barcode generator in vb.net codeproject Using the Positioned DELETE Statement in Software

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Using the Positioned DELETE Statement
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The positioned DELETE statement, like the positioned UPDATE statement, requires a WHERE clause that must include the CURRENT OF option. (A regular DELETE statement, as you ll recall, does not require the WHERE clause.) A positioned DELETE statement uses the following syntax: DELETE <table name> WHERE CURRENT OF <cursor name> As you can see, you need to define a DELETE clause that identifies the table and a WHERE clause that identifies the cursor. The WHERE clause in a positioned DELETE statement works just like the WHERE clause in a positioned UPDATE statement: The row returned by the last FETCH statement is the row that is modified. In this case, the row is deleted. Now let s look at an example of a positioned DELETE statement. The following SQL statements declare the CD_4 cursor, open the cursor, return a row from the cursor, delete that row, and close the cursor:
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DECLARE CD_4 CURSOR FOR SELECT *
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Using SQL Cursors
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FROM CD_INVENTORY FOR UPDATE; OPEN CD_4; FETCH CD_4 INTO :CD, :Category, :Price, :On_Hand; DELETE CD_INVENTORY WHERE CURRENT OF CD_4; CLOSE CD_4;
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You should be familiar with most of these statements. The only new one is the positioned DELETE statement. This statement deletes the row returned by the FETCH statement, which is the Famous Blue Raincoat row. Once the row is deleted, the cursor is closed using a CLOSE statement. As stated previously, it is always a good idea to explicitly close cursors when they are no longer needed.
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Working with SQL Cursors
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In this chapter, we looked at how to declare cursors, open those cursors, retrieve data from them, and then close them. In addition, we reviewed positioned UPDATE and DELETE statements. However, as I said earlier, cursors are used primarily in embedded SQL, which makes it difficult to fully test cursor functionality if you re limited to directly invoking SQL statements (as we are in this Try This exercise). Ideally, it would be best to embed the cursor-related SQL statements in a host language, but that is beyond the scope of this book. What complicates this issue even further is the fact that different SQL implementations support the use of cursors in an interactive environment in different ways, which can make it difficult to directly invoke cursor-related statements. Still, you should be able to execute most cursor-related statements interactively, but know that cursors are designed for use in embedded SQL and SQL client modules, so you might have to modify the statements a great deal in order to execute them. You can download the Try_This_15.txt file, which contains the SQL statements used in this exercise.
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NOTE
Ideally, it would be good to walk you through each step of declaring and opening a cursor, retrieving data, and closing a cursor, but because of the nature of direct invocation, we will use fewer steps and larger blocks of statements.
Step by Step
1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. The first cursor that you ll declare and access is a basic read-only cursor that retrieves data
from the COMPACT_DISCS table. The first thing you ll notice in the set of statements
(continued)
SQL: A Beginner s Guide
you ll be creating is that you ll declare a variable named v_CD_NAME. You ll need to create this variable in order to fully test the FETCH statement. Keep in mind that, depending on the situation, the host language, and the product, you may or may not use this method for defining your variable. Also notice that the variable name in the FETCH statement is not preceded by a colon. This is because you ll be using direct invocation to execute these statements and, for most implementations, the name of the variable in the FETCH statement will have to be the same as the name you declared at the beginning of this set of statements. As with any SQL statement, you will find that the exact language you use to create statements varies from one product to the next. In addition, the fact that you re invoking the statements directly, rather than embedding the statements, can lead to other variations between SQL and the implementation (such as not using a colon in the variable name). For example, if you execute these statements in SQL Server, you ll have to precede your variable names with the at (@) character. Oracle deviates from the standard even more. In Oracle, you declare the cursor and variable in one block of statements. In addition, the CURSOR keyword precedes the name of the cursor, and you must use the IS keyword, rather than FOR. You must also enclose the OPEN, FETCH, and CLOSE statements in a BEGIN...END block. You will also find that not all SQL options are supported in all SQL implementations, and many products include additional features not defined in the SQL standard. Be sure to check your product s documentation before trying to declare and access any cursors. Now let s create the cursor-related statements. Enter and execute the following SQL statements:
DECLARE v_CD_NAME VARCHAR (60); DECLARE CD_cursor_1 CURSOR FOR SELECT CD_TITLE FROM COMPACT_DISCS ORDER BY CD_TITLE ASC; OPEN CD_cursor_1; FETCH CD_cursor_1 INTO v_CD_NAME; CLOSE CD_cursor_1;
In these statements, you first declared a variable named v_CD_NAME. Next, you declared a cursor named CD_cursor_1. The cursor definition contained a SELECT statement that was qualified with an ORDER BY clause. Because you included the ORDER BY clause, your cursor was read-only. After you declared the cursor, you opened it, fetched a row from the cursor s query results, and then closed the cursor. The FETCH statement returned the value After the Rain: The Soft Sounds of Erik Satie, which could have then been used in some other operation, had you embedded these statements. After you executed the statements, you should have received a message saying that the statements were executed successfully.
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