CURSOR:Cursor is a database objects to retrieve data from a result set one row at a time, instead of the T-SQL commands that operate on all the rows in the result set at one time. We use cursor when we need to update records in a database table in singleton fashion means row by row.
Components Of Cursor:
1. Declare Cursor: A cursor is declared by defining the SQL statement that returns a result
set.
2. Open: A Cursor is opened and populated by
executing the SQL statement defined by the cursor.
3. Fetch:
4. Close
After data manipulation, we should
close the cursor explicitly.
5. Deallocate
Finally, we need to delete the cursor
definition and released all the system resources associated with the cursor.
Syntax to Declare Cursor
Declare Cursor SQL
Comaand is used to define the cursor with many options that impact the
scalablity and loading behaviour of the cursor. The basic syntax is given below
DECLARE cursor_name CURSOR
[LOCAL | GLOBAL] --define cursor scope
[FORWARD_ONLY | SCROLL] --define cursor movements (forward/backward) [STATIC | KEYSET | DYNAMIC |
FAST_FORWARD] --basic type of cursor [READ_ONLY | SCROLL_LOCKS |
OPTIMISTIC] --define locks FOR select_statement --define SQL Select statement FOR UPDATE [col1,col2,...coln] --define columns
that need to be updated
Type of Cursor
cursor allow us to retrieve
data from a result set in singleton fashion means row by row. Cursor are
required when we need to update records in a database table one row at a time.
A Cursor impacts
the performance of the SQL Server since it uses the SQL Server instances'
memory, reduce concurrency, decrease network bandwidth and lock resources.
Hence it is mandatory to understand the cursor types and its functions so that
you can use suitable cursor according to your needs.
Types of Cursors
1. Static Cursors
A static cursor populates the result
set at the time of cursor creation and query result is cached for the lifetime
of the cursor. A static cursor can move forward and backward direction. A
static cursor is slower and use more memory in comparison to other cursor.
Hence you should use it only if scrolling is required and other types of
cursors are not suitable.
You can't update, delete data using
static cursor. It is not sensitive to any changes to the original data source.
By default static cursors are scrollable.
2. Dynamic Cursors
A dynamic cursor allows you to see
the data updation, deletion and insertion in the data source while the cursor
is open. Hence a dynamic cursor is sensitive to any changes to the data source
and supports update, delete operations. By default dynamic cursors are
scrollable.
3. Forward Only Cursors
A forward only cursor is the fastest
cursor among the all cursors but it doesn't support backward scrolling. You can
update, delete data using Forward Only cursor. It is sensitive to any changes
to the original data source.
There are three more types of Forward
Only Cursors.Forward_Only KEYSET, FORWARD_ONLY STATIC and FAST_FORWARD.
A FORWARD_ONLY STATIC
Cursor is populated at the time of creation
and cached the data to the cursor lifetime. It is not sensitive to any changes
to the data source.
A FAST_FORWARD Cursor is the fastest cursor and it is not sensitive to any changes to the data
source.
4. Keyset Driven Cursors
A keyset driven cursor is controlled
by a set of unique identifiers as the keys in the keyset. The keyset depends on
all the rows that qualified the SELECT statement at the time of cursor was
opened. A keyset driven cursor is sensitive to any changes to the data source
and supports update, delete operations. By default keyset driven cursors are
scrollable.
