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Thursday, March 15, 2012

Cursors in SQL


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 the active set.
In SQL procedures, a cursor make it possible to define a result set (a set of data rows) and perform complex logic on a row by row basis. By using the same mechanics, an SQL procedure can also define a result set and return it directly to the caller of the SQL procedure or to a client application.


A cursor can be viewed as a pointer to one row in a set of rows. The cursor can only reference one row at a time, but can move to other rows of the result set as needed.


To use cursors in SQL procedures, you need to do the following:
Declare a cursor that defines a result set.
Open the cursor to establish the result set.
Fetch the data into local variables as needed from the cursor, one row at a time.
Close the cursor when done
To work with cursors you must use the following SQL statements:
DECLARE CURSOR
OPEN
FETCH
CLOSE


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. 
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 below :- 



  •  %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.


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. 



example : SQL%FOUND 



  • %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.


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. 



example: SQL%NOTFOUND 



  • %ROWCOUNT :



Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT


example : 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. 



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