Cursors

CURSORS

Cursor is a pointer to memory location which is called as context area which contains the information necessary for processing, including the number of rows processed by the statement, a pointer to the parsed representation of the statement, and the active set which is the set of rows returned by the query.

Cursor contains two parts:

• Header
• Body

Header includes cursor name, any parameters and the type of data being loaded.

Body includes the select statement.

Ex:

Cursor c(dno in number) return dept%rowtype is select *from dept;

           In the above 

Header – cursor c(dno in number) return dept%rowtype

Body – select *from dept

Cursor Types:

1. Implicit Cursor(SQL)
2. Explicit Cursor

               a) Parameterized Cursors

               b) REF cursors

Cursor Stages:

• Open
• Fetch
• Close

Cursor Attributes:

1. %found
2. %notfound
3. %rowcount
4. %isopen
5. %bulk_rowcount
6. %bulk_exceptions

Cursor Declaration:

Syntax:

       Cursor <cursor_name> is select statement;

   EX: 

       Cursor c is select * from dept;

Cursor Loops:

1. Simple Loop
2. While Loop
3. For Loop

1.Simple Loop:

  Syntax:

     Loop

Fetch <cursor_name> into <record_variable>;

Exit when <cursor_name> % notfound;

    <statements>;

End loop;

Ex:

DECLARE

     cursor c is select * from student;

     v_stud student%rowtype;

BEGIN

     Open c;

     Loop

        fetch c into v_stud;

        exit when c%notfound;

        dbms_output.put_line('Name = ' || v_stud.name);

     end loop;

     close c;

END;

Output:

Name = saketh

Name = srinu

Name = satish

Name = sudha

2.While Loop:

  Syntax:

     While <cursor_name> % found 

      loop

         Fetch <cursor_name> into <record_variable>;

          <statements>;

      End loop;

Ex:

DECLARE

     cursor c is select * from student;

     v_stud student%rowtype;

BEGIN

     open c;

     fetch c into v_stud;

     while c%found loop

          fetch c into v_stud;

          dbms_output.put_line('Name = ' || v_stud.name);

     end loop;

     close c;

END;

Output:

Name = saketh

Name = srinu

Name = satish

Name = sudha

3.For Loop:

    Syntax:

       for <record_variable> in <cursor_name> 

        loop

            <statements>;

        End loop;

Ex:

     declare

     cursor c is select * from student;

     begin

         for v_stud in c

     loop

         dbms_output.put_line('Name = ' || v_stud.name);

     end loop;

    end;

Output:

Name = saketh

Name = srinu

Name = satish

Name = sudha

Implicit Cursors:
============

• Implicit cursors are automatically created by Oracle whenever an SQL statement is executed, when there is no explicit cursor for the statement. Programmers cannot control the implicit cursors and the information in it.
• Whenever a DML statement (INSERT, UPDATE and DELETE) is issued, an implicit cursor is associated with this statement. For INSERT operations, the cursor holds the data that needs to be inserted. For UPDATE and DELETE operations, the cursor identifies the rows that would be affected
• In PL/SQL, you can refer to the most recent implicit cursor as the SQL cursor, which always has the attributes like %FOUND, %ISOPEN, %NOTFOUND, and %ROWCOUNT. The SQL cursor has additional attributes, %BULK_ROWCOUNT and %BULK_EXCEPTIONS, designed for use with the FORALL statement. The following table provides the description of the most used attributes:

     Ex:

      The following program would update the table and increase salary of each customer by 500 and use the SQL%ROWCOUNT attribute to determine the number of rows affected.

DECLARE 

   total_rows number(2);

BEGIN

   UPDATE customers

   SET salary = salary + 500;

   IF sql%notfound THEN

      dbms_output.put_line('no customers selected');

   ELSIF sql%found THEN

      total_rows := sql%rowcount;

      dbms_output.put_line( total_rows || ' customers selected ');

   END IF; 

END;

/

PL/SQL provides some attributes, which allow you to evaluate what happened when the implicit cursor was last used. You can use these attributes in PL/SQL statements like functions but you cannot use then within SQL statements.

%ROWCOUNT	The number of rows processed by a SQL statement.
%FOUND	TRUE if at least one row was processed.
%NOTFOUND	TRUE if no rows were processed.
%ISOPEN	TRUE if cursor is open or FALSE if cursor has not been opened or has been closed. Only used with explicit cursors.

An Example of PL/SQL Attribute

DECLARE
rows_deleted NUMBER;
BEGIN
DELETE * FROM emp;
rows_deleted := SQL%ROWCOUNT;
END;

The implicit cursor has the following drawbacks:

• It is less efficient than an explicit cursor.
• It is more vulnerable to data errors.
• It gives you less programmatic control.

