barcode generator in vb.net codeproject Creating a Cursor Declaration in Software

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Creating a Cursor Declaration
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Now that we ve looked at each component of the DECLARE CURSOR statement, let s take a look at a few examples that help illustrate how to declare a cursor. For these examples, we ll use the CD_INVENTORY table, shown in Figure 15-3.
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CATEGORY: VARCHAR (15) Vocal Vocal Vocal Instrumental Instrumental Vocal Vocal Vocal Instrumental Instrumental Vocal Vocal Vocal Instrumental
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Figure 15-3 Declaring cursors on the CD_INVENTORY table
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Using SQL Cursors
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The first example that we ll review is a basic cursor declaration that includes only the required elements plus an ORDER BY clause, as shown in the following DECLARE CURSOR statement:
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DECLARE CD_1 CURSOR FOR SELECT * FROM CD_INVENTORY ORDER BY COMPACT_DISC;
In this statement, I ve declared a cursor named CD_1 and defined a SELECT statement. The cursor name follows the DECLARE keyword. After the cursor name, I ve included the CURSOR keyword and the FOR keyword. The only additional element is the SELECT statement, which includes an ORDER BY clause. The statement returns all rows and columns from the CD_INVENTORY table. The rows are then ordered according to the values in the COMPACT_DISC column. Because I did not specify the ASC or DESC keyword, the rows are returned in ascending order.
NOTE
In 7, when discussing the SELECT statement, I explain that, although an asterisk can be used to return all columns from a table, it is a better practice to identify each column that you want returned. This is especially important in embedded SQL because the host language relies on certain values a specified number in a specified order being returned from the database. If the database should change, your application may not operate properly, and the application code would have to be modified. For the examples in this chapter, I often use an asterisk to simplify the code and conserve space, but know that, in the real world, I would usually specify each column.
The ORDER BY clause is an important element because the order in which the rows are returned affects which rows are retrieved when using a FETCH statement. (I discuss the FETCH statement later in the chapter, in the Retrieve Data from a Cursor section.) This is especially true if defining a scrollable cursor, such as the one in the following example:
DECLARE CD_2 SCROLL CURSOR FOR SELECT * FROM CD_INVENTORY ORDER BY COMPACT_DISC FOR READ ONLY;
Notice that I ve added two new elements to this statement: the SCROLL keyword and the FOR READ ONLY clause. The SCROLL keyword signals to the FETCH statement that the cursor is scrollable. As a result, additional options can be used within the FETCH statement that extend how your application can move through the cursor results. The FOR READ ONLY clause indicates that neither an UPDATE nor a DELETE statement can be used to modify data returned by the cursor. However, this clause is not necessary. Because the cursor
SQL: A Beginner s Guide
declaration includes the SCROLL keyword and the SELECT statement includes an ORDER BY clause, the cursor is automatically limited to read-only operations. The use of either of these two options or the use of the INSENSITIVE option automatically overrides the cursor s default updatability. The next type of read-only declaration that we ll look at also includes the INSENSITIVE keyword, as shown in the following example:
DECLARE CD_3 SCROLL INSENSITIVE CURSOR FOR SELECT * FROM CD_INVENTORY ORDER BY COMPACT_DISC FOR READ ONLY;
The CD_3 cursor declaration is exactly like the CD_2 cursor declaration except that CD_3 has also been defined as an insensitive cursor. This means that no modifications made to the data in the underlying table while the cursor is open will be reflected in the query results returned by the cursor. Of course, if you close the cursor and then reopen it, any modifications that had been made when the cursor was originally open will be reflected in the data returned by the reopened cursor. The three preceding cursor declarations that we ve looked at have all been read-only. Now let s take a look at an updatable cursor. In the following cursor declaration, the SELECT statement again returns all rows and columns from the CD_INVENTORY table:
DECLARE CD_4 CURSOR FOR SELECT * FROM CD_INVENTORY FOR UPDATE;
Notice that this DECLARE CURSOR statement does not include the SCROLL keyword, the INSENSITIVE keyword, or an ORDER BY clause, any of which would have prevented us from creating an updatable cursor. We could have specified the NO SCROLL and SENSITIVE options, but they re not necessary. Also notice, however, that the cursor declaration does include the FOR UPDATE clause. The clause is also not necessary in this particular statement because the cursor is, by default, updatable, since it contains no options to limit the updatability. However, if you want your cursor to be updatable only for a certain column, you must include the FOR UPDATE clause, along with the column name, as shown in the following example:
DECLARE CD_5 CURSOR FOR SELECT * FROM CD_INVENTORY FOR UPDATE OF COMPACT_DISC;
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