Ch4 Cursors

8/6/2019 Ch4 Cursors

1/34

Cursors


2/34

SQL Cursor

A cursor is a private SQL work area. There are two types of cursors:

o Implicit cursorso Explicit cursors

The Oracle Server uses implicit cursors toparse and execute your SQL statements.

Explicit cursors are explicitly declared bythe programmer.


3/34

Cursors

Pointer to memory location on databaseservero DBMS uses to process a SQL query

Use to:o Retrieve and manipulate database data in

PL/SQL programs

Types:o Implicito Explicit


4/34

Cursor

Context Area

activeset

Numberofrows

processed

Parsedcommandstatement

Database ServerMemory


5/34

ImplicitCursors

Context areao Contains information about queryo Created by INSERT, UPDATE, DELETE, or

SELECT Active set

o Set of data rows that query retrieves

Implicit cursoro Pointer to context area


6/34

ImplicitCursors (continued)

Use to assign output of SELECT query toPL/SQL program variablesoWhen query will return only one record


7/34

ImplicitCursors (continued)

Useful to use %TYPE reference data typeo To declare variables used with implicit cursors

Error ORA-01422: exact fetch returns

more than requested number of rowso Implicit cursor query tried to retrieve multiple

records


8/34

SQL CursorAttributes

Using SQL cursor attributes, you cantest the outcome of your SQLstatements.SQL%ROWCOUNTNumberofrowsaffectedbythemost

recent SQL statement (anintegervalue)

SQL%FOUND BooleanattributethatevaluatestoTRUE ifthemostrecent SQL statementaffectsoneormorerowsSQL%NOTFOUND BooleanattributethatevaluatestoTRUEifthemostrecent SQL statementdoesnotaffectanyrowsSQL%ISOPEN Alwaysevaluatesto FALSE becausePL/SQLclosesimplicitcursorsimmediatelyaftertheyareexecuted


9/34

SQL CursorAttributes

Delete rows that have the specifiedorder number from the ITEM table.Print the number of rows deleted.

ExampleVARIABLE rows_deletedVARCHAR2(30)DECLAREv_ordidNUMBER := 605;BEGINDELETE FROM itemWHERE ordid = v_ordid;

:rows_deleted := (SQL%ROWCOUNT ||' rowsdeleted.');END;/PRINTrows_deleted


10/34

ImplicitCursorSQL%ROWCOUNT Example

SQL> SET SERVEROUTPUT ON;SQL> DECLARErNUMBER;

BEGINDELETE FROM empWHERE empno=7900;r:=SQL%ROWCOUNT;DBMS_OUTPUT.PUT_LINE(r);

END;/1


11/34

ImplicitCursorSQL%FOUNDExample

SQL> DECLARErBOOLEAN;BEGINDELETE FROM empWHERE empno=1000;r:=SQL%FOUND;IF r THENDBMS_OUTPUT.PUT_LINE('Rows are founded');ELSEDBMS_OUTPUT.PUT_LINE('No Rows are founded');ENDIF;

END;

/No Rows are founded


12/34

ImplicitCursorSQL%ISOPENExample

SQL> DECLARErBOOLEAN;BEGINUPDATE emp SET sal=1000WHERE empno>7900;r:=SQL%ISOPEN;IF r THENDBMS_OUTPUT.PUT_LINE('The cursor is opened');ELSEDBMS_OUTPUT.PUT_LINE('The cursor is closed');ENDIF;

END;

/The cursor is closed


13/34

AboutCursors

Every SQL statement executedby the Oracle Server has an

individual cursor associated withit:o Implicit cursors: Declared for allDML and PL/SQL SELECT

statementso Explicit cursors: Declared and

named by the programmer


14/34

ExplicitCursorFunctions

Activeset

CurrentrowCursor

7369 SMITHCLERK

7566 JONES MANAGER7788 SCOTT ANALYST7876 ADAMS CLERK7902 FORD ANALYST


15/34

Controlling ExplicitCursors

CreateanamedSQL

area

DECLARE

Identifytheactiveset

OPEN

Loadthecurrentrowinto

variables

FETCH

Testforexistingrows

EMPTY?

ReturntoFETCHifrowsfound

No

Releasetheactiveset

CLOSE

Yes


16/34

Controlling ExplicitCursors

Openthecursor.

Cursor

Pointer

Fetcharowfromthecursor.

Cursor

Pointer

Continueuntil empty.

Cursor

Pointer

Closethecursor.


17/34

DeclaringtheCursor

Syntax

Do not include the INTO clause in thecursor declaration.

If processing rows in a specific sequence isrequired, use the ORDER BY clause in the

query.

CURSOR cursor_name ISselect_statement;


18/34

DeclaringtheCursor

ExampleDECLARECURSOR emp_cursorISSELECTempno,enameFROM emp;

CURSOR dept_cursorISSELECT *FROM deptWHERE deptno = 10;BEGIN

...


19/34

OpeningtheCursor

Syntax

Open the cursor to execute the query and

identify the active set. If the query returns no rows, no exception israised.

Use cursor attributes to test the outcomeafter a fetch.

