SQLJ: Embedding SQL Commands in Java
13.2.4 SQLJ: Embedding SQL Commands in Java
In the previous subsections, we gave an overview of how SQL commands can be embedded in a traditional programming language, using the C language in our examples. We now turn our attention to how SQL can be embedded in an object-
oriented programming language, 8 in particular, the Java language. SQLJ is a stan- dard that has been adopted by several vendors for embedding SQL in Java. Historically, SQLJ was developed after JDBC, which is used for accessing SQL data- bases from Java using function calls. We discuss JDBC in Section 13.3.2. In this sec- tion, we focus on SQLJ as it is used in the Oracle RDBMS. An SQLJ translator will generally convert SQL statements into Java, which can then be executed through the
JDBC interface. Hence, it is necessary to install a JDBC driver when using SQLJ. 9 In this section, we focus on how to use SQLJ concepts to write embedded SQL in a Java program.
Before being able to process SQLJ with Java in Oracle, it is necessary to import sev- eral class libraries, shown in Figure 13.5. These include the JDBC and IO classes (lines 1 and 2), plus the additional classes listed in lines 3, 4, and 5. In addition, the program must first connect to the desired database using the function call getConnection , which is one of the methods of the oracle class in line 5 of Figure
1) import java.sql.* ;
Figure 13.5
2) import java.io.* ; Importing classes needed for including 3) import sqlj.runtime.* ;
SQLJ in Java programs in Oracle, and 4) import sqlj.runtime.ref.* ;
establishing a connection and default 5) import oracle.sqlj.runtime.* ;
context.
... 6) DefaultContext cntxt = 7)
oracle.getConnection("<url name>", "<user name>", "<password>", true) ; 8) DefaultContext.setDefaultContext(cntxt) ; ...
8 This section assumes familiarity with object-oriented concepts (see Chapter 11) and basic JAVA con- cepts.
460 Chapter 13 Introduction to SQL Programming Techniques
13.5. The format of this function call, which returns an object of type default
context, 10 is as follows: public static DefaultContext
getConnection(String url, String user, String password,
Boolean autoCommit) throws SQLException ;
For example, we can write the statements in lines 6 through 8 in Figure 13.5 to con- nect to an Oracle database located at the url <url name> using the login of <user
name> and <password> with automatic commitment of each command, 11 and then set this connection as the default context for subsequent commands.
In the following examples, we will not show complete Java classes or programs since it is not our intention to teach Java. Rather, we will show program segments that illustrate the use of SQLJ. Figure 13.6 shows the Java program variables used in our examples. Program segment J1 in Figure 13.7 reads an employee’s Ssn and prints some of the employee’s information from the database.
Notice that because Java already uses the concept of exceptions for error handling,
a special exception called SQLException is used to return errors or exception con- ditions after executing an SQL database command. This plays a similar role to SQLCODE and SQLSTATE in embedded SQL. Java has many types of predefined exceptions. Each Java operation (function) must specify the exceptions that can be thrown —that is, the exception conditions that may occur while executing the Java code of that operation. If a defined exception occurs, the system transfers control to the Java code specified for exception handling. In J1, exception handling for an SQLException is specified in lines 7 and 8. In Java, the following structure
try {<operation>} catch (<exception>) {<exception handling
code>} <continuation code>
is used to deal with exceptions that occur during the execution of < operation >. If no exception occurs, the < continuation code > is processed directly. Exceptions
Figure 13.6
1) string dname, ssn , fname, fn, lname, ln, Java program vari-
bdate, address ;
ables used in SQLJ
2) char sex, minit, mi ;
examples J1 and J2.
3) double salary, sal ; 4) integer dno, dnumber ;
10 A default context, when set, applies to subsequent commands in the program until it is changed. 11 Automatic commitment roughly means that each command is applied to the database after it is exe- cuted. The alternative is that the programmer wants to execute several related database commands and
13.2 Embedded SQL, Dynamic SQL, and SQLJ 461
//Program Segment J1:
Figure 13.7
1) ssn = readEntry("Enter a Social Security Number: ") ; Program segment J1, 2) try {
a Java program seg- 3)
#sql { select Fname, Minit, Lname, Address, Salary ment with SQLJ. 4)
into :fname, :minit, :lname, :address, :salary 5)
from EMPLOYEE where Ssn = :ssn} ; 6) } catch (SQLException se) { 7)
System.out.println("Social Security Number does not exist: " + ssn) ; 8)
Return ; 9)
} 10) System.out.println(fname + " " + minit + " " + lname + " " + address + " " + salary)
that can be thrown by the code in a particular operation should be specified as part of the operation declaration or interface—for example, in the following format:
<operation return type> <operation name> (<parameters>) throws SQLException, IOException ;
In SQLJ, the embedded SQL commands within a Java program are preceded by #sql , as illustrated in J1 line 3, so that they can be identified by the preprocessor. The #sql is used instead of the keywords EXEC SQL that are used in embedded SQL with the C programming language (see Section 13.2.1). SQLJ uses an INTO clause— similar to that used in embedded SQL—to return the attribute values retrieved from the database by an SQL query into Java program variables. The program vari- ables are preceded by colons (:) in the SQL statement, as in embedded SQL.
