In PL/SQL, cursors are pointers to the context area in memory where SQL query results are stored.
They are used to fetch and manipulate query results
row-by-row. Implicit cursors are automatically
created for single-row queries and DML statements, while
explicit cursors must be explicitly declared
and controlled for multi-row queries.
Explicit cursors provide fine control over the fetching
process,
allowing the use of attributes like %FOUND, %NOTFOUND,
%ROWCOUNT, and %ISOPEN.
Cursors can also be parameterized to accept arguments dynamically.
IMPLICIT CURSOR
Implicit cursors are automatically created by Oracle when a
SQL statement is executed.
They are used for single-row queries and do not need to be
explicitly declared or managed by the programmer.
Implicit cursors are suitable for simple queries that return
a single result.
DECLARE
v_emp_name
employees.first_name%TYPE;
BEGIN
SELECT first_name
INTO v_emp_name FROM employees WHERE employee_id = 100;
DBMS_OUTPUT.PUT_LINE('Employee name: ' || v_emp_name);
END;
In PL/SQL, an implicit cursor is automatically created and
managed by the Oracle engine when you execute a SQL query that returns a single
row.
· DECLARE: This keyword
starts the declaration section where variables (v_emp_name in this case) are defined.
- v_emp_name
employees.first_name%TYPE;: Declares v_emp_name as a variable of type employees.first_name%TYPE. This uses the %TYPE attribute to associate v_emp_name with the data type of the first_name column in the employees table.
· BEGIN: Marks the beginning
of the executable section of the PL/SQL block, where actual logic and
statements are written.
· SELECT first_name INTO
v_emp_name FROM employees WHERE employee_id = 100;: This is the SQL query
that uses an implicit cursor.
- SELECT
first_name INTO v_emp_name: Executes a query to retrieve the first_name of an employee whose employee_id is 100 from the employees table. The INTO clause assigns the retrieved
value to the variable v_emp_name.
· DBMS_OUTPUT.PUT_LINE('Employee
name: ' || v_emp_name);: Outputs the value of v_emp_name to the console using the DBMS_OUTPUT.PUT_LINE procedure. This displays the
employee's first name based on the query result.
· END;: Marks the end of the
PL/SQL block.
EXPLICIT CURSOR
Explicit cursors are defined and managed by the programmer.
They provide more control and flexibility, especially for
queries that return multiple rows.
Explicit cursors have to be explicitly declared, opened,
fetched, and closed.
Declaration: Define the cursor's structure and query in the
declaration section.
Opening: Open the cursor to establish the result set.
Fetching: Retrieve rows one by one from the result set.
Closing: Close the cursor when done
DECLARE
CURSOR emp_cursor
IS
SELECT
employee_id, first_name, last_name FROM employees;
v_emp_id
employees.employee_id%TYPE;
v_emp_fname
employees.first_name%TYPE;
v_emp_lname
employees.last_name%TYPE;
BEGIN
OPEN emp_cursor;
LOOP
FETCH emp_cursor
INTO v_emp_id, v_emp_fname, v_emp_lname;
EXIT WHEN
emp_cursor%NOTFOUND;
DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_emp_id || ', Name: ' ||
v_emp_fname || ' ' || v_emp_lname);
END LOOP;
CLOSE emp_cursor;
END;
This PL/SQL code snippet demonstrates the use of a cursor to fetch and
process multiple rows from the employees
table:
1. DECLARE:
This keyword starts the declaration section where variables (emp_cursor, v_emp_id, v_emp_fname, v_emp_lname
in this case) are defined.
o
CURSOR
emp_cursor IS: Defines a cursor named emp_cursor that retrieves data from the employees table.
o
SELECT
employee_id, first_name, last_name FROM employees;: Specifies the
SQL query that the cursor will execute to fetch data.
o
v_emp_id
employees.employee_id%TYPE;: Declares v_emp_id as a variable of type employees.employee_id%TYPE, which
matches the data type of the employee_id
column in the employees
table.
o
v_emp_fname
employees.first_name%TYPE;: Declares v_emp_fname as a variable of type employees.first_name%TYPE, matching the
data type of the first_name
column.
o
v_emp_lname
employees.last_name%TYPE;: Declares v_emp_lname as a variable of type employees.last_name%TYPE, matching the
data type of the last_name
column.
2. BEGIN:
Marks the beginning of the executable section of the PL/SQL block, where actual
logic and statements are written.
3. OPEN
emp_cursor: Opens the cursor emp_cursor
to start fetching rows from the result set defined by the query.
4. LOOP:
Initiates a loop to fetch each row returned by the cursor until all rows have
been processed.
o
FETCH emp_cursor INTO v_emp_id,
v_emp_fname, v_emp_lname: Retrieves the next row from emp_cursor and assigns the values of employee_id, first_name, and last_name to v_emp_id, v_emp_fname, and v_emp_lname
respectively.
o
EXIT WHEN emp_cursor%NOTFOUND:
Checks if there are no more rows to fetch (%NOTFOUND
attribute of the cursor). If no more rows are found, the loop is exited.
o
DBMS_OUTPUT.PUT_LINE: Outputs
the fetched data (Employee ID,
Name) to the console using DBMS_OUTPUT.PUT_LINE.
