What is a cursor?
Oracle uses a work area to execute SQL commands
and store processing information. PL/SQL
allows you to access this area through a name using a cursor.
Cursors that you use in PL/SQL are of two types:
* Implicit cursor
* Explicit cursor
Implicit Cursor
PL/SQL declares an implicit cursor for every DML
command, and queries that return a single row.
The name of the implicit cursor is SQL. You can directly use this cursor without any
declaration. We will see more about this
later in this chapter.
Explicit Cursor
PL/SQL’s implicit cursor can handle only
single-row queries. If you ever need to select more than one row using SELECT
in PL/SQL then you have to use explicit cursor.
When do we need an explicit cursor?
SELECT command in PL/SQL block can retrieve only
one row. If SELECT command retrieves no row then NO_DATA_FOUND exception will
be raised. If SELECT retrieves more than one row then TOO_MANY_ROWS exception
occurs.
So, a SELECT command will succeed only when it
retrieves a single row. The reason for this is; SELECT command copies the
values of columns that it retrieved into variables. If multiple rows are
retrieved then multiple values for each column are to be copied to a single
variable and that creates the problem.
declare
v_ccode varchar2(5);
v_fee
number(5);
begin
select ccode,fee into
v_ccode, v_fee
from courses
where duration > 25;
end;
SELECT command in the above example will raise
TOO_MANY_ROWS exception if more than one course is having duration more than
25.
An explicit cursor is the solution to the
problem. A cursor can store a collection of records retrieved by a query. Then
it allows us to fetch one record from cursor at a time and thereby enabling to
process all the records in the cursor.
As you can see in figure 1, SELECT retrieves
rows from database into cursor. Cursor stores the collection of record
retrieved by SELECT. Then the program can fetch one row at a time from cursor
and apply the required process to it.
Handling explicit cursor
Explicit cursor is a name used to refer to an
area where you can place multiple rows retrieved by SELECT. You must use an
explicit cursor whenever you have to use a multi-row query in PL/SQL.
The following are the steps required to create
and use an explicit cursor:
* Declare the cursor in Declare section
* Open the cursor using OPEN statement in Executable part
* Fetch one row at a time using FETCH statement.
* Close the cursor after all the records in the cursor are fetched and
processed by using CLOSE.
The following section will discuss each step in
detail.
Declaring a cursor
A cursor is declared in Declare section using
CURSOR statement. At the time of declaration the name of the cursor and the
associated SELECT statement are mentioned.
CURSOR cursor_name [(parameter[,
parameter]...)]
IS select_statement
[FOR UPDATE [OF column,column, . . . ];
The following example shows how to declare a
cursor.
declare
cursor
course_det is
select ccode, name, fee
from courses;
begin
end;
COURSE_DET is the name of the cursor, which will
be populated with the rows retrieved by the given SELECT at the time of opening
the cursor.
We will discuss more about parameters in
declaration of cursor in the section “Input Arguments”.
The following example declares a cursor that
takes course code and number of batches of the course in the ascending order of
number of batches.
Opening a cursor using OPEN statement
OPEN statement is used to execute the SELECT
command associated with the cursor and place the rows retrieved by the query
into cursor.
OPEN
cursor_name [(input arguments)];
Cursor_name is the name of the cursor that is to be opened.
Input_arguments are the values to be passed to the parameters of the cursor. More
on this later in the section “Input Arguments”.
The following statement opens the cursor COURSE_DET and places the rows retrieved by
the query into the cursor.
declare
cursor
course_det is
select ccode, name, fee
from courses;
begin
open
course_det;
end;
Fetching rows using FETCH statement
Once cursor is opened using OPEN statement,
cursor has a set of rows, which can be fetched
using FETCH statement.
FETCH statement takes the data of the current row in the cursor and
copies the values of the columns into variables given after INTO keyword.
FETCH
cursor_name INTO variable-1,
variable-2, . . .;
For each column in the cursor there should be a
corresponding variable in FETCH
statement. Also make sure the data types of variables and corresponding
columns are matching.
The following snippet demonstrates how to fetch
and copy data from current row of the cursor to variables given after INTO.
declare
cursor
course_det is
select ccode, name, fee
from courses;
v_ccode courses.ccode%type;
v_name courses.name%type;
v_fee courses.fee%type;
begin
open
course_det;
loop
fetch course_det into
v_ccode, v_name, v_fee;
. . .
end
loop;
end;
FETCH statement is to be used inside the loop to
repeatedly fetch rows from the cursor. The process of fetching should stop once
all rows of the cursor are fetch (reached end of cursor).
The following code will show how to exit cursor
when cursor is completely processed.
loop
fetch course_det into
v_ccode, v_name, v_fee;
exit when course_det%notfound;
end
loop;
NOTFOUND attribute of the cursor returns TRUE
when previous FETCH doesn’t successfully read a row from cursor.
