Database ApplicationDevelopment

Chapter6

Overview

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

Why accessing to databasesvia programming languages ? SQL is a direct query language; as

such, it has limitations. via programming languages :

– Complex computational processing ofthe data.

– Specialized user interfaces.– Access to more than one database at

a time.

SQL in Application Code

SQL commands can be called from within a hostlanguage (e.g., C++ or Java) program.

– SQL statements can refer to host variables (includingspecial variables used to return status).

– Must include a statement to connect to the right database.

Impedance Mismatch. SQL relations are (multi-) setsof records, with no a priori bound on the number ofrecords. No such data structure exist traditionally inprocedural programming languages such as C++.(Though now: STL)

– SQL supports a mechanism called a cursor to handle this.

Desirable features of suchsystems:

Ease of use.

Conformance to standards for existingprogramming languages, databasequery languages, and developmentenvironments.

Interoperability: the ability to use acommon interface to diverse databasesystems on different operating systems

Vendor specific solutions Oracle PL/SQL: A proprietary PL/1-like

language which supports the execution ofSQL queries:

Advantages:– Many Oracle-specific features, not common to

other systems, are supported.– Performance may be optimized to Oracle-based

systems.

Disadvantages:– Ties the applications to a specific DBMS.– The application programmer must depend upon

the vendor for the application developmentenvironment.

– It may not be available for all platforms.

Vendor Independent solutionsbased on SQL

There are three basic strategieswhich may be considered:– Embed SQL in the host language (Embedded

SQL, SQLJ)– SQL modules.– SQL call level interfaces

Embedded SQL Approach: Embed SQL in the host language.

– A preprocessor converts the SQL statements intospecial API calls.

– Then a regular compiler is used to compile thecode.

Language constructs:– Connecting to a database:EXEC SQL CONNECT

– Declaring variables:EXEC SQL BEGIN (END) DECLARE SECTION

– Statements:EXEC SQL Statement;

Embedded SQL: Variables

EXEC SQL BEGIN DECLARE SECTIONchar c_sname[20];long c_sid;short c_rating;float c_age;

EXEC SQL END DECLARE SECTION

Two special “error” variables:‒ SQLCODE (long, is negative if an error has

occurred)‒ SQLSTATE (char[6], predefined codes for

common errors)

Cursors Can declare a cursor on a relation or query

statement (which generates a relation). Can open a cursor, and repeatedly fetch a

tuple then move the cursor, until all tupleshave been retrieved.– Can use a special clause, called ORDER BY, in

queries that are accessed through a cursor, tocontrol the order in which tuples are returned. Fields in ORDER BY clause must also appear in SELECT

clause.

– The ORDER BY clause, which orders answer tuples, isonly allowed in the context of a cursor.

Can also modify/delete tuple pointed to bya cursor.

Cursor that gets names ofsailors who’ve reserved a red

boat, in alphabetical orderEXEC SQL DECLARE sinfo CURSOR FOR

SELECT 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

Note that it is illegal to replace S.sname by, say,S.sid in the ORDER BY clause! (Why?)

Can we add S.sid to the SELECT clause andreplace S.sname by S.sid in the ORDER BY clause?

Embedding SQL in C: AnExample

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 S WHERE S.rating > :c_minrating ORDER BY S.sname;

do { EXEC SQL FETCH sinfo INTO :c_sname, :c_age; printf(“%s is %d\n”,c_sname,c_age);} while (SQLSTATE != ‘02000’);

EXEC SQL CLOSE sinfo;

Dynamic SQL SQL query strings are not always known at compile

time (e.g., spreadsheet, graphical DBMS frontend):Allow construction of SQL statements on-the-fly

Example:char c_sqlstring[]={“DELETE FROM Sailors WHERE rating>5”};

EXEC SQL PREPARE readytogo FROM :c_sqlstring;EXEC SQL EXECUTE readytogo;

Disadvantages:– It is a real pain to debug preprocessed programs.– The use of a program-development environment is

compromised substantially.– The preprocessor must be vendor and platform specific.

SQL Modules In the module approach, invocations to SQL are made via

libraries of procedures , rather than via preprocessing

Special standardized interface: procedures/objects

Pass SQL strings from language, presents result sets in alanguage-friendly way

Supposedly DBMS-neutral– a “driver” traps the calls and translates them into DBMS-

specific code– database can be across a network

SQL Modules (cont.)

Example:– Sun’s JDBC: Java API– Part of the java.sql package

Advantages over embedded SQL:– Clean separation of SQL from the host

programming language.– Debugging is much more straightforward,

since no preprocessor is involved.

Disadvantages:– The module libraries are specific to the

programming language and environment.Thus, portability is compromised greatly.

