Cursors - Implicit & Explicit

Oracle creates a memory area, known as context area, for processing an SQL statement, which contains all information needed for processing the statement, for example, number of rows processed, etc.

A cursor is a pointer to this context area. PL/SQL controls the context area through a cursor. A cursor holds the rows (one or more) returned by a SQL statement. The set of rows the cursor holds is referred to as the active set.

You can name a cursor so that it could be referred to in a program to fetch and process the rows returned by the SQL statement, one at a time. There are two types of cursors:

·        Implicit cursors

·        Explicit cursors

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:

Attribute	Description
%FOUND	Returns TRUE if an INSERT, UPDATE, or DELETE statement affected one or more rows or a SELECT INTO statement returned one or more rows. Otherwise, it returns FALSE.
%NOTFOUND	The logical opposite of %FOUND. It returns TRUE if an INSERT, UPDATE, or DELETE statement affected no rows, or a SELECT INTO statement returned no rows. Otherwise, it returns FALSE.
%ISOPEN	Always returns FALSE for implicit cursors, because Oracle closes the SQL cursor automatically after executing its associated SQL statement.
%ROWCOUNT	Returns the number of rows affected by an INSERT, UPDATE, or DELETE statement, or returned by a SELECT INTO statement.

Any SQL cursor attribute will be accessed as sql%attribute_name as shown below in the example.

Example:

We will be using the CUSTOMERS table we had created and used in the previous chapters.

Select * from customers;

+----+----------+-----+-----------+----------+

| ID | NAME     | AGE | ADDRESS   | SALARY   |

+----+----------+-----+-----------+----------+

|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |

|  2 | Khilan   |  25 | Delhi     |  1500.00 |

|  3 | kaushik  |  23 | Kota      |  2000.00 |

|  4 | Chaitali |  25 | Mumbai    |  6500.00 |

|  5 | Hardik   |  27 | Bhopal    |  8500.00 |

|  6 | Komal    |  22 | MP        |  4500.00 |

+----+----------+-----+-----------+----------+

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;

/

When the above code is executed at SQL prompt, it produces the following result:

6 customers selected

PL/SQL procedure successfully completed.

If you check the records in customers table, you will find that the rows have been updated:

Select * from customers;

+----+----------+-----+-----------+----------+

| ID | NAME     | AGE | ADDRESS   | SALARY   |

+----+----------+-----+-----------+----------+

|  1 | Ramesh   |  32 | Ahmedabad |  2500.00 |

|  2 | Khilan   |  25 | Delhi     |  2000.00 |

|  3 | kaushik  |  23 | Kota      |  2500.00 |

|  4 | Chaitali |  25 | Mumbai    |  7000.00 |

|  5 | Hardik   |  27 | Bhopal    |  9000.00 |

|  6 | Komal    |  22 | MP        |  5000.00 |

+----+----------+-----+-----------+----------+

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. For example:

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. For example, we will open above-defined cursor as follows:

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:

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:

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;

/

When the above code is executed at SQL prompt, it produces the following result:

1 Ramesh Ahmedabad

2 Khilan Delhi

3 kaushik Kota   

4 Chaitali Mumbai

5 Hardik Bhopal 

6 Komal MP 

PL/SQL procedure successfully completed.