We will discuss more about attributes of
explicit and implicit cursors later in his chapter.
Closing a cursor using CLOSE command
CLOSE statement is used to close cursor after
the cursor is processed. Closing a
cursor would release the resources associated with cursor.
CLOSE cursor_name;
The following example closes COURSE_DET cursor:
declare
begin
open ..
loop
...
end
loop;
close
course_det;
end;
Cursor For Loop
In order to process a cursor, you can use cursor
FOR loop to automate the following steps.
q Opening cursor
q Fetching rows from the cursor
q Terminating loop when all rows in the cursor are fetched
q Closing cursor
The following is the syntax of cursor for loop.
This for loop is specifically meant to process cursors.
FOR
rowtype_variable IN cursor_name
LOOP
Statements;
END
LOOP;
rowtype_variable
is automatically declared by cursor for loop. It is of ROWTYPE of the cursor.
It has columns of the cursor as fields. These fields can be accessed using rowtype_variable.fieldname.
The following example shows the process involved
without using cursor for loop and using for loop.
DECLARE
cursor courses_cursor is
select ccode,fee
from courses;
BEGIN
-- open
cursor
open
courses_cursor;
loop
fetch courses_cursor into
v_ccode, v_fee;
--
if previous fetch failed then exit loop
--
NOTFOUND attribute of the cursor return true if
--
previous fetch has failed.
exit when courses_cursor%notfound;
--
process the record fetched from cursor
.
. .
end
loop;
close courses_cursor;
END;
The same program can also be written using
cursor for loop as follows:
declare
cursor courses_cursor is
select ccode,fee
from courses;
begin
--
cursor is opened and one row at a time is fetched
--
loop is automatically terminated if all records are fetched
for
rec in courses_cursor
loop
--
process the record fetched from cursor
if rec.fee > 5000 then
-- do something here
end if;
end
loop;
end;
The following are the important steps in the
above program:
* Cursor COURSES_CURSOR is automatically opened by cursor for loop.
* REC is declared automatically by cursor for loop as:
REC courses_cursor%ROWTYPE;
But REC
is available only inside the cursor for loop. It contains the same columns as
the cursor. In order to access a column of the current row of the cursor, in
the cursor for loop, use the format:
rowtype_variable.columnname
* Statements in the cursor for loop are executed once for each row of
the cursor. And for each row of the
cursor, row is copied into rowtype_variable.
* Loop is terminated once end of cursor is reached. And cursor is closed.
The following section will summarize what we
have seen so far by showing how to write a simple program using cursor.
Sample program
Let us now put all pieces together and write a
complete program that makes use of cursor.
The sample program is to update FEE of each
course based on the following table.
|
No. of students
|
Percentage
of change
|
|
> 10
|
10%
increase
|
|
> 20
|
15%
increase
|
|
Otherwise
|
5%
decrease
|
/* Author : P.Srikanth
Date : 22-10-2001
Place : Vizag.
Purpose: Sample cursor program to illustrate how to handle a cursor*/
declare
cursor
courses_cursor is
select c.ccode, count(*) count
from
batches b, students s
where
c.ccode = b.ccode and b.bcode = s.bcode
group
by c.ccode;
v_per number(5,2);
begin
-- rec is automatically declared
for rec in courses_cursor
loop
if
rec.count > 20 then
v_per:= 1.15;
elsif rec.count > 10 then
v_per := 1.10;
else
v_per := 0.90;
end if;
-- update row in the table
update courses set fee = fee *
v_per
where ccode = rec.ccode;
end loop;
end;
/
The above program is used to declare a cursor
that takes course code and no. of students joined into that course using a
SELECT that joins BATCHES and STUDENTS table. Depending upon the no. of
students joined into each course, it updates the course fee of the course.
It uses cursor for loop to take one row at a
time from COURSES_CURSOR and updates the FEE of COURSES table after the
process.
Implicit cursor
Oracle implicitly opens a cursor to process each
SQL statement not associated with an explicitly declared cursor. You can refer
to this cursor using the name SQL.
You cannot use the
OPEN, FETCH, and CLOSE
statements with SQL cursor. But, you can use cursor attributes to get
information about the most recently executed SQL statement.
The following example shows how to use implicit
cursor to know whether the most recent UPDATE has updated any rows or not.
declare
begin
update . . .
if SQL%NOTFOUND then
statements;
end
if;
end;
NOTFOUND is an attribute of implicit cursor that
returns true if previous UPDATE command has not affected any row.
Cursor attributes
Cursor attributes allow you to get information
regarding cursor. For example, you can get the number of rows fetched so far
from a cursor using ROWCOUNT attribute and whether a row is fetched or not
using FOUND attribute.
