barcode generator in vb.net codeproject Using SQL Cursors in Software

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Using SQL Cursors
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Figure 15-2
Embedding cursor-related SQL statements
NOTE
For most application programming languages, an embedded SQL statement is preceded by EXEC SQL. This signals to a preprocessor that the following statement is SQL and must be processed separately from the host language. The preprocessor, provided by the RDBMS vendor, analyzes the SQL code and converts it into a form that can be used by the SQL implementation. The host language is compiled in the normal way. For more information about embedded SQL, see 17.
Declare a Cursor
The first statement that we ll look at is the DECLARE CURSOR statement. The cursor must be declared before you can use the cursor to retrieve data. You can declare a cursor at any point in your application code, as long as it s declared before the cursor is referenced by any other statements.
SQL: A Beginner s Guide
NOTE
Many programmers prefer to declare all cursors and variables at the beginning of the program so that all declarations are kept together. The cursors and variables can then be referenced at any point in the program.
The syntax for a cursor declaration includes many elements, as shown in the following syntax: DECLARE <cursor name> [ SENSITIVE | INSENSITIVE | ASENSITIVE ] [ SCROLL | NO SCROLL ] CURSOR [ WITH HOLD | WITHOUT HOLD ] [ WITH RETURN | WITHOUT RETURN ] FOR <query expression> [ ORDER BY <sort specification> ] [ FOR { READ ONLY | UPDATE [ OF <column list> ] } ]
NOTE
Oracle uses the keyword IS instead of FOR preceding the query expression in the cursor declaration.
As you can see, most of the elements that make up the declaration are optional. And as always, you need to check the documentation for your SQL implementation to see which ones are supported. We ll look at these elements in greater detail in the following section. For now, let s focus on those elements that are required. To do so, we can synthesize the syntax down to the following basic elements: DECLARE <cursor name> CURSOR FOR <query expression> This syntax shows only those parts of the cursor declaration that are mandatory. As you can see, this is a much more manageable chunk of code. All you re required to provide is a name for the cursor and the query expression that is invoked when the cursor is opened. The name must be different from the name of any other cursor declared within the same program. The query expression is basically a SELECT statement, as you have seen throughout this book. That s all there is to the basic syntax. In the following section, we ll take a look at each of the optional elements that make up the cursor declaration. After that, we ll look at some examples.
Working with Optional Syntax Elements
If you refer back to the full syntax for a cursor declaration (shown in the previous section), you ll see that the majority of the elements are optional. In this section, we ll look at each of these elements. Later in the chapter, after we ve completed this discussion, you might find that you ll want to refer back to this section for details about specific options.
15:
Cursor Sensitivity
Using SQL Cursors
The first optional element of the DECLARE CURSOR statement that we ll look at is cursor sensitivity, which is represented with the following syntax: [ SENSITIVE | INSENSITIVE | ASENSITIVE ] Cursor sensitivity is concerned with statements outside the cursor that affect the same rows as those returned by the cursor. For example, suppose your cursor returns rows from the CDS_ IN_STOCK table. While the cursor is open, another statement within the same transaction deletes some of the same rows in the CDS_IN_STOCK table that were returned by the cursor. Whether or not the cursor can see these deletions depends on the cursor sensitivity. As you can see in the syntax, SQL supports three cursor sensitivity options:
SENSITIVE Significant changes made by statements outside the cursor immediately affect the query results within the cursor. INSENSITIVE Significant changes made by statements outside the cursor do not affect the query results within the cursor. ASENSITIVE Cursor sensitivity is implementation-defined. Significant changes may or may not be visible within the cursor.
If no cursor sensitivity option is specified, ASENSITIVE is assumed, in which case the SQL implementation can take whatever action it has been designed to take.
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