Chapter Sixteen Cursors Objective: – Introduction to cursors – Use of cursors in a database record – Implicit & explicit cursors – Cursors & loops – Cursors

Chapter SixteenCursors

Objective:– Introduction to cursors

– Use of cursors in a database record

– Implicit & explicit cursors

– Cursors & loops

– Cursors with parameters

COSC 641 Chapter 16: PL/SQL Cursors

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Cursors

DEF:A cursor is a pointer to the context area.

Context area: Memory location containing info needed to complete the PL/SQL processing


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Cursors (Example 1)

DECLAREV_ID Student.id%TYPE;V_Name Student.name%TYPE;V_Major Student.major%TYPE=‘COSC’;

CURSOR C_Student ISSELECT id, nameFROM StudentWHERE major=V_Major;

BEGINOPEN C_Student;LOOP FETCH C_Student INTO V_ID, V_Name;

EXIT WHEN C_Student%NOTFOUND;END LOOP;CLOSE C_Student;

END;


4

Cursors (Example 2)

CURSOR My_cursor IS

SELECT a.Name, b.Name, c.Name

FROM faculty a, student b, staff c

WHERE a.ID = b.ID AND

b.ID = c.ID;


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Cursors

• Every SQL statement executed by the Oracle Server has an individual cursor associated with it.

– Explicit cursors– Implicit cursors


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Explicit Cursors (from example 1)

• Keep track of the current row that is processing• Process row by row• Programmer can manually control the cursor• All in an active set

Active Set

1111 Mark

Cursor 1231 Cathy Current row

1421 Jack

1789 Sandy


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Controlling Explicit Cursors:

1. Create a named SQL area; (DECLARE)

2. Identify the active set (OPEN)

3. Load the current row into variables (FETCH)

4. Test for existing rowsa. If found goto step 3

b. If not found goto step 5

5. Release the active set (CLOSE)


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Explicit Cursor

OPEN

FETCH

How Many Rows

CLOSE


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Cursors (Example 1)

DECLAREV_ID Student.id%TYPE;V_Name Student.name%TYPE;V_Major Student.major%TYPE=‘COSC’;

CURSOR C_Student ISSELECT id, nameFROM StudentWHERE major=V_Major;

BEGINOPEN C_Student;LOOP FETCH C_Student INTO V_ID, V_Name;

EXIT WHEN C_Student%NOTFOUND;END LOOP;CLOSE C_Student;

END;


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Example

CREATE OR REPLACE FUNCTION myFac(Name_In IN student.Name%TYPE) RETURN NUMBER ASCURSOR a IS

SELECT MajorFROM studentWHERE Name = UPPER(Name_In);

a_rec a%ROWTYPE;Ret_val NUMBER;BEGIN

OPEN a;FETCH a INTO a_rec;If a%FOUND THEN

IF a_rec.Major = ‘COSC’ THENRet_val = 10;

ELSIF a_Rec.major = ‘MATH’ THENRet_val = 5;

END IF;END IF;CLOSE a;RETURN Ret_Val;

END;/


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Declaring a Cursor

Syntax:

CURSOR C_Name [([Parameter [, parameter])]

IS

Select_Statement [RETURN ret]

[For UPDATE [of [COLUMN List]];


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Example

CURSOR C_Faculty IS

SELECT *FROM FacultyWHERE Major=UPPER(‘COSC’);-----------------------------

CURSOR C_Dept IS

SELECT ID, NoFROM DeptWHERE Name=UPPER(‘COSC’);

-----------------------------


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Opening the Cursor

Syntax:OPEN C_Name;

OPEN is an executable statement that:

1. Dynamically allocates memory for a context area2. Parses the SELECT statement3. Binds the input variables: (by obtaining their memory address)4. Identifies the active set

5. Positions the pointer just before the 1st row in the active set


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Example

DECLARECURSOR C_Faculty IS

SELECT *FROM FacultyWHERE Major=‘COSC’;

CURSOR C_Dept ISSELECT ID, NoFROM DeptWHERE Name=‘COSC’;

V_F_Name Faculty.Name%TYPE;V_F_Salary Faculty.Salary%TYPE;V_D_Rec Dept%ROWTYPE;

BEGINOPEN C_Faculty;OPEN C_Dept;….