Use the following syntax to access cursor
attributes:
cursor_name%Attribute
Explicit cursor attributes
The following is the list of available cursor
attributes and their meaning for explicit cursor.
|
Attribute
|
What it returns?
|
|
NOTFOUND
|
True, if previous fetch failed.
|
|
FOUND
|
True, if previous fetch succeeded.
|
|
ROWCOUNT
|
Number of rows fetched from cursor so far.
|
|
ISOPEN
|
True, if cursor is currently open
|
Table 1: Explicit Cursor Attributes.
Implicit cursor attributes
Cursor attributes do not have the same meaning
for both explicit and implicit cursors. The following table shows the meaning
of cursor attributes with implicit cursor.
|
Attribute
|
What it returns?
|
|
NOTFOUND
|
True, if previous DML operation didn’t affect
any row.
|
|
FOUND
|
True, if previous DML operation affected any
row.
|
|
ROWCOUNT
|
Number of rows affected by the most recent DML
operation.
|
Table 2: Implicit Cursor Attributes.
The following example shows how to use ROWCOUNT
attribute with implicit cursor to know how many rows were updated with most
recent UPDATE command.
begin
update courses set fee = fee
& 1.1
where duration > 25;
/* if more than 5 rows are
effected
then rollback updation */
if SQL%ROWCOUNT > 5
then
rollback;
else
commit;
end if;
end;
The following is how you use cursor attributes
with explicit cursors. Attribute NOTFOUND returns true if previous FETCH
statement couldn’t fetch any row.
LOOP
fetch courses_cursor into v_ccode, v_fee;
/*
exit loop if previous FETCH failed */
exit when students_cursor%NOTFOUND;
/*
process the record fetched */
END LOOP;
In the above example EXIT is executed when
NOTFOUND attribute of cursor
CORUSES_CURSOR returns TRUE.
Input arguments to cursor
Input arguments are the values that are passed
to cursor at the time of opening the cursor. These values are passed to
parameters that are declared in cursor and they are used while executing the
query.
The following example will illustrate the
process.
declare
cursor batch_cursor ( pccode
varchar2) is
select * from batches where ccode = pccode;
begin
--
open the cursor and get all batches of course oracle.
open batch_cursor(‘ora’);
--
process the cursor
close batch_cursor;
--
open the same cursor but with a different course code
open batch_cursor(‘vb’);
--
process the cursor, which contains details of batches of vb
close batch_cursor;
end;
Cursor BATCH_CURSOR is declared to take a
parameter or input argument – PCCODE. The parameter is used to retrieve details
of the batches of the given course.
At the time of opening the cursor, the value for
parameter PCCODE is to be passed by enclosing the value within parentheses. The
value is passed to PCCODE, which is then used in the query of the cursor.
The advantage of the input arguments, as you can
see in the example above, is that you can use the same cursor to represent
different sets of records at different points of time. In the above example,
first cursor contains the batches of Oracle course then that cursor is closed
and reopened with course code VB to take the details of batches of VB.
In case of cursor for loop, values to input
arguments are passed at the time of using cursor in for loop as follows:
for rec in batch_cursor(‘ora’)
for rec in batch_cursor(‘ora’)
loop
...
end
loop;
1.
Note: The size of data type of input arguments should not be given. That
means, we should give only VARCHAR2 and not VARCHAR2 (10).
FOR UPDATE and CURRENT OF
By default, Oracle locks rows while manipulating
the rows. But it is possible to override default locking by using FOR UPDATE
option of SELECT command.
FOR UPDATE option clause can be used with SELECT
while declaring cursor to lock all records retrieved by cursor to make sure
they are not locked by others before we update or delete them. As Oracle automatically locks rows for you,
FOR UPDATE clause is required only when you want to lock rows ahead of update
or delete - at the time of opening cursor.
CURRENT OF clause with UPDATE and DELETE
commands can be used to refer to the current row in the cursor.
2.
Note: FOR UPDATE must be given if you
want to use CURRENT OF clause to refer to current row in the cursor.
The following example shows how to use FOR
UPDATE OF and CURRENT
OF clauses:
DECLARE
CURSOR
course_details IS
SELECT ccode, name, fee from courses
FOR
UPDATE OF fee;
...
BEGIN
OPEN
course_details;
LOOP
FETCH course_details INTO...
...
/*
Update the current record in the cursor
course_details */
UPDATE courses SET fee = new_fee
WHERE CURRENT OF course_details;
END
LOOP;
CLOSE course_details;
END;
Summary
A cursor is always used by PL/SQL to execute
single-row queries and DML command. But, in order to use multi-row query, you
have to use an explicit cursor. An explicit cursor contains a row-set, which is
retrieved by multi-row query. Implicit cursor is used to get information about
the most recent DML operation.
Cursor FOR loop is used to open, fetch rows
until end of cursor, and close the cursor.
Input arguments of the cursor can be used to pass
values to cursor at the time of opening cursor so that these values are used by
SELECT command of the cursor. FOR UPDATE clause is used to override default
locking and CURRENT OF is used to refer to current record in the cursor.
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