Explicit Cursors:
============
Explicit cursors are programmer defined cursors for gaining more control over the context area. An explicit cursor should be defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row.

The syntax for creating an explicit cursor is :
      CURSOR cursor_name IS select_statement;

Working with an explicit cursor involves four steps:

• Declaring the cursor for initializing in the memory
• Opening the cursor for allocating memory
• Fetching the cursor for retrieving data
• Closing the cursor to release allocated memory

Declaring the Cursor:

Declaring the cursor defines the cursor with a name and the associated SELECT statement.

Ex: CURSOR c_customers IS

   SELECT id, name, address FROM customers;

Opening the Cursor:

Opening the cursor allocates memory for the cursor and makes it ready for fetching the rows returned by the SQL statement into it.

Ex: OPEN c_customers;

Fetching the Cursor:

Fetching the cursor involves accessing one row at a time. 

For example we will fetch rows from the above-opened cursor as follows:

Ex:FETCH c_customers INTO c_id, c_name, c_addr;

Closing the Cursor:

Closing the cursor means releasing the allocated memory.

For example, we will close above-opened cursor as follows:

Ex:CLOSE c_customers;

Example:

Following is a complete example to illustrate the concepts of explicit cursors:

DECLARE

   c_id customers.id%type;

   c_name customers.name%type;

   c_addr customers.address%type;

   CURSOR c_customers is

      SELECT id, name, address FROM customers;

BEGIN

   OPEN c_customers;

   LOOP

      FETCH c_customers into c_id, c_name, c_addr;

      EXIT WHEN c_customers%notfound;

      dbms_output.put_line(c_id || ' ' || c_name || ' ' || c_addr);

   END LOOP;

   CLOSE c_customers;

END;

/

Parameterized cursors:

• This was used when you are going to use the cursor in more than one place with different values for the same where clause.
• Cursor parameters must be in mode.
• Cursor parameters may have default values.
• The scope of cursor parameter is within the select statement.

Ex:

     DECLARE

         cursor c(dno in number) is select * from dept where deptno = dno;

         v_dept dept%rowtype;

      BEGIN

         open c(20);

         loop

             fetch c into v_dept;

             exit when c%notfound;

            dbms_output.put_line('Dname = ' || v_dept.dname || ' Loc = ' || v_dept.loc);

         end loop;

         close c;

     END;

/

Output:

    Dname = RESEARCH Loc = DALLAS

Reference Cursors:

This is unconstrained cursor which will return different types depends upon the user input

• Ref Cursors cannot be closed implicitly
• Ref Cursors with return type is called strong cursor
• Ref Cursors with out return type is called weak cursor
• you can declare Ref cursor type in package spec as well as body
• you can declare ref cursor types in local subprograms or anonymous blocks. 

Ex:

CREATE OR REPLACE PROCEDURE REF_CURSOR(

    TABLE_NAME IN VARCHAR)

IS

type t

IS

  ref

  CURSOR;

    c t;

    v_dept dept%rowtype;

  type r

IS

  record

  (

    ename emp.ename%type,

    job emp.job%type,

    sal emp.sal%type);

  v_emp r;

  v_stud student.name%type;

BEGIN

  IF table_name = 'DEPT' THEN

    OPEN c FOR SELECT * FROM dept;

  elsif table_name = 'EMP' THEN

    OPEN c FOR SELECT ename,job,sal FROM emp;

  elsif table_name = 'STUDENT' THEN

    OPEN c FOR SELECT name FROM student;

  END IF;

  LOOP

    IF table_name = 'DEPT' THEN

      FETCH c INTO v_dept;

    EXIT

  WHEN c%notfound;

    dbms_output.put_line('Deptno = ' || v_dept.deptno || ' Dname = ' || v_dept.dname || ' Loc = ' || v_dept.loc);

  elsif table_name = 'EMP' THEN

    FETCH c INTO v_emp;

    EXIT

  WHEN c%notfound;

    dbms_output.put_line('Ename = ' || v_emp.ename || ' Job = ' || v_emp.job || ' Sal = ' || v_emp.sal);

  elsif table_name = 'STUDENT' THEN

    FETCH c INTO v_stud;

    EXIT

  WHEN c%notfound;

    dbms_output.put_line('Name = ' || v_stud);

  END IF;

END LOOP;

CLOSE c;

END;

Output: 

SQL> EXEC ref_cursor('DEPT') 

