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Database Application Development

Chapter 6

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Overview

SQL in application code Embedded SQL Cursors Dynamic SQL JDBC Stored procedures

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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 …)

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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.

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Embedded SQL

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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 languageEXEC SQL BEGIN DECLARE SECTION

…

EXEC SQL END DECLARE SECTION

Executing SQL statementsEXEC SQL …

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Embedding SQL in C: Oracle

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <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, “me”); userid.len = strlen (userid.arr);strcpy (passwd.arr, “no-me”);passwd.len = strlen (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 FORSELECT S.snameFROM Sailors S, Boats B, Reserves RWHERE 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 SECTIONchar 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 SWHERE S.rating > :c_minratingORDER 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 cursorEXEC 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 asmake -f $ORACLE_HOME/precomp/demo/proc/demo_proc.mk build EXE=test OBJS=test.o

(or)$ORACLE_HOME/bin/proc iname=test.pc code=cpp parse=noneg++ -I$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

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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 JDBCWe 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

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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 cursorWhile (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

getTimestamp()java.sql.TimeStamp

TIMESTAMP

getTime()java.sql.TimeTIME

getDate()java.sql.DateDATE

getFloat()DoubleREAL

getInt()IntegerINTEGER

getDouble()DoubleFLOAT

getDouble()DoubleDOUBLE

getString()StringVARCHAR

getString()StringCHAR

getBoolean()BooleanBIT

ResultSet get method

Java classSQL Type

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A (Semi-)Complete ExampleConnection con = // connect DriverManager.getConnection ( url, ”login", ”pass“ ); Statement stmt = con.createStatement(); // set up stmtString 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

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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 byDROP PROCEDURE <procedureName>

In PL/SQL, you can replace procedure byCREATE OR REPLACE PROCEDURE <procedureName> …

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PL/SQL Engine

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Stored Procedures: Examples

Stored procedure do not have to be in SQL:

CREATE PROCEDURE TopSailors(IN num INTEGER)

LANGUAGE JAVAEXTERNAL NAME “file:///c:/storedProcs/rank.jar”

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Example: Procedure in PSMCREATE PROCEDURE testProcedure

BEGININSERT INTO Student VALUES (5, ‘Joe’);

END;

Oracle PL/SQL:CREATE PROCEDURE testProcedure IS

BEGININSERT INTO Student VALUES (5, ‘Joe’);

END;.run;

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Calling Stored Procedures

EXEC SQL BEGIN DECLARE SECTIONInt sid;Int rating;EXEC SQL END DECLARE SECTION

// now increase the rating of this sailorEXEC CALL IncreaseRating(:sid,:rating);

Embedded SQL:

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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 Procedurescall <procedureName> [(<paramList>)];

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PL/SQL

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Local Declarations

CREATE PROCEDURE testProcedure (num IN int, name IN varchar) ISnum1 int; -- local variableBEGIN

num1 := 10;INSERT INTO Student VALUES (num1,

name);END;

.run;

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Other PL/SQL features

Assignment statements: PL/SQL<varName> := <expression>

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Control Structures: IF THEN ELSE

IF <condition> THEN<statementList>ELSIF <condition> THEN<statementList>ELSIF…ELSE <statementList>END IF;

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Loops

LOOP<statementList>

END LOOP;

To exit from a loop use EXIT;

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Loops: ExampleCREATE PROCEDURE testProcedure (num IN int, name

IN varchar) IS num1 int;

BEGINnum1 := 10;LOOP

INSERT INTO Student VALUES (num1, name);

num1 := num1 + 1;IF (num1 > 15) THEN EXIT; END IF;

END LOOP;END;

.run;

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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 ISnum1 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 ExampleCREATE OR REPLACE FUNCTION rateSailor

(sailorId IN INTEGER) RETURN INT ISrating 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;

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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 isSELECT * 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 AttributesThe 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

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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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NOWAITIf 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

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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;

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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)