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Fetching the Data

Syntax:FETCH C_Name INTO [record_name| V1, V2, …];

1. Retrieve the current row values into output variables2. Match each variable to the columns3. Test to see if the cursor points to a row

FETCH statement performs:1. Advance the pointer to the next row in the active set2. Reads the data from the current row into the output variables3. Exits the loop, if the pointer points to the end of the active set


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Example


SELECT *FROM FacultyWHERE Major=UPPER(‘COSC’);

CURSOR C_Dept ISSELECT ID, NoFROM DeptWHERE

Name=‘COSC’;V_F_Name Faculty.Name%TYPE;V_F_Salary Faculty.Salary%TYPE;V_D_Rec Dept%ROWTYPE;

BEGINOPEN C_Faculty;OPEN C_Dept;

FOR I IN 1..5 LOOP FETCH C_Faculty INTO V_F_Name, V_F_Salary

………END LOOP;

LOOP FETCH C_Dept INTO V_D_Rec EXIT WHEN C_Dept%NOTFOUNDEND LOOP;END;/


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Closing the Cursor

Syntax:CLOSE C_Name;

1. Close the cursor after the process is completed

2. Reopen the cursor, if it is needed

• OPEN_CURSORS = 50;


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Illegal Cursor

DECLARECURSOR C_Student IS

SELECT id, name

FROM Student

WHERE major=V_Major;V_Major Student.major%TYPE=‘COSC’;


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ROWNUM• Return a number indicating the order in which a row was selected from a table

DECLARECURSOR C1 IS

SELECT ID, NameFROM FacultyWHERE salary > 40000 AND ROWNUM <=100; --100 rows

CURSOR C2 ISSELECT *FROM (SELECT ID, Name

FROM faculty_temp WHERE salary> 40000 ORDER BY salary DESC)

WHERE ROWNUM <10; --first 9 sorted rowsBEGIN

………UPDATE faculty_tempSET ID = ROWNUM; --each row gets a different number

END;/


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Explicit Cursor Attributes

• %FOUND Boolean TypeC_Dept%FOUND;

• %NOTFOUND Boolean TypeC_Dept%NOTFOUND;

• %ISOPEN Boolean Type C_Dept%ISOPEN;

• %ROWCOUNT Number TypeC_Dept%ROWCOUNT;

• %BULK_ROWCOUNTC_Dept% BULK_ROWCOUNT;


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%ISOPEN

• You can fetch rows only when it is open• Use %ISOPEN to check

IF NOT C_faculty%ISOPEN THEN OPEN C_faculty;

END IF;LOOP

FETCH C_faculty into V_ID, V_Name;…


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Explicit Cursor Attributes

• Use %ROWCOUNT cursor attribute to retrieve an exact number of rows• Use %NOTFOUND cursor attribute to determine when to exit the loop

SET SERVEROUTPUT ON;BEGIN

….OPEN C_faculty;LOOP

FETCH C_faculty INTO V_ID, V_Name;DBMS_OUTPUT.PUT_LINE (‘faculty’ || V_Name || ’has ID = ‘ || V_ID);EXIT WHEN

C_faculty %ROWCOUNT >5 ORC_faculty %NOTFOUND;

END LOOP;CLOSE C_faculty;

END;/


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Cursor Exception

• INVALID_CURSOR

• TOO_MANY_ROWS

• %BULK_EXCEPTION


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Example


SELECT *FROM Faculty;

V_F_Rec C_Faculty%ROWTYPE;BEGIN

AOPEN C_Faculty;

BFETCH C_Faculty INTO V_F_Rec;

CFETCH C_Faculty INTO V_F_Rec;

DCLOSE C_Faculty;

E F

END;

NameSalaryMary 40,000Mark 38,000


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%FOUND & %NOTFOUND

C_Faculty%FOUND C_Faculty%NOTFOUND

A: ORA_1001 ORA_1001

B: NULL NULL

C: TRUE FALSE

D: TRUE FALSE

E: FALSE TRUE

F: ORA_1001 ORA_1001


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%ISOPEN & %ROWCOUNT

C_Faculty%ISOPEN C_Faculty%ROWCOUNT

A: FALSE ORA_1001

B: TRUE 0

C: TRUE 1

D: TRUE 2

E: FALSE ORA_1001


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• NO_DATA_FOUND– SELECT INTO….