Deptno = 10 Dname = ACCOUNTING Loc = NEW YORK 

Deptno = 20 Dname = RESEARCH Loc = DALLAS 

Deptno = 30 Dname = SALES Loc = CHICAGO 

Deptno = 40 Dname = OPERATIONS Loc = BOSTON 

SQL> EXEC ref_cursor('EMP') 

Ename = SMITH Job = CLERK Sal = 800

 Ename = ALLEN Job = SALESMAN Sal = 1600 

 Ename = WARD Job = SALESMAN Sal = 1250 

 Ename = JONES Job = MANAGER Sal = 2975

 Ename = MARTIN Job = SALESMAN Sal = 1250 

 Ename = BLAKE Job = MANAGER Sal = 2850 

 Ename = CLARK Job = MANAGER Sal = 2450 

 Ename = SCOTT Job = ANALYST Sal = 3000 

 Ename = KING Job = PRESIDENT Sal = 5000

 Ename = TURNER Job = SALESMAN Sal = 1500

 Ename = ADAMS Job = CLERK Sal = 1100 

 Ename = JAMES Job = CLERK Sal = 950 

 Ename = FORD Job = ANALYST Sal = 3000 

 Ename = MILLER Job = CLERK Sal = 1300 

 

 SQL> EXEC ref_cursor('STUDENT') 

 Name = saketh 

 Name = srinu 

 Name = satish 

 Name = sudha

Nested Cursors:

• Oracle opens the nested cursor defined by a cursor expression implicitly as soon as it fetches the data containing the cursor expression from the parent or outer cursor
• Nested cursor closes if you closed explicitly
• Nested cursor closes whenever the outer or parent cursor is executed again or closed or cancelled
• Nested cursor closes whenever an exception is raised while fetching data from a parent cursor

Ex:

DECLARE

cursor c is select ename,cursor(select dname from dept d where e.empno = d.deptno)  from emp e;

type t is ref cursor;

c1 t;

c2 t;

v1 emp.ename%type;

v2 dept.dname%type;

BEGIN

open c;

loop

     fetch c1 into v1;

          exit when c1%notfound;

          fetch c2 into v2;

          exit when c2%notfound;

          dbms_output.put_line('Ename = ' || v1 || ' Dname = ' || v2);

end loop;

end loop;

close c;

END;

CURSOR CLAUSES:

                        1     Return

2     For update

3     Where current of

4     Bulk collect

RETURN:

         Cursor c return dept%rowtype is select *from dept;

Or

Cursor c1 is select *from dept;

Cursor c  return c1%rowtype is select *from dept;

Or

Type t is record(deptno dept.deptno%type, dname dept.dname%type);

Cursor c return t is select deptno, dname from dept;

FOR UPDATE AND WHERE CURRENT OF:

 Normally, a select operation will not take any locks on the rows being accessed. This will allow other sessions connected to the database to change the data being selected. The result set is still consistent. At open time, when the active set is determined, oracle takes a snapshot of the table. Any changes that have been committed prior to this point are reflected in the active set. Any changes made after this point, even if they are committed, are not reflected unless the cursor is reopened, which will evaluate the active set again.

However, if the FOR UPDATE clause is present, exclusive row locks are taken on the rows in the active set before the open returns. These locks prevent other sessions from changing the rows in the active set until the transaction is committed or rolled back. If another session already has locks on the rows in the active set, then SELECT … FOR UPDATE operation will wait for these locks to be released by the other session. There is no time-out for this waiting period. The SELECT…FOR UPDATE will hang until the other session releases the lock. To handle this situation, the NO WAIT clause is available.

 Syntax:

Select …from … for update of column_name [wait n];

If the cursor is declared with the FOR UPDATE clause, the WHERE CURRENT OF clause can be used in an update or delete statement.

Syntax:

Where current of cursor;

Ex:

DECLARE

       cursor c is select * from dept for update of dname;

BEGIN

       for v in c loop

             update dept set dname = 'aa' where current of c;

             commit;

       end loop;

END;

BULK COLLECT:

1     This is used for array fetches

2     With this you can retrieve multiple rows of data with a single round trip.

3     This reduces the number of context switches between the pl/sql and sql engines.

4     Reduces the overhead of retrieving data.

5     You can use bulk collect in both dynamic and static sql.

6     You can use bulk collect in select, fetch into and returning into clauses.

7   SQL engine automatically initializes and extends the collections you reference in the bulk collect clause.

8    Bulk collect operation empties the collection referenced in the into clause before executing the query.

9     You can use the limit clause of bulk collect to restrict the no of rows retrieved.

10  You can fetch into multiple collections with one column each.

11  Using the returning clause we can return data to the another collection.

BULK COLLECT IN FETCH

 Ex:

