Cursors

                          Cursors

What are Cursors?

A cursor is a temporary work area created in the system memory when a SQL statement is executed. A cursor contains information on a select statement and the rows of data accessed by it. This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called theactive set.

There are two types of cursors in PL/SQL:

Implicit cursors:

These are created by default when DML statements like, INSERT, UPDATE, and DELETE statements are executed. They are also created when a SELECT statement that returns just one row is executed. 

Explicit cursors:

They must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.

Both implicit and explicit cursors have the same functionality, but they differ in the way they are accessed. 


Implicit Cursors:

When you execute DML statements like DELETE, INSERT, UPDATE and SELECT statements, implicit statements are created to process these statements.

Oracle provides few attributes called as implicit cursor attributes to check the status of DML operations. The cursor attributes available are %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN. 

For example, When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us whether any rows are affected and how many have been affected.
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.

The status of the cursor for each of these attributes are defined in the below table. 

Attributes	Return Value	Example
%FOUND	The return value is TRUE, if the DML statements like INSERT, DELETE and UPDATE affect at least one row and if SELECT ….INTO statement return at least one row.	SQL%FOUND
	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE do not affect row and if SELECT….INTO statement do not return a row.
%NOTFOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE at least one row and if SELECT ….INTO statement return at least one row.	SQL%NOTFOUND
	The return value is TRUE, if a DML statement like INSERT, DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement does not return a row.
%ROWCOUNT	Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT	SQL%ROWCOUNT

For Example: Consider the PL/SQL Block that uses implicit cursor attributes as shown below:

DECLARE  var_rows number(5);

BEGIN

  UPDATE employee 

  SET salary = salary + 1000;

  IF SQL%NOTFOUND THEN

    dbms_output.put_line('None of the salaries where updated');

  ELSIF SQL%FOUND THEN

    var_rows := SQL%ROWCOUNT;

    dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');

  END IF; 

END; 

In the above PL/SQL Block, the salaries of all the employees in the ‘employee’ table are updated. If none of the employee’s salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 1000 employees are updated' if there are 1000 rows in ‘employee’ table. 

Explicit Cursors

An explicit cursor is defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row. We can provide a suitable name for the cursor.

The General Syntax for creating a cursor is as given below:

CURSOR cursor_name IS select_statement;

• cursor_name – A suitable name for the cursor.
• select_statement – A select query which returns multiple rows.

How to use Explicit Cursor?

There are four steps in using an Explicit Cursor. 

• DECLARE the cursor in the declaration section.
• OPEN the cursor in the Execution Section.
• FETCH the data from cursor into PL/SQL variables or records in the Execution Section.
• CLOSE the cursor in the Execution Section before you end the PL/SQL Block.

1) Declaring a Cursor in the Declaration Section:

   DECLARE

   CURSOR emp_cur IS 

   SELECT * 

   FROM emp_tbl

   WHERE salary > 5000; 

      In the above example we are creating a cursor ‘emp_cur’ on a query which returns the records of all the
      employees with salary greater than 5000. Here ‘emp_tbl’ in the table which contains records of all the
      employees.
2) Accessing the records in the cursor:
      Once the cursor is created in the declaration section we can access the cursor in the execution
      section of the PL/SQL program.

How to access an Explicit Cursor?

These are the three steps in accessing the cursor.
1) Open the cursor.
2) Fetch the records in the cursor one at a time.
3) Close the cursor.
General Syntax to open a cursor is:

OPEN cursor_name;

General Syntax to fetch records from a cursor is:

FETCH cursor_name INTO record_name;

OR 

FETCH cursor_name INTO variable_list;

General Syntax to close a cursor is:

CLOSE cursor_name;

When a cursor is opened, the first row becomes the current row. When the data is fetched it is copied to the record or variables and the logical pointer moves to the next row and it becomes the current row. On every fetch statement, the pointer moves to the next row. If you want to fetch after the last row, the program will throw an error. When there is more than one row in a cursor we can use loops along with explicit cursor attributes to fetch all the records.
Points to remember while fetching a row:
· We can fetch the rows in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL Block.
· If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
· If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.
General Form of using an explicit cursor is:

 DECLARE

    variables;

    records;

    create a cursor;

 BEGIN 

   OPEN cursor;

   FETCH cursor;

     process the records;

   CLOSE cursor;

 END;

Lets Look at the example below

Example 1:

1> DECLARE 

2>    emp_rec emp_tbl%rowtype;

3>    CURSOR emp_cur IS 

4>    SELECT *

5>    FROM 

6>    WHERE salary > 10; 

7> BEGIN 

8>    OPEN emp_cur; 

9>    FETCH emp_cur INTO emp_rec; 

10>      dbms_output.put_line (emp_rec.first_name || '  ' || emp_rec.last_name); 

11>   CLOSE emp_cur; 

12> END; 

In the above example, first we are creating a record ‘emp_rec’ of the same structure as of table ‘emp_tbl’ in line no 2. We can also create a record with a cursor by replacing the table name with the cursor name. Second, we are declaring a cursor ‘emp_cur’ from a select query in line no 3 - 6. Third, we are opening the cursor in the execution section in line no 8. Fourth, we are fetching the cursor to the record in line no 9. Fifth, we are displaying the first_name and last_name of the employee in the record emp_rec in line no 10. Sixth, we are closing the cursor in line no 11.