5. END
LOOP: Marks the end of the loop structure.
6. CLOSE
emp_cursor: Closes the cursor emp_cursor
to release associated resources and complete the operation.
7. END;:
Marks the end of the PL/SQL block.
Explicit Cursor: Parameterized Cursors
Explicit cursors can also accept parameters, allowing you to
pass values into the query when opening the cursor.
DECLARE
CURSOR emp_cursor
(dept_id NUMBER) IS
SELECT
employee_id, first_name, last_name FROM employees WHERE department_id =
dept_id;
v_employee_id
employees.employee_id%TYPE;
v_first_name
employees.first_name%TYPE;
v_last_name
employees.last_name%TYPE;
BEGIN
OPEN
emp_cursor(10);
LOOP
FETCH
emp_cursor INTO v_employee_id, v_first_name, v_last_name;
EXIT WHEN
emp_cursor%NOTFOUND;
DBMS_OUTPUT.PUT_LINE('Employee ID: ' || v_employee_id || ', Name: ' ||
v_first_name || ' ' || v_last_name);
END LOOP;
CLOSE emp_cursor;
END;
This PL/SQL code snippet demonstrates the use of a cursor with parameters (a
parameterized cursor) to fetch and process rows based on a specific department
ID:
1. DECLARE:
This keyword starts the declaration section where variables (emp_cursor, v_employee_id, v_first_name, v_last_name in this case) are defined.
o
CURSOR
emp_cursor (dept_id NUMBER) IS: Defines a cursor named emp_cursor that accepts a parameter dept_id of type NUMBER.
o
SELECT
employee_id, first_name, last_name FROM employees WHERE department_id =
dept_id;: Specifies the SQL query that the cursor will execute.
It retrieves employee_id, first_name, and last_name for employees belonging to a
specific department identified by dept_id.
o
v_employee_id
employees.employee_id%TYPE;: Declares v_employee_id as a variable of type employees.employee_id%TYPE, matching the
data type of the employee_id
column in the employees
table.
o
v_first_name
employees.first_name%TYPE;: Declares v_first_name as a variable of type employees.first_name%TYPE, matching the
data type of the first_name
column.
o
v_last_name
employees.last_name%TYPE;: Declares v_last_name as a variable of type employees.last_name%TYPE, matching the
data type of the last_name
column.
2. BEGIN:
Marks the beginning of the executable section of the PL/SQL block, where actual
logic and statements are written.
3. OPEN
emp_cursor(10): Opens the cursor emp_cursor
with the parameter value 10.
This executes the cursor query for employees belonging to the department with department_id = 10.
4. LOOP:
Initiates a loop to fetch each row returned by the cursor until all rows have
been processed.
o
FETCH emp_cursor INTO v_employee_id,
v_first_name, v_last_name: Retrieves the next row from emp_cursor and assigns the values of employee_id, first_name, and last_name to v_employee_id, v_first_name, and v_last_name respectively.
o
EXIT WHEN emp_cursor%NOTFOUND:
Checks if there are no more rows to fetch (%NOTFOUND
attribute of the cursor). If no more rows are found, the loop is exited.
o
DBMS_OUTPUT.PUT_LINE: Outputs
the fetched data (Employee ID,
Name) to the console using DBMS_OUTPUT.PUT_LINE.
5. END
LOOP: Marks the end of the loop structure.
6. CLOSE
emp_cursor: Closes the cursor emp_cursor
to release associated resources and complete the operation.
7. END;:
Marks the end of the PL/SQL block.
Explicit Cursor: Cursor FOR Loops
A cursor FOR
loop simplifies cursor handling by implicitly declaring, opening, fetching, and
closing the cursor.
BEGIN
FOR emp_record IN (SELECT employee_id, first_name, last_name
FROM employees WHERE department_id = 10)
LOOP DBMS_OUTPUT.PUT_LINE('Employee ID: ' ||
emp_record.employee_id || ', Name: ' || emp_record.first_name || ' ' ||
emp_record.last_name);
END LOOP; END;
This PL/SQL code snippet demonstrates the use of a FOR loop to iterate over a
result set obtained from a SQL query:
· FOR emp_record IN (SELECT ...):
This line starts a FOR loop where emp_record is the loop index variable. The loop
iterates over each row returned by the SQL query (SELECT employee_id, first_name,
last_name FROM employees WHERE department_id = 10).
- SELECT
employee_id, first_name, last_name FROM employees WHERE department_id = 10: This SQL query retrieves employee_id, first_name, and last_name from the employees table where department_id =
10.
- emp_record is a record type implicitly
declared to match the structure of the columns selected in the query (employee_id, first_name, last_name).
· LOOP: Begins the loop
structure where each iteration processes one row fetched by the cursor
implicitly created by the FOR loop.
· DBMS_OUTPUT.PUT_LINE:
Outputs each employee's ID and full name (employee_id, first_name, last_name) to the console using DBMS_OUTPUT.PUT_LINE.
- emp_record.employee_id, emp_record.first_name, emp_record.last_name: These refer to the specific
fields (employee_id, first_name, last_name) of the current row being processed in the loop.
· END LOOP: Marks the end of
the loop structure.
· END;: Marks the end of the
PL/SQL block.
Comments
Post a Comment