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mismatch exists between SQL and the programming languages. Impedance mismatch refers to differences between SQL and other programming languages. As you might recall from 3, one example of impedance mismatch is the way in which SQL data types differ from data types in other programming languages. These differences can lead to the loss of information when an application extracts data from an SQL database. Another example of impedance mismatch is the fact that SQL returns data in sets but other programming languages cannot handle sets. Generally, they can process only a few pieces of data (a single record) at the same time. The way in which SQL deals with this type of impedance mismatch is through the use of cursors. A cursor serves as a pointer that allows the application programming language to deal with query results one row at a time, much like the way these programming languages handle records from traditional (flat) data files. Although the cursor can traverse all the rows of a query result, it focuses on only one row at a time. A cursor still returns a full result set, but allows the programming language to call only one row from that set. For example, suppose your query results are derived from the following SELECT statement:
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SELECT PERFORMER_NAME, PLACE_OF_BIRTH FROM PERFORMERS;
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The query results from this statement will return all rows from the PERFORMERS table, which includes the PERFORMER_NAME column and the PLACE_OF_BIRTH column. However, your application programming language can deal with only one row at a time, so the cursor is declared as an embedded SQL statement within the application programming language. The cursor is then opened, much like the way these application languages open files, and a row is retrieved from the query results. Figure 15-1 illustrates how a cursor acts as a pointer to retrieve only one row of data. In this case, the row that is retrieved through the cursor is the Bing Crosby row. However, you can retrieve any row from the query results, and you can continue to retrieve rows, as long as they re retrieved one at a time and the cursor remains open. Once you close the cursor, you cannot retrieve any more rows from the query results.
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Declaring and Opening SQL Cursors
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Most application programming languages support the use of cursors to retrieve data from an SQL database. The cursor language is embedded in the programming code in much the same way you would embed any SQL statement. When using a cursor in a programming language, you must first declare the cursor similar to how you would declare a variable and then use the declaration name (the name you ve assigned to the cursor) in other embedded SQL statements to open the cursor, retrieve individual rows through the cursor, and close the cursor.
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You can also use cursors in SQL client modules, which are sets of SQL statements that can be called from within an application programming language. Client modules, along with embedded SQL and interactive SQL, provide one more method to invoke SQL statements. Because client modules are not implemented as widely as embedded SQL, I focus on using cursors in embedded SQL. For more information about SQL client modules, see 17.
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PERFORMER_NAME: PLACE_OF_BIRTH: VARCHAR (60) VARCHAR (60) Jennifer Warnes Joni Mitchell William Acherman Kitaro Cursor Bing Crosby Patsy Cline Jose Carreras Luciano Pavarotti Placido Domingo Seattle, Washington, USA Fort MacLeod, Alberta, Canada Germany Toyohashi, Japan Tacoma, Washington, United States Winchester, Virginia, United States Barcelona, Spain Modena, Italy Madrid, Spain
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Figure 15-1
Using a cursor to access the PERFORMERS table
Although declaring a cursor is pivotal in using that cursor in your application, the declaration alone is not enough to extract data from an SQL database. In fact, full cursor functionality is supported through the use of four SQL statements, each of which are embedded in the application programming language, or host language. The following descriptions provide an overview of these four statements:
DECLARE CURSOR Declares the SQL cursor by defining the cursor name, the cursor s characteristics, and a query expression that is invoked when the cursor is opened. OPEN Opens the cursor and invokes the query expression, making the query results available to FETCH statements. FETCH Retrieves data into variables that pass the data to the host programming language or to other embedded SQL statements. CLOSE Closes the cursor. Once the cursor is closed, data cannot be retrieved from the cursor s query results.
The four statements are called from within the host language. Figure 15-2 illustrates how the cursor-related statements are used. The embedded SQL statements are shown in the boxes that are shaded gray. As you can see, you must first declare the cursor, and then you open it. Once you ve opened the cursor, you can use the FETCH statement to retrieve rows of data. You can use this statement as many times as necessary, usually within some sort of looping structure defined by the host language. Once you ve retrieved the necessary data, you should close the cursor.
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