JDBC: Architecture Four architectural

components:– Application (initiates and

terminates connections,submits SQL statements)

– Driver manager (loadJDBC driver)

– Driver (connects to datasource, transmitsrequests andreturns/translates resultsand error codes)

– Data source (processesSQL statements)

Java Application

JDBC driver manager

JDBC/nativebridge

JDBC/OBDCbridge

JDBC driver(DBMS specific)

JDBC middleware(various DBMS)

DBMS

Native driver(DBMS specific)

OBDCdriver

JDBC Architecture (Cont.)Four types of drivers:Direct translation to native API, non-Java driver:

– Translates JDBC function calls (with SQL commands) tonative API of data source. Need OS-specific binary oneach client.

Bridge:– Translates JDBC calls (with SQL commands) into non-

native API.Example: JDBC-ODBC bridge. Code for ODBC and JDBCdriver needs to be available on each client.

Direction translation to native API via Java driver:– Converts JDBC calls directly to network protocol used by

DBMS. Needs DBMS-specific Java driver at each client.Network bridge:

– The JDBC driver talks over a network to a middlewareserver that translates JDBC requests into DBMS-specificmethod invocations, i.e. the middleware server talks to thedata source. Needs only small JDBC driver at each client.

JDBC Classes andInterfaces

Steps to submit a database query:– Load the JDBC driver– Connect to the data source– Execute SQL statements

JDBC Driver Loading All drivers are managed by the

DriverManager class Loading a JDBC driver:

– In the Java code:Class.forName(“oracle/jdbc.driver.Oracledriver”);

– When starting the Java application:Djdbc.drivers=oracle/jdbc.driver

Connections in JDBC We interact with a data source through

sessions. Each connection identifies a logicalsession.

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 excpt { …}

Connection Class Interface public int getTransactionIsolation() and

void setTransactionIsolation(int level)Gets/Sets isolation level for the current connection.

public boolean getReadOnly() andvoid setReadOnly(boolean b)Specifies if transactions in this connection are read-only

public boolean getAutoCommit() andvoid setAutoCommit(boolean b)If autocommit is set, then each SQL statement is considered itsown transaction. Otherwise, a transaction is committed usingcommit(), or aborted using rollback().

public boolean isClosed()Checks whether connection is still open.

Executing SQL Statements Three different ways of executing SQL

statements:– Statement (both static and dynamic SQL

statements)– PreparedStatement (semi-static SQL

statements)– CallableStatment (stored procedures)

PreparedStatement class:Precompiled, parametrized SQL statements:– Structure is fixed– Values of parameters are determined at run-time

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);

//we know no. rows are returned, thus we use executeUpdate()int numRows = pstmt.executeUpdate();

ResultSets PreparedStatement.executeUpdate only returns

the 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}

A ResultSet is a very powerful cursor:‒ previous(): moves one row back‒ absolute(int num): moves to the row with number num‒ relative (int num): moves forward or backward‒ first() and last()

Matching Java and SQLData Types

getTimestamp()java.sql.TimeStampTIMESTAMP

getTime()java.sql.TimeTIME

getDate()java.sql.DateDATE

getFloat()DoubleREAL

getInt()IntegerINTEGER

getDouble()DoubleFLOAT

getDouble()DoubleDOUBLE

getString()StringVARCHAR

getString()StringCHAR

getBoolean()BooleanBIT

ResultSet get methodJava classSQL Type

Examining DatabaseMetadata

DatabaseMetaData object givesinformation about the databasesystem and the catalog.

DatabaseMetaData md = con.getMetaData();// print information about the driver:System.out.println(“Name:” + md.getDriverName() +“version: ” + md.getDriverVersion()

);

Database Metadata (Cont.)DatabaseMetaData md=con.getMetaData();ResultSet trs=md.getTables(null,null,null,null);String tableName;

while(trs.next()) {tableName = trs.getString(“TABLE_NAME”);System.out.println(“Table: “ + tableName);

//print all attributes ResultSet crs =

md.getColumns(null,null,tableName, null); while (crs.next()) {

System.out.println(crs.getString(“COLUMN_NAME”+ “, “);

}}

A (Semi-)CompleteExampleimport java.sql.*;

/* This is a sample program with jdbc odbc Driver */public class localdemo { public static void main(String[] args) { try { // Register JDBC/ODBC Driver in jdbc DriverManager // On some platforms with some java VMs, newInstance() is needed

Class.forName("sun.jdbc.odbc.JdbcOdbcDriver").newInstance(); // Test with MS Access database (sailors ODBC data source) String url = "jdbc:odbc:mysailors";

java.sql.Connection c = DriverManager.getConnection(url);java.sql.Statement st = c.createStatement();java.sql.ResultSet rs = st.executeQuery("select * from Sailors");java.sql.ResultSetMetaData md = rs.getMetaData();

while(rs.next()) { System.out.print("\nTUPLE: | "); for(int i=1; i<= md.getColumnCount(); i++) { System.out.print(rs.getString(i) + " | "); } } rs.close(); } catch(Exception e) { e.printStackTrace();} }};