• %NOTFOUND SQL%NOTFOUND– Cursor


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Practice:

Create a PL/SQL block to find the top n salaries– Use a substitution parameter so user can input n.– Use loop to get names and salaries of the top n

faculty from faculty table.– Store the names and salaries in a temporary table.– Test for n=0 , and n > number of faculty– Assume no two faculty members have the same

salary.– Empty the table after each test.


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Practice 2:

– Assume faculty members may have the same salaries. In this case list all of them;

– Mary 10000– Mark 5000– Sandy5000– John 2000– If n=2; list Mary, Mark, and Sandy– If n=3; list Mary, Mark, Sandy, and John


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Cursors with Parameters

Syntax:CURSOR C_Name [ (P1, P2, …) ] IS

Select_Statement;

Where P1,P2: P [IN] datatype [[:= | DEFAULT ] exp]

1. Pass parameter values to a cursor when the cursor is opened & the query is executed

2. Open an explicit cursor several times with a different active set each time.


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Example of Cursors with Parameters

DECLARE CURSOR C_Student (V_ID NUMBER, V_Major VARCHAR2) IS

SELECT ID, MajorFROM StudentWHERE Major = V_Major AND ID = V_ID;

BEGINOPEN C_Student (1111, ‘COSC’);….CLOSE C_Student;OPEN C_Student(2222, ‘MATH’);….CLOSE C_Student;

END;/


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DECLAREV_VAR_ID Student.ID%TYPE;V_VAR_Major Student.Major%TYPE:=‘COSC’;

CURSOR C_Student (V_ID NUMBER, V_Major VARCHAR2) IS

SELECT ….

OPEN C_Student (1111, V_VAR_Major);CLOSE C_Student;OPEN C_Student (2222, ‘VART’);


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CURSOR a (Name_IN VARCHAR2) ISSELECT MajorFROM StudentWHERE Name = UPPER(Name_IN);a_Rec a%ROWTYPE;ret_val NUMBER;

BEGINOPEN a (‘Sandy’);FETCH a INTO a_Rec;IF a%FOUND THEN

IF a_Rec.Major = ‘COSC’ THENRet_val = 10;

ELSIF a_Rec.Major = ‘MATH’ THENRet_val = 5;

END IF;END IF;CLOSE a;

END;


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Open Cursor with Parameters

OPEN a (:My_pack.Name);

OPEN a (‘John’);

OPEN a (‘Mark’);


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Cursor With Parameter:

CURSOR C_staff RETURN staff%ROWTYPE

IS

SELECT *

FROM staff

WHERE dept=UPPER(‘Cosc’);


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Cursor with Default Value

DECLARE

CURSOR C1(low NUMBER DEFAULT 0,

high NUMBER DEFAULT 99) IS

SELECT …


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FOR UPDATE Clause

Syntax:

SELECT …..

FROM …..

FOR UPDATE [OF Col_Ref] [NOWAIT]

- Lock the record before update or delete


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Example

DECLARE

CURSOR C_Student ISSELECT Name, ID, GPA

FROM Student

WHERE Dept=‘COSC’

FOR UPDATE OF id, name;


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WHERE CURRENT OF clause

Syntax:

WHERE CURRENT OF cursor• Use Cursors to update or delete the current row

• Include the FOR UPDATE clause in the cursor query to Lock the rows first

• Use WHERE CURRENT OF clause to reference the current row from an explicit cursor.


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ExampleDECLARE

CURSOR C_Student ISSELECT Major, idFROM StudentWHERE Major = ‘COSC’FOR UPDATE NOWAIT;

BEGINFOR I IN C_StudentLOOP

UPDATE studentSET ID=student.Id * 100WHERE CURRENT OF C_Student;

END LOOP;COMMIT;

END;/


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Cursor with subqueries

DECLARECURSOR C1 is

SELECT id, NameFROM facultyWHERE salary >

( SELECT AVG(Salary) FROM

Faculty );CURSOR C2 is

SELECT *FROM (SELECT Name,Salary

FROM Faculty ORDER BY Salary DESC,Name)

WHERE ROWNUM<21;Continued…


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Cursor FOR loops:

Syntax:

FOR record_name IN cursor_name LOOP

statement1;

statement2;

….

END LOOP;


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Cursor FOR loop:

BEGINFOR I IN C1LOOP

DBMS_OUTPUT.PUT_LINE(‘Above average Salary’ || I.Name);

END LOOP;FOR I IN C2LOOP

DBMS_OUTPUT.PUT_LINE(‘Twenty Top paid Faculty’ || I.NAME || I.SALARY);

END LOOP;END;/


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NO_DATA_FOUND VS. %NOTFOUND

SELECT ….

