IS
BEGIN
INSERT INTO Student VALUES (5, ‘Joe’);
END;
.
run;
49
Calling Stored Procedures
Embedded SQL:
EXEC SQL BEGIN DECLARE SECTION
Int sid;
Int rating;
EXEC SQL END DECLARE SECTION
// now increase the rating of this sailor
EXEC CALL IncreaseRating(:sid,:rating);
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Calling Stored Procedures
JDBC:
CallableStatement cstmt=
con.prepareCall(“{call
ShowSailors});
ResultSet rs =
cstmt.executeQuery();
while (rs.next()) {
…
}
SQLJ:
#sql iterator ShowSailors(…);
ShowSailors showsailors;
#sql showsailors={CALL
ShowSailors};
while (showsailors.next()) {
…
}
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More about Procedures


If there is an error in your procedure, Oracle will
give you a warning. Use command SHOW
ERRORS to show the errors in your procedure.
Calling Procedures
call <procedureName> [(<paramList>)];
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PL/SQL
53
Local Declarations
CREATE PROCEDURE testProcedure (num IN int,
name IN varchar) IS
num1 int; -- local variable
BEGIN
num1 := 10;
INSERT INTO Student VALUES (num1, name);
END;
.
run;
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Other PL/SQL features
Assignment statements: PL/SQL
<varName> := <expression>
55
Control Structures: IF THEN ELSE
IF <condition> THEN
<statementList>
ELSIF <condition> THEN
<statementList>
ELSIF
…
ELSE <statementList>
END IF;
56
Loops
LOOP
<statementList>
END LOOP;
To exit from a loop use
EXIT;
57
Loops: Example
CREATE PROCEDURE testProcedure (num IN int, name IN
varchar) IS
num1 int;
BEGIN
num1 := 10;
LOOP
INSERT INTO Student VALUES (num1, name);
num1 := num1 + 1;
IF (num1 > 15) THEN EXIT; END IF;
END LOOP;
END;
.
run;
58
Functions
CREATE FUNCTION <functionName>
[(<paramList>)] RETURNS type AS
<localDeclarations>
BEGIN <functionBody>; END;
You can call a function as part of an SQL expression
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Functions: Example
CREATE FUNCTION testFunction RETURN int IS
num1 int;
BEGIN
SELECT MAX (sNumber) INTO num1
FROM Student;
RETURN num1;
END;
.
run;
SELECT * from Student where sNumber = testFunction ();
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PL/SQL Example
CREATE OR REPLACE FUNCTION rateSailor
(sailorId IN INTEGER) RETURN INT IS
rating INTEGER;
numRes INTEGER;
BEGIN
SELECT COUNT(*) INTO numRes
FROM Reserves R WHERE R.sid = sailorId;
IF (numRes > 10) THEN rating :=1;
ELSE rating := 0;
END IF;
RETURN rating;
END;
.
run;
63
Cursors
When we execute a statement, a relation is returned.
It is stored in private work area for the statement.
Cursor is a pointer to this area.
To create a cursor
CURSOR c_customers is
SELECT * from CUSTOMERS;
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Cursors
We can open the cursor.
OPEN c_customers;
We can select data from the cursor.
FETCH c_customers into customers_rec;
And we can close the cursor.
CLOSE c_customers;
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Implicit & Explicit Cursors
Every SQL data manipulation statements including
queries that return only one row is an implicit cursor.
An explicit cursor is what we create. For queries that
return more than one row, you must declare an
explicit cursor
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CREATE OR REPLACE PROCEDURE copyProcedure IS
stID INT; name VARCHAR (10);
CURSOR myCursor IS SELECT * FROM STUDENT;
BEGIN
OPEN myCursor;
LOOP
FETCH myCursor INTO stID, name;
EXIT WHEN myCURSOR%NOTFOUND;
INSERT INTO newStudent VALUES (stID, name);
END LOOP;
CLOSE myCursor;
END;
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Cursor Attributes
The SQL cursor attributes are : %ROWCOUNT: The number of rows processed by a
SQL statement.
 %FOUND : TRUE if at least one row was processed.
 %NOTFOUND : TRUE if no rows were processed.
 %ISOPEN : TRUE if cursor is open or FALSE if
cursor has not been opened or has been closed. Only
used with explicit cursors.
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Advanced Explicit Cursor
• Concepts
69
Cursor that uses parameters
CURSOR c_students
(p_Department
classes.department%TYPE
p_Course classes.department%TYPE
) IS
SELECT * FROM classes
WHERE department =
p_Department
AND course = p_Course;
To call the cursor
OPEN c_students('CS',101);
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Cursors for update
The syntax for this parameter in the SELECT
statement is:
SELECT ... FROM ... FOR UPDATE [OF
column_reference] [NOWAIT]
where column_reference is a column in
the table against which the query
is performed. A list of columns
can also be used.
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Example…for update
DECLARE
CURSOR c_AllStudents IS
SELECT *
FROM students
FOR UPDATE OF first_name, last_name;
Or the cursor can select every column by not specifing a range
DECLARE
CURSOR c_AllStudents IS
SELECT *
FROM students
FOR UPDATE;
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NOWAIT
If another session already has locks on the rows in the
active set, then the SELECT FOR UPDATE will hang
until the other session releases the lock.
To handle this situation the parameter NOWAIT is
available, which in case the rows are locked,OPEN will
return the error ORA-54 resource busy and
acquire with NOWAIT specified
73
Other Info Re Oracle :

Oracle stores procedures and functions in
catalog as relational tables:
 Check user_procedures
 Check user_functions
 You may run queries against them such as:
• describe user_procedures;
• select object_name from user_procedures;
74
Summary of Whirlwind Tour







Embedded SQL allows execution of parameterized static
queries within host language
Dynamic SQL allows execution of completely ad-hoc queries
within a host language
Cursor mechanism allows retrieval of one record at a time
and bridges impedance mismatch between HL and SQL
APIs such as JDBC introduce a layer of abstraction between
application and DBMS
SQLJ: Static model, queries checked at compile-time.
Stored procedures execute application logic directly at the
server
SQL/PSM standard for writing stored procedures (and also
for triggers)
76