Tài liệu Using Cursors pptx

A cursor allows you to step through the rows returned by a particular SELECT statement.. Declare variables to store the column values from the SELECT statement.. Declare the cursor, spec

Using Cursors

When you execute a SELECT statement, all the rows are returned in one go This might not always be appropriate For example, you might want to take some action based on the

column values retrieved for a particular row To do this, you can use a cursor to process

rows retrieved from the database one row at a time A cursor allows you to step through the rows returned by a particular SELECT statement

You follow these steps when using a cursor:

1 Declare variables to store the column values from the SELECT statement

2 Declare the cursor, specifying your SELECT statement

3 Open your cursor

4 Fetch the rows from your cursor

5 Close your cursor

You'll learn the details of these steps in the following sections

Step 1: Declare Variables to Store the Column Values from the SELECT Statement

These variables must be compatible with the column types for the retrieved rows For example, you'll want to use an int variable to store the value from an int column, and so

on

The following example declares three variables to store the ProductID, ProductName, and UnitPrice columns from the Products table:

DECLARE @MyProductID int

DECLARE @MyProductName nvarchar(40)

DECLARE @MyUnitPrice money

Step 2: Declare the Cursor

A cursor declaration consists of a name that you assign to the cursor and the SELECT statement that you want to execute This SELECT statement is not actually run until you open the cursor You declare your cursor using the DECLARE statement

The following example declares a cursor named ProductCursor with a SELECT

statement that retrieves the ProductID, ProductName, and UnitPrice columns for the first

10 products from the Products table:

DECLARE ProductCursor CURSOR FOR

SELECT ProductID, ProductName, UnitPrice

FROM Products

WHERE ProductID <= 10

Step 3: Open the Cursor

Now it's time to open your cursor, which runs the SELECT statement previously defined

in the DECLARE statement You open a cursor using the OPEN statement The

following example opens ProductCursor, and therefore also runs the SELECT statement that retrieves the rows from the Products table:

OPEN ProductCursor

Step 4: Fetch the Rows from the Cursor

Now you must read each row from your cursor To do this, you use the FETCH

statement Your cursor may contain many rows, and therefore a WHILE loop is required

is to read each row in turn To determine when the loop is to end, you can use the

@@FETCH_STATUS function This function returns one of the possible values shown

in Table 4.1

Table 4.1: RETURN VALUES FROM THE @@FETCH_STATUS FUNCTION

VALUE DESCRIPTION

0 FETCH statement successfully returned a row

-1 FETCH statement failed or the requested row was outside the result set

-2 Row fetched was missing

The following example shows a loop that reads each row from ProductCursor:

FETCH NEXT FROM ProductCursor

INTO @MyProductID, @MyProductname, @MyUnitPrice

PRINT '@MyProductID = ' + CONVERT(nvarchar, @MyProductID)

PRINT '@MyProductName = ' + CONVERT(nvarchar, @MyProductName)

PRINT '@MyUnitPrice = ' + CONVERT(nvarchar, @MyUnitPrice)

WHILE @@FETCH_STATUS = 0

BEGIN

FETCH NEXT FROM ProductCursor

INTO @MyProductID, @MyProductname, @MyUnitPrice

PRINT '@MyProductID = ' + CONVERT(nvarchar, @MyProductID)

PRINT '@MyProductName = ' + CONVERT(nvarchar, @MyProductName)

PRINT '@MyUnitPrice = ' + CONVERT(nvarchar, @MyUnitPrice)

END

You'll notice that the condition @@FETCH_STATUS = 0 is used in the WHILE loop to check that the FETCH statement successfully returned a row When this condition is no longer true, the loop ends

Tip You can get the number of rows stored in a cursor using the @@CURSOR_ROWS

function You'll learn more about functions later in the "Using Functions" section

Step 5: Close the Cursor

Close your cursor using the CLOSE statement The following example closes

ProductCursor:

CLOSE ProductCursor

You should also remove the reference to your cursor using the DEALLOCATE

statement This frees the system resources used by your cursor The following example removes the reference to ProductCursor using the DEALLOCATE statement:

DEALLOCATE ProductCursor

The following section shows a complete example script that you may run using Query Analyzer This script contains all five steps for using a cursor

Complete Example: ProductCursor.sql

Listing 4.1 shows the ProductCursor.sql script You can load this file into Query

Analyzer and run it

Listing 4.1: USING CURSORS

/*

ProductCursor.sql uses a cursor to display

the ProductID, ProductName, and UnitPrice columns

from the Products table

*/

USE Northwind

step 1: declare the variables

DECLARE @MyProductID int

DECLARE @MyProductName nvarchar(40)

DECLARE @MyUnitPrice money

step 2: declare the cursor

DECLARE ProductCursor CURSOR FOR

SELECT ProductID, ProductName, UnitPrice

FROM Products

WHERE ProductID <= 10

step 3: open the cursor

OPEN ProductCursor

step 4: fetch the rows from the cursor

FETCH NEXT FROM ProductCursor

INTO @MyProductID, @MyProductname, @MyUnitPrice

PRINT '@MyProductID = ' + CONVERT(nvarchar, @MyProductID)

PRINT '@MyProductName = ' + CONVERT(nvarchar, @MyProductName) PRINT '@MyUnitPrice = ' + CONVERT(nvarchar, @MyUnitPrice)

WHILE @@FETCH_STATUS = 0

BEGIN

FETCH NEXT FROM ProductCursor

INTO @MyProductID, @MyProductName, @MyUnitPrice

PRINT '@MyProductID = ' + CONVERT(nvarchar, @MyProductID)

PRINT '@MyProductName = ' + CONVERT(nvarchar, @MyProductName) PRINT '@MyUnitPrice = ' + CONVERT(nvarchar, @MyUnitPrice)

END

step 5: close the cursor

CLOSE ProductCursor

DEALLOCATE ProductCursor

The output for the first two rows read by the cursor is as follows:

@MyProductID = 1

@MyProductName = Chai

@MyUnitPrice = 18.00

@MyProductID = 2

@MyProductName = Chang

@MyUnitPrice = 19.00

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