INTO …..

FROM …..

WHERE …..

• NO_DATA_FOUND :Exception• %NOTFOUND :Flag


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Implicit Cursor

INSERT

DELETE

UPDATE

SELECT INTO


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Implicit Cursor

DECLARE

a faculty%ROWTYPE;

BEGIN

SELECT *

INTO a

FROM faculty

WHERE ID = 1111;

END;


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Error Handling with Implicit Cursor

• NO_DATA_FOUND

• TOO_MANY_ROWS


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Implicit Cursor Attributes

• SQL%FOUND

• SQL%NOTFOUND

• SQL%ROWCOUNT

• SQL%ISOPEN --Always Return False


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Implicit Cursor

BEGIN

UPDATE classroom

SET NoOfSeats= 250

WHERE roomId=111;

IF SQL%NOTFOUND THEN

INSERT INTO classroom (roomId, NoOfSeats)

VALUES ( 111, 250);

END IF;


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UPDATE, DELETE, INSERT

SET SERVEROUTPUT ON;BEGIN

UPDATE FacultySET Salary = Salary * 0.05WHERE Dept = ‘COSC’;DBMS_OUTPUT.PUT_LINE(‘Update’

|| SQL%ROWCOUNT);

END;/


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Cursor Variable

• CURSOR

• CURSOR Variable


52

Cursor Variable

TYPE ref_Type IS REF CURSOR [RETURN TYPE];

Example:

DECLARE

TYPE stud_type REF CURSOR RETURN student%ROWTYPE;

stud_cv stud_type; -- Declar Cursor Variable

TYPE fac_type REF CURSOR RETURN faculty%ROWTYPE;

fac_cv fac_type;


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Cursor Variable

Example:DECLARE

TYPE FacCur_Type IS REF CURSOR RETURN faculty%ROWTYPE;Fac FacCur_Type;-- Declar Cursor Variable--Process all rowsPROCEDURE p1(fac_cv IN FacCur_Type) IS

person faculty%ROWTYPE;BEGIN LOOP

FETCH fac_cv INTO person;EXIT WHEN fac_cv%NOTFOUND;DBMS_OUTPUT.PUT_LINE( person.name);

END LOOP;END;


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Cursor Variable

BEGINOPEN Fac FOR

SELECT * FROM FacultyWHERE ROWNUM<=10;p1(fac);CLOSE Fac;

OPEN Fac FOR SELECT * FROM FacultyWHERE name LIKE ‘X%’;p1(Fac);CLOSE Fac;

END;/


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Controlling Cursor VariablesCursor variables are controlled by OPEN-FOR, FETCH, CLOSE

DECLARETYPE FacCur_type IS REF CURSOR RETURN

faculty%ROWTYPE;Fac FacCur_type; -- Declar Cursor VariableFacRec faculty%ROWTYPE;

BEGINOPEN Fac FOR

SELECT * FROM FacultyWHERE salary>40000;

LOOPFETCH Fac INTO FacRec;EXIT WHEN Fac%NOTFOUND;DBMS_OUTPUT.PUT_LINE( FacRec.name);

END LOOP;CLOSE Fac;

END;/


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EXAMPLE

DECLARETYPE Company_Type IS REF CURSOR

RETURN Company%ROWTYPE;company_cv Company_Type;company_Rec Company%ROWTYPE;

BEINGOPEN company_cv FOR

SELECT *FROM Company;

FETCH company_cv INTO company_Rec;CLOSE company_cv;

END;


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Example:

DECLARECURSOR O_CUR IS

SELECT Customer_ID, Room_NoFROM OccupancyWHERE R_Date = TRUNC(SYSDATE);

O_Rec O_CUR%ROWTYPE;

BEGINOPEN O_CUR

LOOP FETCH O_CUR INTO O_Rec;EXIT WHEN O_CUR%NOTFOUND;F1 (O_Rec.customer_ID, O_Rec.Room_No);

END LOOP;CLOSE O_CUR;

END;/

Documents

Chapter Sixteen Cursors Objective: – Introduction to cursors – Use of cursors in a database record – Implicit & explicit cursors – Cursors & loops – Cursors