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

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Using SQL Cursors
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3. Now you will declare and access a second cursor. This time you will specify that the cursor
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is insensitive and scrollable. In addition, you will specify that the cursor is read-only, although this clause is optional because you re making the cursor scrollable and insensitive. You will also fetch the last row from the cursor s query results, rather than the first. Enter and execute the following SQL statements:
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DECLARE v_CD_NAME VARCHAR(60); DECLARE CD_cursor_2 SCROLL INSENSITIVE CURSOR FOR SELECT CD_TITLE FROM COMPACT_DISCS ORDER BY CD_TITLE ASC FOR READ ONLY; OPEN CD_cursor_2; FETCH LAST FROM CD_cursor_2 INTO v_CD_NAME; CLOSE CD_cursor_2;
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This time the FETCH statement retrieved the value That Christmas Feeling because LAST was specified. This value was inserted into the v_CD_NAME variable. After you executed the statements, you should have received a message saying that the statements were executed successfully.
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4. Your next cursor will be updatable, which means that it cannot include an ORDER BY
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clause and cannot be defined as insensitive or scrollable. Because the cursor is updatable, you will also create an UPDATE statement that doubles the value of the IN_STOCK column for the row returned by the FETCH statement. Enter and execute the following SQL statements:
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DECLARE v_CD_NAME VARCHAR(60); DECLARE CD_cursor_3 CURSOR FOR SELECT CD_TITLE FROM COMPACT_DISCS FOR UPDATE; OPEN CD_cursor_3; FETCH CD_cursor_3 INTO v_CD_NAME; UPDATE COMPACT_DISCS SET IN_STOCK = IN_STOCK * 2 WHERE CURRENT OF CD_cursor_3; CLOSE CD_cursor_3;
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Notice that your UPDATE statement includes a WHERE clause that contains the CURRENT OF option, which specifies the CD_cursor_3 cursor. This clause is mandatory. Because no ORDER BY clause was used, the first row in your cursor s query results was Famous Blue Raincoat. This is the row that was updated. After you executed the statements, you should have received a message indicating that a row had been updated.
5. Now let s take a look at the COMPACT_DISCS table to verify that the change you made is
correct. Enter and execute the following SQL statement:
SELECT * FROM COMPACT_DISCS;
The IN_STOCK value of the Famous Blue Raincoat row should now be 26, double its original amount.
6. Let s return the database to its original state. Enter and execute the following SQL
statement:
UPDATE COMPACT_DISCS SET IN_STOCK = 13 WHERE COMPACT_DISC_ID = 101;
You should receive a message indicating that the row has been updated.
7. Close the client application.
Try This Summary
In this Try This exercise, you declared and accessed three cursors, two that were read-only and one that was updatable. For all three cursors you declared a variable. The variable was then used in the FETCH statement to receive the value returned by that statement. For the updatable cursor, you created an UPDATE statement that modified the IN_STOCK value for the row returned by the FETCH statement. After you updated the COMPACT_DISCS table, you updated it once more to return the database to its original state. Because no other changes were made to the database, your data should be as it was before you started this exercise.
15 Self Test
1. What is a cursor 2. Which invocation methods support the use of cursors 3. What form of impedance mismatch is addressed through the use of cursors 4. A(n) ____________ serves as a pointer that allows the application programming language
to deal with query results one row at a time.
15:
Using SQL Cursors
5. When using cursors in embedded SQL, what is the first step you must take before you can
retrieve data through that cursor
A Fetch the cursor. B Declare the cursor. C Close the cursor. D Open the cursor. 6. What are the four cursor-related statements that you can embed in a host language 7. Which options can be used only in read-only cursor declarations A SCROLL B WITH HOLD C ORDER BY D INSENSITIVE 8. What are the required elements of a DECLARE CURSOR statement 9. What cursor type does not see changes made by statements outside the cursor 10. Which option should you use in a cursor declaration to extend the retrieving capabilities of
a FETCH statement
A WITHOUT HOLD B ASENSITIVE C SCROLL D FOR UPDATE 11. Cursor ____________ refers to a characteristic in cursors that is concerned with whether
a cursor is automatically closed when the transaction in which the cursor was opened is committed.
12. You re creating a cursor declaration. The SELECT statement includes an ORDER BY
clause. Which clauses cannot be included in the SELECT statement
A SELECT B HAVING C GROUP BY D WHERE 13. What option should you include in a cursor declaration to define that cursor as holdable
SQL: A Beginner s Guide
14. Your cursor declaration includes a FOR UPDATE clause that does not specify any columns.
Which columns in the underlying table can be updated
15. What SQL statement should you use if you want to open the CD_ARTISTS cursor 16. Which SQL statement executes the SELECT statement in a cursor 17. A(n) ____________ statement retrieves rows from a cursor s query results once you open
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