DECLARE

     Type t is table of dept%rowtype;

     nt t;

     Cursor c is select *from dept;

BEGIN

     Open c;

     Fetch c bulk collect into nt;

     Close c;

     For i in nt.first..nt.last loop

           dbms_output.put_line('Dname = ' || nt(i).dname || ' Loc = ' ||

                                                          nt(i).loc);

     end loop;

END;

Output:

Dname = ACCOUNTING Loc = NEW YORK

Dname = RESEARCH Loc = DALLAS

Dname = SALES Loc = CHICAGO

Dname = OPERATIONS Loc = BOSTON

BULK COLLECT IN SELECT

 Ex:

DECLARE

     Type t is table of dept%rowtype;

     Nt t;

BEGIN

     Select * bulk collect into nt from dept;

     for i in nt.first..nt.last loop

           dbms_output.put_line('Dname = ' || nt(i).dname || ' Loc = ' ||

                                                                nt(i).loc);

     end loop;

END;

Output:

Dname = ACCOUNTING Loc = NEW YORK

Dname = RESEARCH Loc = DALLAS

Dname = SALES Loc = CHICAGO

Dname = OPERATIONS Loc = BOSTON

LIMIT IN BULK COLLECT

 You can use this to limit the number of rows to be fetched.

 Ex:

DECLARE

     Type t is table of dept%rowtype;

     nt t;

     Cursor c is select *from dept;

BEGIN

     Open c;

     Fetch c bulk collect into nt limit 2;

     Close c;

     For i in nt.first..nt.last loop

           dbms_output.put_line('Dname = ' || nt(i).dname || ' Loc = ' || nt(i).loc);

     end loop;

END;

Output:

Dname = ACCOUNTING Loc = NEW YORK

Dname = RESEARCH Loc = DALLAS

MULTIPLE FETCHES IN  INTO CLAUSE

 Ex1:

     DECLARE

           Type t is table of dept.dname%type;

           nt t;

           Type t1 is table of dept.loc%type;

           nt1 t;

           Cursor c is select dname,loc from dept;

      BEGIN

           Open c;

           Fetch c bulk collect into nt,nt1;

           Close c;

           For i in nt.first..nt.last loop

                  dbms_output.put_line('Dname = ' || nt(i));

           end loop;

           For i in nt1.first..nt1.last loop

                  dbms_output.put_line('Loc = ' || nt1(i));

           end loop;

      END;

Output:

Dname = ACCOUNTING

Dname = RESEARCH

Dname = SALES

Dname = OPERATIONS

Loc = NEW YORK

Loc = DALLAS

Loc = CHICAGO

Loc = BOSTON

Ex2:

DECLARE

      type t  is table of dept.dname%type;

      type t1 is table of dept.loc%type;

      nt t;

      nt1 t1;

BEGIN

      Select dname,loc bulk collect into nt,nt1 from dept;

      for i in nt.first..nt.last loop

           dbms_output.put_line('Dname = ' || nt(i));

      end loop;

      for i in nt1.first..nt1.last loop

           dbms_output.put_line('Loc = ' || nt1(i));

      end loop;

END;

Output:

Dname = ACCOUNTING

Dname = RESEARCH

Dname = SALES

Dname = OPERATIONS

Loc = NEW YORK

Loc = DALLAS

Loc = CHICAGO

Loc = BOSTON

RETURNING CLAUSE IN BULK COLLECT

 You can use this to return the processed data to the output variables or typed variables.

 Ex:

DECLARE

       type t is table of number(2);

       nt t := t(1,2,3,4);

       type t1 is table of varchar(2);

       nt1 t1;

       type t2 is table of student%rowtype;

       nt2 t2;

BEGIN

       select name bulk collect into nt1 from student;

       forall v in nt1.first..nt1.last

        E           update student set no = nt(v) where name = nt1(v) returning 

                              no,name,marks bulk collect into nt2;

       for v in nt2.first..nt2.last loop

               dbms_output.put_line('Marks = ' || nt2(v));

       end loop;

END;

Output:

Marks = 100

Marks = 200

Marks = 300

Marks = 400

POINTS TO REMEMBER

     1     Cursor name can be up to 30 characters in length.

2   Cursors declared in anonymous blocks or subprograms closes automatically when that block terminates execution.

3     %bulk_rowcount and %bulk_exceptions can be used only with forall construct.

4     Cursor declarations may have expressions with column aliases.

5     These expressions are called virtual columns or calculated columns.