SQL SERVER – Examples of Cursors
-------Create Table-----------------
CREATE TABLE EmployeeDetails(
EmployeeID int PRIMARY KEY,
EmployeeName varchar (50) NOT NULL,
Salary int NOT NULL,
Address varchar (200) NOT NULL,
)
-----Insert values in Table---------
insert into EmployeeDetails values(5,'Ravi',11000,'Noida'),(6,'Naveen',12000,'Delhi'),
(7,'shivam',13000,'UP'),(4,'Kavita',14000,'Kanpur'),(5,'Mukesh',15000,'Orissa')
1.Static Cursor - Example
2.Dynamic Cursor - Example
---------Dynamic Cursor For Update------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Dynamic_cur_employeeupdate
CURSOR DYNAMIC FOR
SELECT EmployeeID,EmployeeName from Employeedetails ORDER BY EmployeeName
OPEN Dynamic_cur_employeeupdate
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Dynamic_cur_employeeupdate INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='naveen'
Update EmployeeDetails SET Salary=25000
WHERE CURRENT OF Dynamic_cur_employeeupdate
FETCH NEXT FROM Dynamic_cur_employeeupdate INTO @EmployeeId,@Employeename
END
END
CLOSE Dynamic_cur_employeeupdate
DEALLOCATE Dynamic_cur_employeeupdate
select * from EmployeeDetails
---------Dynamic Cursor For Delete------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Dynamic_cur_employeedelete
CURSOR DYNAMIC FOR
SELECT EmployeeID,EmployeeName from EmployeeDetails ORDER BY EmployeeName
OPEN Dynamic_cur_employeedelete
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Dynamic_cur_employeedelete INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='mukesh'
DELETE EmployeeDetails WHERE CURRENT OF Dynamic_cur_employeedelete
FETCH NEXT FROM Dynamic_cur_employeedelete INTO @EmployeeId,@Employeename
END
END
CLOSE Dynamic_cur_employeedelete
DEALLOCATE Dynamic_cur_employeedelete
select * from EmployeeDetails
3.Forward Only Cursor - Example
-------------Forward Only Cursor for Update--------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Forward_cur_employeeupdate
CURSOR FORWARD_ONLY FOR
SELECT EmployeeID,EmployeeName from Employeedetails
ORDER BY EmployeeName
OPEN Forward_cur_employeeupdate
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Forward_cur_employeeupdate INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='Ravi'
Update EmployeeDetails SET Salary=24000 WHERE CURRENT OF Forward_cur_employeeupdate
FETCH NEXT FROM Forward_cur_employeeupdate INTO @EmployeeId,@Employeename
END
END
CLOSE Forward_cur_employeeupdate
DEALLOCATE Forward_cur_employeeupdate
Select * from EmployeeDetails
------------- Forward Only Cursor for Delete------------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Forward_cur_employeedelete CURSOR
FORWARD_ONLY
FOR
SELECT EmployeeID,EmployeeName from EmployeeDetails ORDER BY EmployeeName
OPEN Forward_cur_employeedelete
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Forward_cur_employeedelete INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='shivam'
DELETE EmployeeDetails WHERE CURRENT OF Forward_cur_employeedelete
FETCH NEXT FROM Forward_cur_employeedelete INTO @EmployeeId,@Employeename
END
END
CLOSE Forward_cur_employeedelete
DEALLOCATE Forward_cur_employeedelete
\
Select * from EmployeeDetails
4.Keyset Driven Cursor - Example
------------- Keyset driven Cursor for Update----------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Keyset_cur_employeeupdate
CURSOR KEYSET FOR
SELECT EmployeeID,EmployeeName from EmployeeDetails ORDER BY EmployeeName
OPEN Keyset_cur_employeeupdate
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Keyset_cur_employeeupdate INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='Naveen'
Update EmployeeDetails SET Salary=27000 WHERE CURRENT OF Keyset_cur_employeeupdate
FETCH NEXT FROM Keyset_cur_employeeupdate INTO @EmployeeId,@Employeename
END
END
CLOSE Keyset_cur_employeeupdate
DEALLOCATE Keyset_cur_employeeupdate
Select * from EmployeeDetails
------------ Keyset Driven Cursor for Delete--------------
DECLARE @EmployeeId int
DECLARE @Employeename varchar(50)
DECLARE Keyset_cur_employeedelete
CURSOR KEYSET FOR
SELECT EmployeeID,EmployeeName from Employeedetails ORDER BY EmployeeName
OPEN Keyset_cur_employeedelete
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM Keyset_cur_employeedelete INTO @EmployeeId,@Employeename
WHILE @@Fetch_status = 0
BEGIN
IF @Employeename='shivam'
DELETE EmployeeDetails WHERE CURRENT OF Keyset_cur_employeedelete
FETCH NEXT FROM Keyset_cur_employeedelete INTO @EmployeeId,@Employeename
END
END
CLOSE Keyset_cur_employeedelete
DEALLOCATE Keyset_cur_employeedelete
Select * from EmployeeDetails
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