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CHAPTER
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Integrating SQL into Applications
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In this chapter, we ll have a look at how SQL is used in applications. An application is a set of computer programs designed to solve a particular business problem, such as an order-entry system, a payroll-processing system, or an accounting system. Applications can be built using a general-purpose procedural programming language such as Java, C, or C++, or a programming environment such as the Microsoft .NET Framework. A procedural programming language speci es an explicit sequence of steps to follow in order to complete a transaction. In applications that use relational databases, the application developers use various techniques for communicating with the database, which of course includes sending SQL statements to the database for processing and dealing with the results. This chapter explores some of those techniques.
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SQL Demysti ed
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Cursor Processing
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Procedural programming languages are designed to handle one record at a time (one object instance or collection at a time in the case of object-oriented languages such as Java). This presents a dilemma when SQL is used in conjunction with the programming language because SQL queries typically produce result sets that contain multiple records (rows) of data. There is a mismatch that must be addressed. To overcome the mismatch, most relational databases support the concept of a cursor, which is merely a pointer to a single row in the result set. In Oracle, cursor support is included in a procedural language SQL extension called PL/SQL (Procedural Language/SQL) and similarly is included in Transact-SQL in Sybase Adaptive Server and Microsoft SQL Server. These procedural SQL extensions are discussed later in this chapter. The use of a cursor parallels the use of a traditional at le in that the cursor must be de ned and opened before it may be used, it may be read from by fetching rows in a programming loop, and it should be closed when the program no longer needs it.
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The DECLARE CURSOR Statement
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A cursor must be declared before it can be referenced by any other SQL statements. Here is the general syntax of the DECLARE CURSOR statement that de nes a cursor:
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DECLARE cursor_name CURSOR FOR select_statement [FOR UPDATE [OF column_name [, column_name...]]]
N OTE : The exact syntax will vary from one DBMS to another. For example, the SQL standard speci es the keyword FOR, but the Oracle syntax uses the keyword IS, and the Microsoft SQL Server syntax makes the keyword optional. Also, Oracle requires the keyword CURSOR to appear before the cursor name instead of after it. As usual, consult your DBMS documentation for details. The DECLARE CURSOR statement only de nes the cursor no data is selected until the cursor is subsequently opened and used. This is very much
Integrating SQL into Applications
like de ning a le in a traditional programming language nothing really happens until the le is opened and used. The cursor_name must be unique within the program and will be used to reference the declared cursor in subsequent statements. The SQL statement included in the cursor declaration can contain most SELECT statement clauses. However, the INTO clause may not be included here because it is used in the FETCH statement as shown later in this section. The FOR UPDATE clause is optional, but some SQL implementations require it if subsequent SQL statements update and/or delete data selected by the cursor. As usual, consult your RDBMS documentation for speci cs. Here is an example of a DECLARE CURSOR statement written in Oracle syntax. It selects movie rentals that are currently overdue.
DECLARE CURSOR overdue_rentals IS SELECT MOVIE_ID, COPY_NUMBER, TRANSACTION_ID, DUE_DATE FROM MOVIE_RENTAL WHERE DUE_DATE < CURRENT_DATE FOR UPDATE OF DUE_DATE;
You can tell that this is an Oracle example because of the syntax and the use of CURRENT_DATE, which is an Oracle function that returns the current date and time. For other SQL implementations, substitute the expression that yields the current date and time in the RDBMS and adjust other syntax as required. For example, for Microsoft SQL Server, the statement would be
DECLARE overdue_rentals CURSOR FOR SELECT MOVIE_ID, COPY_NUMBER, TRANSACTION_ID, DUE_DATE FROM MOVIE_RENTAL WHERE DUE_DATE < getdate() FOR UPDATE OF DUE_DATE;
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