Another Exampleimport java.sql.*; //import all the JDBC classespublic class CreateJoltData {

static String[] SQL = {"create table JoltData ("+

"programmer varchar (32),"+ "day varchar (3),"+ "cups integer);", "insert into JoltData values ('Gilbert', 'Mon', 1);", "insert into JoltData values ('Wally', 'Mon', 2);", "insert into JoltData values ('Edgar', 'Tue', 8);", "insert into JoltData values ('Wally', 'Tue', 2);", "insert into JoltData values ('Eugene', 'Tue', 3);", "insert into JoltData values ('Josephine', 'Wed', 2);", "insert into JoltData values ('Eugene', 'Thu', 3);", "insert into JoltData values ('Gilbert', 'Thu', 1);", "insert into JoltData values ('Clarence', 'Fri', 9);", "insert into JoltData values ('Edgar', 'Fri', 3);", "insert into JoltData values ('Josephine', 'Fri', 4);", }; public static void main(String[] args) { String URL = "jdbc:odbc:CafeJolt"; String username = "";

String password = "";

Another Example (cont.)try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");}catch (Exception e) {

System.out.println("Failed to load JDBC/ODBC driver."); return; } Statement stmt = null; Connection con=null; try { con = DriverManager.getConnection(URL,username,password); stmt = con.createStatement(); } catch (Exception e) { System.err.println("problems connecting to "+URL); } try { // execute SQL commands to create table, insert data for (int i=0; i<SQL.length; i++) {stmt.execute(SQL[i]);} con.close(); } catch (Exception e) { System.err.println("problems with SQL sent to "+URL+ ": "+e.getMessage()); }

}}

SQLJ Complements JDBC with a (semi-)static query

model: Compiler can perform syntax checks,strong type checks, consistency of the querywith the schema– All arguments always bound to the same variable:

#sql x = { SELECT name, rating INTO :name, :rating FROM Books WHERE sid = :sid;

– Compare to JDBC:sid=rs.getInt(1);if (sid==1) {sname=rs.getString(2);}else { sname2=rs.getString(2);}

SQLJ (part of the SQL standard) versusembedded SQL (vendor-specific)

SQLJ CodeInt sid; String name; Int rating;

// named iterator#sql iterator Sailors(Int sid, String name, Int rating);Sailors sailors;

// assume that the application sets rating#sailors = { SELECT sid, sname INTO :sid, :name

FROM Sailors WHERE rating = :rating};

// retrieve resultswhile (sailors.next()) { System.out.println(sailors.sid + “ “ + sailors.sname));}sailors.close();

SQLJ Iterators Two types of iterators (“cursors”):

– Named iterator Need both variable type and name, and then

allows retrieval of columns by name. See example on previous slide.

– Positional iterator Need only variable type, and then uses FETCH ..

INTO construct:#sql iterator Sailors(Int, String, Int);Sailors sailors;#sailors = …while (true) { #sql {FETCH :sailors INTO :sid, :name} ; if (sailors.endFetch()) { break; } // process the sailor}

SQL call level interfaces

A call-level interface provides a library of functions foraccess to DBMS’s.

The DBMS drivers are stored separately; thus the libraryused by the programming language is DBMS-independent.

The programming language functions provided only aninterface to the DBMS drivers.

Advantages:– The development environment is not tied to a particular DBMS,

operating sytem, or even a particular developmentenvironment.

Disadvantages:– Some low-level optimization may be more difficult or impossible

to achieve.

Key example:

The SQL CLI (X/Open CLI) Microsoft ODBC (Open Database

Connectivity) The two are closely aligned.

Open DataBaseConnectivity

Shorten to ODBC, a standard database access method

The goal: make it possible to access any data from anyapplication, regardless of which (DBMS).

ODBC manages this by inserting a middle layer, called adatabase driver , between an application and the DBMS.

The purpose of this layer is to translate the application'sdata queries into commands that the DBMS understands.

For this to work, both the application and the DBMS mustbe ODBC-compliant -- that is, the application must becapable of issuing ODBC commands and the DBMS mustbe capable of responding to them.

Configuring a datasource(Access) under Windows

Open the ODBC menu in the control panel. Click on the User DSN tab.

– click on Add. From the menu in the new window,

– select Microsoft Access Driver (sailors.mdb),– click on Finish.

From the menu in the new window,– type in a data source name (mysailors), and optionally, a

description.– Then click on either Select or Create, depending upon

whether you want to link to an existing database, or createa new blank one.

In the new window, give the path to the database. “OK” away the pile of subwindows; the new

database should appear under the top-level ODBCUser DSN tab.