OPENcursor_name;


20/34

FetchingDatafromtheCursor

Syntax

Retrieve the current row values into variables.

Include the same number of variables. Match each variable to correspond to the columns

positionally. Test to see if the cursor contains rows.

The FETCH statement performs thefollowing operations:

1. Advances the pointer to the next row in the activeset.2. Reads the data for the current row into the outputPL/SQL variables.

FETCH cursor_name INTO [variable1, variable2, ...]| record_name];


21/34

FetchingDatafromtheCursor

Examples

FETCHemp_cursorINTO v_empno,v_ename;

...OPENdefined_cursor;LOOPFETCHdefined_cursorINTO defined_variablesEXIT WHEN...;

...-- Processtheretrieveddata

...END;


22/34

ClosingtheCursor

Syntax

Close the cursor after completing theprocessing of the rows. Reopen the cursor, if required. Do not attempt to fetch data from a cursor

once it has been closed. The CLOSE statement releases the context

area.

CLOSE cursor_name;


23/34

ExplicitCursorExample

SQL> DECLAREv_num emp.empno%TYPE;v_name emp.ename%TYPE;CURSOR my_cursorIS SELECT empno,ename FROM empWHEREempno>7900;

BEGINOPENmy_cursor;LOOPFETCHmy_cursorINTO v_num,v_name;EXITWHENmy_cursor%NOTFOUND;DBMS_OUTPUT.PUT_LINE(v_num || ' has the name ' ||v_name);END LOOP;CLOSE my_cursor;END;/7902 has the name FORD7934 has the name MILLER


24/34

ExplicitCursorAttributes

Obtain status information about acursor.AttributeTypeDescription

%ISOPEN Boolean EvaluatestoTRUE ifthecursorisopen%NOTFOUND Boolean EvaluatestoTRUE ifthemostrecentfetchdoesnotreturnarow

%FOUND Boolean EvaluatestoTRUE ifthemostrecentfetchreturnsarow; complementof%NOTFOUND%ROWCOUNTNumberEvaluatestothetotal numberofrowsreturnedsofar


25/34

Controlling Multiple Fetches

Process several rows from an explicitcursor using a loop.

Fetch a row with each iteration.

Use the %NOTFOUND attribute to write atest for an unsuccessful fetch.

Use explicit cursor attributes to test thesuccess of each fetch.


26/34


27/34

The %NOTFOUND

and %ROWCOUNT Attributes

Use the %ROWCOUNT cursor attribute toretrieve an exact number of rows.

The value of %ROWCOUNT beforefetching any row is NULL.

Use the %NOTFOUND cursor attribute todetermine when to exit the loop.


28/34

ExplicitCursor%ISOPENExample

SQL> DECLAREv_num emp.empno%TYPE;v_name emp.ename%TYPE;rBOOLEAN;CURSOR my_cursorIS SELECT empno,ename FROM empWHERE

empno>7900;BEGINOPENmy_cursor;r:=my_cursor%ISOPEN;IF r THENDBMS_OUTPUT.PUT_LINE('The Cursor is opened after the open statement');ELSEDBMS_OUTPUT.PUT_LINE('The Cursor is closed after the open statement');ENDIF;


29/34

ExplicitCursor%ISOPENExampleCont.

LOOPFETCHmy_cursorINTO v_num,v_name;EXITWHENmy_cursor%NOTFOUND;END LOOP;CLOSE my_cursor;

r:=my_cursor%ISOPEN;IF r THENDBMS_OUTPUT.PUT_LINE('The Cursor is opened after the close statement');ELSEDBMS_OUTPUT.PUT_LINE('The Cursor is closed after the close statement');ENDIF;END;/The Cursor is opened after the open statementThe Cursor is closed after the close statement


30/34


31/34

Cursorsand Records

Process the rows of the active setconveniently by fetching values into aPL/SQL RECORD.

Exam

pleDECLARECURSOR emp_cursorISSELECTempno,enameFROM emp;emp_recordemp_cursor%ROWTYPE;

BEGI

NOPENemp_cursor;LOOPFETCHemp_cursorINTO emp_record;...


32/34

Syntax

The cursor FOR loop is a shortcut toprocess explicit cursors.

Implicit open, fetch, and close occur. The record is implicitly declared.

CursorFOR Loops

FOR record_name INcursor_name LOOPstatement1;statement2;...

END LOOP;


33/34

CursorFOR Loops

Retrieve employees one by one untilno more are left.

Example

DECLARECURSOR emp_cursorISSELECTename,deptnoFROM emp;BEGINFOR emp_recordINemp_cursorLOOP

-- implicitopenandimplicitfetchoccurIF emp_record.deptno = 30 THEN...END LOOP; -- implicitcloseoccursEND;


34/34

Summary

Cursor types:o Implicit cursors: Used for all DML statements

and single-row queries.o Explicit cursors: Used for queries of zero, one, orm

ore rows. You can manipulate explicit cursors. You can evaluate the cursor status by using

cursor attributes. You can use cursor FOR loops.

Documents

Ch4 Cursors