qr barcode generator vb.net The CLOSE Statement in Software

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The CLOSE Statement
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The CLOSE statement, shown in Figure 17-28, conceptually closes the table of query results created by the OPEN statement, ending access by the application program. Its single parameter is the name of the cursor associated with the query results, which must be
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17:
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FIGURE 17-27 Cursor positioning with OPEN, FETCH, and CLOSE
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OPEN FETCH FETCH FETCH FETCH (NOT FOUND) CLOSE Query Results (three rows) MFR ID PRODUCT ID DESCRIPTION
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a cursor previously opened by an OPEN statement. The CLOSE statement can be executed at any time after the cursor has been opened. In particular, it is not necessary to FETCH all rows of query results before closing the cursor, although this will usually be the case. All cursors are automatically closed at the end of a transaction. Once a cursor is closed, its query results are no longer available to the application program.
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Scroll Cursors
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The SQL1 standard specifies that a cursor can only move forward through the query results. For a number of years, most commercial SQL products also supported only this form of forward, sequential cursor motion. If a program wants to re-retrieve a row once the cursor has moved past it, the program must CLOSE the cursor and reOPEN it (causing the DBMS to perform the query again), and then FETCH through the rows until the desired row is reached. In the early 1990s, a few commercial SQL products extended the cursor concept with the concept of a scroll cursor. Unlike standard cursors, a scroll cursor provides random access to the rows of query results. The program specifies which row it wants to retrieve through an extension of the FETCH statement, shown in Figure 17-29: FETCH FIRST retrieves the first row of query results. FETCH LAST retrieves the last row of query results. FETCH PRIOR retrieves the row of query results that immediately precedes the current row of the cursor. FETCH NEXT retrieves the row of query results that immediately follows the current row of the cursor. This is the default behavior if no motion is specified and corresponds to the standard cursor motion. FETCH ABSOLUTE retrieves a specific row by its row number. FETCH RELATIVE moves the cursor forward or backward a specific number of rows relative to its current position.
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PART V
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FIGURE 17-28
The CLOSE statement syntax diagram
Part V:
Programming with SQL
FIGURE 17-29
An extended FETCH statement for scroll cursors
Scroll cursors can be especially useful in programs that allow a user to browse database contents. In response to the user s request to move forward or backward through the data a row or a screenful at a time, the program can simply fetch the required rows of the query results. However, scroll cursors are also a great deal harder for the DBMS to implement than a normal, unidirectional cursor. To support a scroll cursor, the DBMS must keep track of the previous query results that it provided for a program and of the order in which it supplied those rows of results. The DBMS must also ensure that no other concurrently executing transaction modifies any data that has become visible to a program through a scroll cursor, because the program can use the extended FETCH statement to re-retrieve the row, even after the cursor has moved past the row. If you use a scroll cursor, you should be aware that certain FETCH statements on a scroll cursor may have a very high overhead for some DBMS brands. If the DBMS brand normally carries out a query step-by-step as your program FETCHes its way down through the query results, your program may wait a much longer time than normal if you request a FETCH NEXT operation when the cursor is positioned at the first row of query results. It s best to understand the performance characteristics of your particular DBMS brand before writing programs that depend on scroll cursor functionality for production applications. Because of the usefulness of scroll cursors, and because a few DBMS vendors had begun to ship scroll cursor implementations that were slightly different from one another, the SQL standard was expanded to include support for scroll cursors. The Entry SQL level of the standard requires only the older-style, sequential forward cursor, but conformance at the Intermediate SQL or Full SQL levels requires full support for the scroll cursor syntax shown in Figure 17-29. The standard also specifies that if any motion other than FETCH NEXT (the default) is used on a cursor, its DECLARE CURSOR statement must explicitly identify it as a scroll cursor. Using the standard syntax, the cursor declaration in Figure 17-23 would appear as:
exec sql declare select from where order repcurs scroll cursor for name, quota, sales salesreps sales > quota by name;
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