What are Explicit Cursor Attributes?

Oracle provides some attributes known as Explicit Cursor Attributes to control the data processing while using cursors. We use these attributes to avoid errors while accessing cursors through OPEN, FETCH and CLOSE Statements.

When does an error occur while accessing an explicit cursor?

a) When we try to open a cursor which is not closed in the previous operation.
b) When we try to fetch a cursor after the last operation.
These are the attributes available to check the status of an explicit cursor.

Attributes	Return values	Example
%FOUND	TRUE, if fetch statement returns at least one row.	Cursor_name%FOUND
	FALSE, if fetch statement doesn’t return a row.
%NOTFOUND	TRUE, , if fetch statement doesn’t return a row.	Cursor_name%NOTFOUND
	FALSE, if fetch statement returns at least one row.
%ROWCOUNT	The number of rows fetched by the fetch statement	Cursor_name%ROWCOUNT
	If no row is returned, the PL/SQL statement returns an error.
%ISOPEN	TRUE, if the cursor is already open in the program	Cursor_name%ISNAME
	FALSE, if the cursor is not opened in the program.

Using Loops with Explicit Cursors:

Oracle provides three types of cursors namely SIMPLE LOOP, WHILE LOOP and FOR LOOP. These loops can be used to process multiple rows in the cursor. Here I will modify the same example for each loops to explain how to use loops with cursors.

Cursor with a Simple Loop:

1> DECLARE 

2>   CURSOR emp_cur IS 

3>   SELECT first_name, last_name, salary FROM emp_tbl; 

4>   emp_rec emp_cur%rowtype; 

5> BEGIN 

6>   IF NOT sales_cur%ISOPEN THEN 

7>      OPEN sales_cur; 

8>   END IF; 

9>   LOOP 

10>     FETCH emp_cur INTO emp_rec; 

11>     EXIT WHEN emp_cur%NOTFOUND; 

12>     dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 

13>     || ' ' ||emp_cur.salary); 

14>  END LOOP; 

15>  END; 

16>  / 

In the above example we are using two cursor attributes %ISOPEN and %NOTFOUND.
In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9.
In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues.
We can use %FOUND in place of %NOTFOUND and vice versa. If we do so, we need to reverse the logic of the program. So use these attributes in appropriate instances.

Cursor with a While Loop:

Lets modify the above program to use while loop.

1> DECLARE 

2>  CURSOR emp_cur IS 

3>  SELECT first_name, last_name, salary FROM emp_tbl; 

4>  emp_rec emp_cur%rowtype; 

5> BEGIN 

6>   IF NOT sales_cur%ISOPEN THEN 

7>      OPEN sales_cur; 

8>   END IF; 

9>   FETCH sales_cur INTO sales_rec;  

10>  WHILE sales_cur%FOUND THEN  

11>  LOOP 

12>    dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 

13>    || ' ' ||emp_cur.salary); 

15>    FETCH sales_cur INTO sales_rec; 

16>  END LOOP; 

17> END; 

18> / 

In the above example, in line no 10 we are using %FOUND to evaluate if the first fetch statement in line no 9 returned a row, if true the program moves into the while loop. In the loop we use fetch statement again (line no 15) to process the next row. If the fetch statement is not executed once before the while loop the while condition will return false in the first instance and the while loop is skipped. In the loop, before fetching the record again, always process the record retrieved by the first fetch statement, else you will skip the first row.

Cursor with a FOR Loop:

When using FOR LOOP you need not declare a record or variables to store the cursor values, need not open, fetch and close the cursor. These functions are accomplished by the FOR LOOP automatically.

General Syntax for using FOR LOOP:

FOR record_name IN cusror_name 

LOOP 

    process the row...

END LOOP; 

Let’s use the above example to learn how to use for loops in cursors.

1> DECLARE 

2>  CURSOR emp_cur IS 

3>  SELECT first_name, last_name, salary FROM emp_tbl; 

4>  emp_rec emp_cur%rowtype; 

5> BEGIN 

6>  FOR emp_rec in sales_cur 

7>  LOOP  

8>  dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name 

9>    || ' ' ||emp_cur.salary);  

10> END LOOP; 

11>END;

12> /

In the above example, when the FOR loop is processed a record ‘emp_rec’of structure ‘emp_cur’ gets created, the cursor is opened, the rows are fetched to the record ‘emp_rec’ and the cursor is closed after the last row is processed. By using FOR Loop in your program, you can reduce the number of lines in the program.
NOTE: In the examples given above, we are using backward slash ‘/’ at the end of the program. This indicates the oracle engine that the PL/SQL program has ended and it can begin processing the statements.