In J1 a single tuple is retrieved by the embedded SQLJ query; that is why we are able to assign its attribute values directly to Java program variables in the INTO clause in line 4 in Figure 13.7. For queries that retrieve many tuples, SQLJ uses the concept of an iterator, which is similar to a cursor in embedded SQL.
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» Fundamentals_of_Database_Systems,_6th_Edition
» Characteristics of the Database Approach
» Advantages of Using the DBMS Approach
» A Brief History of Database Applications
» Schemas, Instances, and Database State
» The Three-Schema Architecture
» The Database System Environment
» Centralized and Client/Server Architectures for DBMSs
» Classification of Database Management Systems
» Domains, Attributes, Tuples, and Relations
» Key Constraints and Constraints on NULL Values
» Relational Databases and Relational Database Schemas
» Integrity, Referential Integrity, and Foreign Keys
» Update Operations, Transactions, and Dealing with Constraint Violations
» SQL Data Definition and Data Types
» Specifying Constraints in SQL
» The SELECT-FROM-WHERE Structure of Basic SQL Queries
» Ambiguous Attribute Names, Aliasing, Renaming, and Tuple Variables
» Substring Pattern Matching and Arithmetic Operators
» INSERT, DELETE, and UPDATE Statements in SQL
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» Discussion and Summary of SQL Queries
» Specifying General Constraints as Assertions in SQL
» Introduction to Triggers in SQL
» Specification of Views in SQL
» View Implementation, View Update, and Inline Views
» Schema Change Statements in SQL
» Sequences of Operations and the RENAME Operation
» The UNION, INTERSECTION, and MINUS Operations
» The CARTESIAN PRODUCT (CROSS PRODUCT) Operation
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» The Role of Information Systems
» The Database Design and Implementation Process
» Use of UML Diagrams as an Aid to Database Design Specification 6
» Rational Rose: A UML-Based Design Tool
» Automated Database Design Tools
» Introduction to Object-Oriented Concepts and Features
» Object Identity, and Objects versus Literals
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» Database Programming: Techniques
» Retrieving Single Tuples with Embedded SQL
» Retrieving Multiple Tuples with Embedded SQL Using Cursors
» Specifying Queries at Runtime Using Dynamic SQL
» SQLJ: Embedding SQL Commands in Java
» Retrieving Multiple Tuples in SQLJ Using Iterators
» Database Programming with SQL/CLI Using C
» JDBC: SQL Function Calls for Java Programming
» Database Stored Procedures and SQL/PSM
» PHP Variables, Data Types, and Programming Constructs
» Overview of PHP Database Programming
» Imparting Clear Semantics to Attributes in Relations
» Redundant Information in Tuples and Update Anomalies
» Normal Forms Based on Primary Keys
» General Definitions of Second and Third Normal Forms
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» Inference Rules for Functional Dependencies
» Minimal Sets of Functional Dependencies
» Properties of Relational Decompositions
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» Dependency-Preserving and Nonadditive (Lossless) Join Decomposition into 3NF Schemas
» Problems with NULL Values and Dangling Tuples
» Discussion of Normalization Algorithms and Alternative Relational Designs
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» Some General Issues Concerning Indexing
» Algorithms for External Sorting
» Implementing the SELECT Operation
» Implementing the JOIN Operation
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» Notation for Query Trees and Query Graphs
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» Catalog Information Used in Cost Functions
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» Examples of Cost Functions for JOIN
» Example to Illustrate Cost-Based Query Optimization
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» An Overview of Database Tuning in Relational Systems
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» Introduction to Database Security Issues 1
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» Introduction to Statistical Database Security
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» Challenges of Database Security
» Distributed Database Concepts 1
» Types of Distributed Database Systems
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» Data Replication and Allocation
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» Distributed Databases in Oracle 13
» Generalized Model for Active Databases and Oracle Triggers
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» Examples of Statement-Level Active Rules
» Time Representation, Calendars, and Time Dimensions
» Incorporating Time in Relational Databases Using Tuple Versioning
» Incorporating Time in Object-Oriented Databases Using Attribute Versioning
» Temporal Querying Constructs and the TSQL2 Language
» Spatial Database Concepts 24
» Multimedia Database Concepts
» Clausal Form and Horn Clauses
» Datalog Programs and Their Safety
» Evaluation of Nonrecursive Datalog Queries
» Introduction to Information Retrieval
» Types of Queries in IR Systems
» Evaluation Measures of Search Relevance
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» Grouping, Aggregation, and Database Modification in QBE
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