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Using SQL Cursors
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Now the FOR UPDATE clause includes the OF keyword and the column name, COMPACT_DISC. If you were to try to modify data in the cursor results in columns other than the COMPACT_DISC column, you would receive an error. Once you ve declared your cursor, you can open it and retrieve data from the query results. However, as you have seen in the preceding cursor declarations, the actions that you can take are limited to the restrictions defined with the DECLARE CURSOR statement.
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Open and Close a Cursor
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The process of opening a cursor is very straightforward. You need to provide only the keyword OPEN and the name of the cursor, as shown in the following syntax: OPEN <cursor name> For example, to open the CD_1 cursor, you invoke the following SQL statement:
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OPEN CD_1;
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You cannot open a cursor until you have declared it. Once you ve declared it, you can open it anywhere within your program. The SELECT statement within the cursor is not invoked until you actually open the cursor. That means that any data modifications made between the time the cursor is declared and the time the cursor is opened are reflected in the query results returned by the cursor. If you close the cursor and then reopen it, data modifications that took place between the time you close it and the time you reopen it are reflected in the new query results. Once you have finished using your cursor, you should close it so that you can free up system resources. To close a cursor, you can use the CLOSE statement, as shown in the following syntax: CLOSE <cursor name> The CLOSE statement does nothing more than close the cursor, which means that the query results from the cursor s SELECT statement are released. For example, to close the CD_1 cursor, use the following SQL statement:
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CLOSE CD_1;
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Once you close the cursor, you cannot retrieve any more rows from the cursor s query results. In other words, you cannot use a FETCH statement to retrieve data from a closed cursor. If you reopen the cursor, you can again retrieve data, but you will get a new result set and (assuming no scrolling options) will start with the first row in the query results, which can mean retrieving rows that have already been processed by a prior invocation of the cursor.
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Retrieve Data from a Cursor
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So far, you ve learned how to declare a cursor, open it, and then close it. However, these actions alone do not allow you to retrieve any of the data that is provided by the cursor. In order to do that, you must use a FETCH statement.
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SQL: A Beginner s Guide
Before we take a look at the syntax for the FETCH statement, let s briefly review the purpose of a cursor and its related statements. As I said earlier, one of the problems with embedding SQL statements in a programming host language is the impedance mismatch. One form of that mismatch is that SQL returns data in sets and traditional application programming languages cannot handle sets of data. In general, they can deal only with individual values. In order to address this form of impedance mismatch, you can use cursors to retrieve data one row at a time regardless of how many rows are returned from which you can extract individual values that can be used by the host language. As you have seen, a cursor declaration includes a SELECT statement that returns a set of data. The OPEN statement executes the SELECT statement, and the CLOSE statement releases the query results from the SELECT statement. However, it is the FETCH statement that identifies individual rows within that set of data and extracts individual values from those rows, which are then passed to host variables. A host variable is a type of parameter that passes a value to the host language. One or more FETCH statements can be executed while a cursor is open. Each statement points to a specific row in the query results, and values are then extracted from those rows. The following syntax shows the basic elements that make up the FETCH statement: FETCH [ [ <fetch orientation> ] FROM ] <cursor name> INTO <host variables> As you can see by the syntax, you must specify the FETCH keyword, the name of the cursor, and an INTO clause that identifies the host variables that will receive the values returned by the FETCH statement. These values are derived from the query results that are generated by the cursor s SELECT statement when that cursor is opened. If your FETCH statement includes more than one host variable, you must separate the variables with commas. In addition to the mandatory components of the FETCH statement, the syntax also includes the optional <fetch orientation> placeholder and the FROM keyword. If you specify a fetch orientation option in your FETCH statement, you must include the FROM keyword, or you can specify FROM without the fetch orientation. SQL supports six fetch orientation options that identify which row is selected from the cursor s query results. Most of these options are available only if you declare the cursor as scrollable. A scrollable cursor, as you ll recall, is one that extends the ability of the FETCH statement to move through the cursor s query results. A cursor is scrollable if the cursor declaration includes the SCROLL keyword. If you include a fetch orientation in your FETCH statement, you can choose from one of the following options:
NEXT Retrieves the next row from the query results. If you use NEXT in your first FETCH statement after you open your cursor, the first row in the query results will be returned. A second FETCH NEXT statement will return the second row. PRIOR Retrieves the row directly preceding the one that had last been retrieved. If you use PRIOR in your first FETCH statement after you open the cursor, no row will be returned because no row precedes the first row.
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