Saturday, June 25, 2016

Types of Cursor in MSSQL

SQL Server Different Types of Cursors

           
A 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. I have already explained the basic of cursor.
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.
You should avoid the use of cursor. Basically you should use cursor alternatives like as WHILE loop, sub queries, Temporary tables and Table variables. We should use cursor in that case when there is no option except cursor.

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.
    No UPDATE, INSERT, or DELETE operations are reflected in a static cursor (unless the cursor is closed and reopened). By default static cursors are scrollable. SQL Server static cursors are always read-only.
  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

  1. CREATE TABLE Employee
  2. (
  3. EmpID int PRIMARY KEY,
  4. EmpName varchar (50) NOT NULL,
  5. Salary int NOT NULL,
  6. Address varchar (200) NOT NULL,
  7. )
  8. GO
  9. INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(1,'Mohan',12000,'Noida')
  10. INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(2,'Pavan',25000,'Delhi')
  11. INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(3,'Amit',22000,'Dehradun')
  12. INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(4,'Sonu',22000,'Noida')
  13. INSERT INTO Employee(EmpID,EmpName,Salary,Address) VALUES(5,'Deepak',28000,'Gurgaon')
  14. GO
  15. SELECT * FROM Employee

Static Cursor - Example

  1. SET NOCOUNT ON
  2. DECLARE @Id int
  3. DECLARE @name varchar(50)
  4. DECLARE @salary int
  5. DECLARE cur_emp CURSOR
  6. STATIC FOR
  7. SELECT EmpID,EmpName,Salary from Employee
  8. OPEN cur_emp
  9. IF @@CURSOR_ROWS > 0
  10. BEGIN
  11. FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
  12. WHILE @@Fetch_status = 0
  13. BEGIN
  14. PRINT 'ID : '+ convert(varchar(20),@Id)+', Name : '+@name+ ', Salary : '+convert(varchar(20),@salary)
  15. FETCH NEXT FROM cur_emp INTO @Id,@name,@salary
  16. END
  17. END
  18. CLOSE cur_emp
  19. DEALLOCATE cur_emp
  20. SET NOCOUNT OFF

Dynamic Cursor - Example

  1. --Dynamic Cursor for Update
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Dynamic_cur_empupdate CURSOR
  6. DYNAMIC
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Dynamic_cur_empupdate
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Dynamic_cur_empupdate INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Mohan'
  16. Update Employee SET Salary=15000 WHERE CURRENT OF Dynamic_cur_empupdate
  17. FETCH NEXT FROM Dynamic_cur_empupdate INTO @Id,@name
  18. END
  19. END
  20. CLOSE Dynamic_cur_empupdate
  21. DEALLOCATE Dynamic_cur_empupdate
  22. SET NOCOUNT OFF
  23. Go
  24. Select * from Employee

  1. -- Dynamic Cursor for DELETE
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Dynamic_cur_empdelete CURSOR
  6. DYNAMIC
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Dynamic_cur_empdelete
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Dynamic_cur_empdelete INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Deepak'
  16. DELETE Employee WHERE CURRENT OF Dynamic_cur_empdelete
  17. FETCH NEXT FROM Dynamic_cur_empdelete INTO @Id,@name
  18. END
  19. END
  20. CLOSE Dynamic_cur_empdelete
  21. DEALLOCATE Dynamic_cur_empdelete
  22. SET NOCOUNT OFF
  23. Go
  24. Select * from Employee

Forward Only Cursor - Example

  1. --Forward Only Cursor for Update
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Forward_cur_empupdate CURSOR
  6. FORWARD_ONLY
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Forward_cur_empupdate
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Forward_cur_empupdate INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Amit'
  16. Update Employee SET Salary=24000 WHERE CURRENT OF Forward_cur_empupdate
  17. FETCH NEXT FROM Forward_cur_empupdate INTO @Id,@name
  18. END
  19. END
  20. CLOSE Forward_cur_empupdate
  21. DEALLOCATE Forward_cur_empupdate
  22. SET NOCOUNT OFF
  23. Go
  24. Select * from Employee

  1. -- Forward Only Cursor for Delete
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Forward_cur_empdelete CURSOR
  6. FORWARD_ONLY
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Forward_cur_empdelete
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Forward_cur_empdelete INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Sonu'
  16. DELETE Employee WHERE CURRENT OF Forward_cur_empdelete
  17. FETCH NEXT FROM Forward_cur_empdelete INTO @Id,@name
  18. END
  19. END
  20. CLOSE Forward_cur_empdelete
  21. DEALLOCATE Forward_cur_empdelete
  22. SET NOCOUNT OFF
  23. Go
  24. Select * from Employee

Keyset Driven Cursor - Example

  1. -- Keyset driven Cursor for Update
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Keyset_cur_empupdate CURSOR
  6. KEYSET
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Keyset_cur_empupdate
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Keyset_cur_empupdate INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Pavan'
  16. Update Employee SET Salary=27000 WHERE CURRENT OF Keyset_cur_empupdate
  17. FETCH NEXT FROM Keyset_cur_empupdate INTO @Id,@name
  18. END
  19. END
  20. CLOSE Keyset_cur_empupdate
  21. DEALLOCATE Keyset_cur_empupdate
  22. SET NOCOUNT OFF
  23. Go
  24. Select * from Employee

  1. -- Keyse Driven Cursor for Delete
  2. SET NOCOUNT ON
  3. DECLARE @Id int
  4. DECLARE @name varchar(50)
  5. DECLARE Keyset_cur_empdelete CURSOR
  6. KEYSET
  7. FOR
  8. SELECT EmpID,EmpName from Employee ORDER BY EmpName
  9. OPEN Keyset_cur_empdelete
  10. IF @@CURSOR_ROWS > 0
  11. BEGIN
  12. FETCH NEXT FROM Keyset_cur_empdelete INTO @Id,@name
  13. WHILE @@Fetch_status = 0
  14. BEGIN
  15. IF @name='Amit'
  16. DELETE Employee WHERE CURRENT OF Keyset_cur_empdelete
  17. FETCH NEXT FROM Keyset_cur_empdelete INTO @Id,@name
  18. END
  19. END
  20. CLOSE Keyset_cur_empdelete
  21. DEALLOCATE Keyset_cur_empdelete
  22. SET NOCOUNT OFF
  23. Go Select * from Employee

 

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