SQL, Embedded SQL, Dynamic SQL and SQLJ

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Embedded SQL, Dynamic SQL
and SQLJ
e-Infochips Institute of Training Research and Academics Limited
Prepared By:-
Dharita Chokshi

Outlines
• What is SQL
• What is Embedded SQL
• Cursors
• Dynamic SQL
• SQLJ
• Summary

What is SQL?
• Structured Query Language (SQL) is a standardized language
used to manipulate database objects and the data they
contain.
• It comprised of several different statements that are used
manipulate data values.
• SQL being a nonprocedural is not a general-purpose
programming language.
UPDATE EMPLOYEE SET LASTNAME = 'Jones' WHERE
EMPID = '001'

What is Embedded SQL?
• As a result, database applications are usually developed by
combining capabilities of a high-level programming language
with SQL.
• The simplest approach is to embed SQL statements directly
into the source code file(s) that will be used to create an
application. This technique is referred to as embedded SQL
programming.
• sqlca.h – header file to be included.

Embedded SQL
• High-level programming language compilers cannot interpret,
SQL statements.
• Hence source code files containing embedded SQL
statements must be preprocessed before compiling.
• Thus each SQL statement coded in a high-level programming
language source code file must be prefixed with the keywords
EXEC SQL and terminated with either a semicolon or the
keywords END_EXEC.

Embedded SQL
• Likewise, the Database Manager cannot work directly with
high-level programming language variables.
• Instead, it must use special variables known as host
variables to move data between an application and a
database.
• Two types of Host variables:-
1. Input Host Variables – Transfer data to database
2. Output Host Variables – receives data from database

Embedded SQL
• Host variables are ordinary programming language variables.
• To be set apart, they must be defined within a special section
known as a declare section.
EXEC SQL BEGIN DECLARE SECTION
char EmployeeID[7];
double Salary;
EXEC SQL END DECLARE SECTION
• Each host variable must be assigned a unique name
even though declared in different declaration section.

Embedded SQL
main() {
EXEC SQL BEGIN DECLARE SECTION;
int OrderID, CustID;
char SalesPerson[10], Status[6];
EXEC SQL END DECLARE SECTION;
printf ("Enter order number: ");
scanf ("%d", &OrderID);
EXEC SQL SELECT CustID, SalesPerson, Status FROM
Orders WHERE OrderID = :OrderID INTO :CustID,
:SalesPerson, :Status;
printf ("Customer number: %d n", CustID);
printf ("Salesperson: %s n", SalesPerson);
printf ("Status: %s n", Status);
}

Embedded SQL
Connecting to Database using embedded sql
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=%dn”,value);

Cursor
• 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.

Cursor
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;

Dynamic SQL
• Dynamic SQL means composing and executing new (not
previously compiled) SQL statements at run-time.
• Although static SQL statements are relatively easy to
incorporate, Dynamic SQL statements are much more flexible
as they can be constructed at run time.
• Dynamic queries can be complex because the type and
number of retrieved attributes are unknown at compile time.
INSERT INTO EMPLOYEES VALUES (?, ?)
DELETE FROM DEPARTMENT WHERE DEPTID = ?

SQLJ
• SQLJ – Standard for embedding SQL in Java
• Similar to existing SQL extensions provided for C, FORTRAN,
and other programming languages.
• IBM, Oracle, and several other companies proposed SQLJ as
a standard and as a simpler and easier-to-use alternative to
JDBC.
• An SQLJ translator converts SQL statements into Java
• These are executed through the JDBC interface
• Certain classes have to be imported E.g., java.sql

SQLJ
• SQLJ precompiles SQL code in a Java program.
• Provides greater compile-time checking of SQL statements.
• Reduces the amount of code needed to execute SQL from
within Java.

SQLJ v/s JDBC
// SQLJ
int n;
#sql { INSERT INTO emp VALUES (:n)};
// JDBC
int n;
Statement stmt = conn.prepareStatement
(“INSERT INTO emp VALUES (?)”);
stmt.setInt(1,n);
stmt.execute ();
stmt.close();

class X {
void myJavaMethod() {
try {
#sql{update EMP set SAL = SAL + 100
where SAL < 1500};
}
SQLJ
import java.sql.*;
import sqlj.runtime.*;
import sqlj.runtime.ref.*;
Imports Needed
SQL statement placed in braces
can throw SQLException
catch (SQLException e) {…}
}

SQLJ
Loading the JDBC Driver
SQLJ requires that the JDBC driver class is loaded. This can be
performed in the same way as for JDBC
try
{
Class.forName("oracle.jdbc.driver.OracleDriver");
}
catch (ClassNotFoundException e)
{
System.out.println("Could not load driver");
}

SQLJ
Specifying a Connection Context
• All SQLJ statements execute in a “connection context”
• Plays similar role as a Connection object does in JDBC.
• Establishes the database we are connecting to, the user
name, and the password.
try
{
Class.forName("oracle.jdbc.driver.OracleDriver");
DefaultContext.setDefaultContext(new DefaultContext(
"jdbc:oracle:thin:@HOSTID:1521:ORCL",
"theUser", "thePassword") );
}

SQLJ
Passing Host Variables into a SQLJ
Statement
• Prefix the java variable name with a colon (:)
#sql {delete from EMP where SAL >= :amt};

SQLJ
Dealing with Query Result Sets
• SQLJ can be used to execute queries that return a result set .
• To process the result set, define an “iterator” type that
specifies the data type of each column
#sql iterator MyIter(String ENAME, String JOB);
class MyClass {
MyIter iter;
#sql iter = { select ENAME, JOB from EMP };
while(iter.next()) {
String ename = iter.ENAME();
String job = iter.JOB();
}

When to Which ?
How do applications connect to a database?
• App ↔ DBMS: Embedded SQL
• App ↔ Driver ↔ DBMS: JDBC/ODBC or SQLJ
What mechanisms exist to retrieve/modify data?
• Static Queries: Embedded SQL, SQLJ
• Dynamic Queries: JDBC/ODBC, Dynamic SQL

SQL, Embedded SQL, Dynamic SQL and SQLJ

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