// program connects to an ODBC data source called “mysailors“// then executes SQL statement “SELECT * FROM Sailors';"#include <windows.h>#include <sqlext.h>#include <stdio.h>

int main(void){HENV hEnv = NULL; // Env Handle from SQLAllocEnv()HDBC hDBC = NULL; // Connection handleHSTMT hStmt = NULL; // Statement handleUCHAR szDSN[SQL_MAX_DSN_LENGTH] = “mysailors";// Data Source Name bufferUCHAR* szUID = NULL; // User ID bufferUCHAR* szPasswd = NULL; // Password bufferUCHAR szname[255]; // bufferSDWORD cbModel; // bytes receivedUCHAR szSqlStr[]="Select * From Sailors”; // SQL stringRETCODE retcode; // Return code

SQLAllocEnv (&hEnv); // Allocate memory for ODBC Environment handle

SQLAllocConnect (hEnv, &hDBC); // Allocate memory for the connection handle

// Connect to the data source “mysailors" using userid and password.retcode = SQLConnect (hDBC, szDSN, SQL_NTS, szUID, SQL_NTS, szPasswd, SQL_NTS);

Example:

if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO){

// Allocate memory for the statement handle retcode = SQLAllocStmt (hDBC, &hStmt); // Assign SQL statement to the statement handle retcode = SQLPrepare (hStmt, szSqlStr, sizeof (szSqlStr)); // Execute the SQL statement handle retcode = SQLExecute (hStmt); // Project only column 2 which is the name SQLBindCol (hStmt, 2, SQL_C_CHAR, szname, sizeof(szname), &cbModel); // Get row of data from the result set defined above in the statement retcode = SQLFetch (hStmt);

while (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) { printf ("\t%s\n", szname); // Print row (sname)

retcode = SQLFetch (hStmt); // Fetch next row from result set }

// Free the allocated statement handle SQLFreeStmt (hStmt, SQL_DROP);

// Disconnect from datasource SQLDisconnect (hDBC);} // Free the allocated connection handleSQLFreeConnect (hDBC); // Free the allocated ODBC environment handleSQLFreeEnv (hEnv);return 0;

}

Example(cont.):

Stored Procedures

What is a stored procedure:– Program executed through a single SQL

statement– Executed in the process space of the server– May possess parameters: IN, OUT, INOUT– Do not have to be written in SQL

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

Stored Procedures:Examples

CREATE PROCEDURE ShowNumReservationsSELECT S.sid, S.sname, COUNT(*)FROM Sailors S, Reserves RWHERE S.sid = R.sidGROUP BY S.sid, S.sname

CREATE PROCEDURE upRating(IN sailor_id INTEGER, IN i INTEGER)UPDATE SailorsSET rating = rating + iWHERE sid = sailor_id

CREATE PROCEDURE TopSailors(IN num INTEGER)LANGUAGE JAVAEXTERNAL NAME “file:///c:/storedProcs/rank.jar”

0 PARAMETERS!

2 PARAMETERS!

procedure em Java

Calling Stored ProceduresEXEC SQL BEGIN DECLARE SECTIONInt sid;Int rating;EXEC SQL END DECLARE SECTION

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

Calling Stored Procedures(Contd.)

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()) { …}

SQL/PSMMost DBMSs allow users to write stored

procedures in a simple, general-purposelanguage (close to SQL) SQL/PSMstandard is a representative

Declare a stored procedure:CREATE PROCEDURE name(p1, p2, …, pn)

local variable declarations

procedure code;

Declare a function:CREATE FUNCTION name (p1, …, pn) RETURNSsqlDataTypelocal variable declarations

function code;

Main SQL/PSM ConstructsCREATE FUNCTION rate Sailor

(IN sailorId INTEGER) RETURNS INTEGER

DECLARE rating INTEGERDECLARE numRes INTEGERSET numRes = (SELECT COUNT(*) FROM Reserves R

WHERE R.sid = sailorId)IF (numRes > 10) THEN rating =1;ELSE rating = 0;END IF;RETURN rating;

Main SQL/PSM Constructs(Contd.)

Local variables (DECLARE) RETURN values for FUNCTION Assign variables with SET Branches and loops:

– IF (condition) THEN statements;ELSEIF (condition) statements;… ELSE statements; END IF;

– LOOP statements; END LOOP

Queries can be parts of expressions Can use cursors naturally without “EXEC

SQL”

Web-based Applicationsvia HTML/PHP

http://www.keithjbrown.co.uk/vworks/php/

Summary Embedded SQL allows execution of parametrized

static queries within a 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 hostlanguage and SQL

APIs such as JDBC introduce a layer of abstractionbetween application and DBMS

SQLJ: Static model, queries checked a compile-time. Stored procedures execute application logic directly

at the server SQL/PSM standard for writing stored procedures