Tài liệu Processing queries by using explicit cursors doc

Return to Controlling Explicit Cursors Open the Cursor Pointer Ea " = Cursor Fetch a Row from the Cursor... Controlling Explicit Cursors continued PARR Reem ee Rete Hae e OEE RARE

* Declare and explain explicit cursors to fetch rows

from the database

* Create an explicit cursor with parameters

¢ Write cursor FOR loops

24-2 Introduction to Oracle: SQL and PL/SQL Using Procedure Builder

Objectives

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You may need to use a multiple row SELECT statement within PL/SQL to

process many rows To accomplish this, you declare and control explicit cursors, which are used in loops, including the cursor FOR loop

At the end of this lesson, you should be able to

e Explain the difference between implicit and explicit cursors

e Declare and use explicit cursors to fetch rows from the database

e Create an explicit cursor containing parameters

e Write cursor FOR loops

Processing Queries by Using Explicit Cursors 24-3

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What Is a Cursor?

* Every SQL statement executed by the Oracle7 Server

has an individual cursor associated with it

Explicit Cursor Functions

* Process beyond the first row returned by the query, row by row

se Keep track of which row is currently being processed

¢ Control cursors manually in the PL/SQL block

Overview

The Oracle7 Server uses work areas called “‘private SQL areas” to execute SQL

statements and store processing information PL/SQL cursors let you name a private SQL area and access its stored information The cursor directs all phases of

processing

Cursor Type Description

Implicit Declared by PL/SQL implicitly for all DML and PL/SQL

SELECT statements

Explicit Declared and named by the programmer and manipulated

through specific statements within the block’s executable actions

Recall that the SELECT statement in PL/SQL must only return a single row PL/SQL actually attempts to fetch two rows from an implicit cursor: one to satisfy the query, and a second to see if further rows were returned One method to eliminate this extra fetch is to use an explicit cursor

Explicit Cursor Functions

e Can process beyond the first row returned by the query, row by row

e Keep track of which row is currently being processed

e Allow the programmer to manually control them in the PL/SQL block

Processing Queries by Using Explicit Cursors 24-5

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Controlling Explicit Cursors

Open the Cursor

Pointer

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Fetch a Row from the Cursor

Controlling Explicit Cursors

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Now that you have a conceptual understanding of cursors, review the steps to use them The syntax for each step follows on the next pages

Controlling Explicit Cursors Using Four Commands

1 Declare the cursor

Declare the cursor by naming it and defining the structure of the query to be performed within it

2 Open the cursor

The OPEN statement executes the query and binds any variables that are

referenced Rows identified by the query are called the active set and are now available for fetching

3 Fetch data from the cursor

The FETCH statement loads the current row from the cursor into variables Each fetch causes the cursor to move its pointer to the next row in the active set

Therefore, each fetch will access a different row returned by the query

In the flow diagram on the left page, each fetch tests the cursor for any existing rows If rows are found, it loads the current row into variables, else it closes the

cursor

4 Close the cursor

The CLOSE statement releases the active set of rows It is now possible to reopen

the cursor to establish a fresh active set

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Declaring the Cursor: Syntax

'* Define parameters to allow substitution of values

into the cursor query

DECLARE CURSOR cursor_name IS

Declaring the Cursor: Example

Retrieve the line items of an order one by one

DECLARE

v_ord id s_item.ord id%TYPE;

v_product_id s_item.product_id%TYPE;

v item total NUMBER (11,2);

CURSOR item cursor IS

SELECT product_id, price*quantity

FROM s_item WHERE ord_id = v_ord_ id;

Controlling Explicit Cursors continued

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Declaring the Cursor

Use the CURSOR statement to declare an explicit cursor You can define parameters

to allow substitution of values into the query when the cursor is opened You can also reference variables within the query, but you must declare them before the cursor

where: cursor_name is a PL/SQL identifier

Select_statement is a SELECT statement without an INTO

clause

Note: Do not include the INTO clause within the cursor declaration because it

appears later within the FETCH statement

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Opening the Cursor: Syntax

¢ Open the cursor to execute the query and © identify the active set

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24-10 Introduction to Oracle: SQL and PL/SQL Using Procedure Builder

Controlling Explicit Cursors continued

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Opening the Cursor

Open the cursor to execute the query and identify the active set after specifying

values for all input variables The cursor will now point to the first row in the active set

an exception However, you can test the cursor’s status after a fetch

Processing Queries by Using Explicit Cursors 24-11

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Fetching Data from the Cursor: Syntax

* Retrieve the current row values into output variables

* Include the same number of variables

¢ Match each variable to correspond to the columns

Fetching Data from the Cursor: Example

Retrieve the line items of an order one-by-one

Controlling Explicit Cursors continued

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Fetching Data from the Cursor

Use the FETCH statement to retrieve the current row values into output variables After the fetch, you can manipulate the variables by further statements

where: cursor name is the name of the previously declared cursor

variable is an output variable to store the results

Guidelines

e Include the same number of variables within the INTO clause of the FETCH statement as output columns in the SELECT statement, and be sure that the

datatypes are compatible

e Match each variable to correspond to the columns positionally

e Alternatively, define a record for the cursor and reference the record in the

FETCH INTO clause

e Test to see if the cursor contains rows If a fetch acquires no values, that is, there are now rows left to process in the active set and no error is recorded

Processing Queries by Using Explicit Cursors 24-13

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Closing the Cursor: Syntax

* Close the cursor after completing the processing

of the rows

CLOSE cursor name;

* Reopen the cursor again, if required

* Do not attempt to fetch data from a cursor once

it has been closed

24-14 Introduction to Oracle: SQL and PL/SQL Using Procedure Builder

Controlling Explicit Cursors continued

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Closing the Cursor

Close the cursor after completing the processing of the SELECT statement This step allows the cursor to be reopened, if required Therefore, you can establish an active set several times

where: cursor_name is the name of the previously declared cursor

Note: Do not attempt to fetch data from a cursor once it has been closed or the

INVALID_CURSOR exception will be raised

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Processing Queries by Using Explicit Cursors 24-15

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Explicit Cursor Attributes

Obtain status information about a cursor by using

Attribute Type Description

%ISOPEN Boolean Evaluates to TRUE if the cursor is

open

%NOTFOUND Boolean Evaluates to TRUE if the most

recent fetch does not return a row

%FOUND Boolean Evaluates to TRUE until the

most recent fetch does not return a row

rows returned so far

Controlling Multiple Fetches

¢ Process several rows from an explicit cursor using a loop

¢ Fetch arow with each iteration

¢ Write atest for an unsuccessful fetch by using the %NOTFOUND attribute

¢ Test success of each fetch using explicit cursor attributes

24-16 Introduction to Oracle: SQL and PL/SQL Using Procedure Builder

Explicit Cursor Attributes

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As with implicit cursors, there are four attributes for obtaining status information about a cursor When used, the attribute name is preceded by the cursor identifier

Cursor Attribute Description

%ISOPEN Boolean attribute that evaluates to TRUE if the cursor is

open

%NOTFOUND Boolean attribute that evaluates to TRUE if the most recent

fetch does not return a row

%FOUND Boolean attribute that evaluates to TRUE until the most

recent fetch does not return a row; complement of

%NOTFOUND

%ROWCOUNT Numeric attribute that evaluates to the total number of rows

retumed so far

Note: Do not reference cursor attributes directly within a SQL statement

Controlling Multiple Fetches from Explicit Cursors

Typically, when you want to process several rows from an explicit cursor, you define

a loop to perform a fetch on each iteration Eventually, all rows in the active set will

be processed, and an unsuccessful fetch sets the %7 NOTFOUND attribute to TRUE Use the explicit cursor attributes to test success of each fetch before any further

references are made to the cursor If you omit an exit criteria, an infinite loop will result

\ For more information, see

iG PL/SQL User’s Guide and Reference, Release 2.3 “Using Cursor Attributes” section

Processing Queries by Using Explicit Cursors 24-17

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The %ISOPEN Attribute: Example

¢ Fetch rows only when the cursor is open

* Test if the cursor is open using %ISOPEN cursor

attribute before performing a fetch

IF item_cursortISOPEN THEN FETCH item_cursor INTO v_quantity, v_price;

The %NOTFOUND and %ROWCOUNT

Attributes: Example

¢ Retrieve an exact number of rows using the

%ROWCOUNT cursor attribute

* Determine when to exit the loop using the

%NOTFOUND cursor attribute

LOOP

FETCH item_cursor INTO v_product_id, v_item_total;

EXIT WHEN item_cursortROWCOUNT > 5

Explicit Cursor Attributes continued

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You can only fetch rows when the cursor is open Determine whether the cursor is open using the %ISOPEN cursor attribute, if necessary

Fetch rows in a loop Determine when to exit the loop by using cursor attributes

To retrieve an exact number of rows, fetch the rows in a numeric FOR loop, or fetch the rows in a simple loop and determine when to exit the loop by using the

%ROWCOUNT cursor attribute

CURSOR item cursor IS

SELECT productid, price * quantity

FETCH item cursor INTO v_product_id, v_item total;

EXIT WHEN item_cursor%ROWCOUNT > 5

Note: If using Z7ROWCOUNT, add a test for no rows in the cursor by using the

%NOTFOUND attribute because the rowcount is not incremented if the fetch does not retrieve any rows

Processing Queries by Using Explicit Cursors 24-19

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Cursors and Records: Example

Process the rows of the active set conveniently by

fetching values into a PL/SQL RECORD

CURSOR emp cursor IS

FROM 3 emp WHERE dept_id = 41;

emp record eømp Cursor$ROWTYPE;

BEGIN

OPEN emp cursor;

FETCH emp cursor INTO emp record;

24-20 Introduction to Oracle: SQL and PL/SQL Using Procedure Builder