Cursors in SQL

Article

Cursors

A cursor is a temporary work area created in system memory when an SQL statement is executed. A cursor is a set of rows together with a pointer that identifies a current row. It is a database object to retrieve data from a result set one row at a time. It is useful when we want to manipulate the record of a table in a singleton method, in other words one row at a time. In other words, 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.

Types of Cursors

There are the following two types of Cursors:

Implicit Cursor
Explicit Cursor

Implicit Cursor

These types of cursors are generated and used by the system during the manipulation of a DML query (INSERT, UPDATE and DELETE). An implicit cursor is also generated by the system when a single row is selected by a SELECT command.

Explicit Cursor

This type of cursor is generated by the user using a SELECT command. An explicit cursor contains more than one row, but only one row can be processed at a time. An explicit cursor moves one by one over the records. An explicit cursor uses a pointer that holds the record of a row. After fetching a row, the cursor pointer moves to the next row.

Main components of Cursors

Each cursor contains the followings 5 parts:

Declare Cursor: In this part we declare variables and return a set of values.
Open: This is the entering part of the cursor.
Fetch: Used to retrieve the data row by row from a cursor.
Close: This is an exit part of the cursor and used to close a cursor.
Deallocate: In this part we delete the cursor definition and release all the system resources associated with the cursor.

Syntax of a Cursor

DECLARE @Variable nvarchar(50) /* Declare All Required Variables */
DECLARE Cursor_Name CURSOR /* Declare Cursor Name*/
[LOCAL | GLOBAL] /* Define Cursor Scope */
[FORWARD_ONLY | SCROLL] /* Define Movement Direction of Cursor */
[ KEYSET | DYNAMIC |STATIC | FAST_FORWARD] /* Define basic type of cursor */
[ SCROLL_LOCKS | OPTIMISTIC |READ_ONLY ] /* Define Locks */
OPEN Cursor_Name /* Open Cursor */
FETCH NEXT FROM Cursor_Name /* Fetch data From Cursor */
Implement SQL QUery
CLOSE Cursor_Name /* Clsoe The Cursor */
DEALLOCATE Cursor_Name /* Deallocate all resources and Memory */

Now we will explain 4 important terminologies of cursors.

Cursor Scope

Microsoft SQL Server supports the GLOBAL and LOCAL keywords on the DECLARE CURSOR statement to define the scope of the cursor name.

GLOBAL: specifies that the cursor name is global to the connection.
LOCAL: specifies that the cursor name is local to the Stored Procedure, trigger or query that holds the cursor.

Data Fetch Option in Cursors

Microsoft SQL Server supports the following two fetch options for data:

FORWARD_ONLY: Specifies that the cursor can only be scrolled from the first to the last row.
SCROLL: It provides 6 options to fetch the data (FIRST, LAST, PRIOR, NEXT, RELATIVE and ABSOLUTE).

Types of cursors

Microsoft SQL Server supports the following 4 types of cursors.

STATIC CURSOR: A static cursor populates the result set during cursor creation and the query result is cached for the lifetime of the cursor. A static cursor can move forward and backward.
FAST_FORWARD: This is the default type of cursor. It is identical to the static except that you can only scroll forward.
DYNAMIC: In a dynamic cursor, additions and deletions are visible for others in the data source while the cursor is open.
KEYSET: This is similar to a dynamic cursor except we can't see records others add. If another user deletes a record, it is inaccessible from our record set.

Types of Locks

Locking is the process by which a DBMS restricts access to a row in a multi-user environment. When a row or column is exclusively locked, other users are not permitted to access the locked data until the lock is released. It is used for data integrity. This ensures that two users cannot simultaneously update the same column in a row.

Microsoft SQL Server supports the following three types of Locks.

READ ONLY: Specifies that the cursor cannot be updated.
SCROLL_LOCKS: Provides data integrity into the cursor. It specifies that the cursor will lock the rows as they are read into the cursor to ensure that updates or deletes made using the cursor will succeed.
OPTIMISTIC: Specifies that the cursor does not lock rows as they are read into the cursor. So, the updates or deletes made using the cursor will not succeed if the row has been updated outside the cursor.

First we create a table as in the following:

GO
CREATE TABLE [dbo].[Employee](
[Emp_ID] [int] NOT NULL,
[Emp_Name] [nvarchar](50) NOT NULL,
[Emp_Salary] [int] NOT NULL,
[Emp_City] [nvarchar](50) NOT NULL,
CONSTRAINT [PK_Employee] PRIMARY KEY CLUSTERED
(
[Emp_ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

Now insert some values into the table as in the following:

Insert into Employee
Select 1,'Pankaj',25000,'Alwar' Union All
Select 2,'Rahul',26000,'Alwar' Union All
Select 3,'Sandeep',25000,'Alwar' Union All
Select 4,'Sanjeev',24000,'Alwar' Union All
Select 5,'Neeraj',28000,'Alwar' Union All
Select 6,'Naru',20000,'Alwar' Union All
Select 7,'Omi',23000,'Alwar'

Select all values from the table as in the following:

Select all value from table

Example 1

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
LOCAL FORWARD_ONLY FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR

Output

EMP_ID: 1 EMP_NAME Pankaj EMP_SALARY 25000 EMP_CITY Alwar
EMP_ID: 2 EMP_NAME Rahul EMP_SALARY 26000 EMP_CITY Alwar
EMP_ID: 3 EMP_NAME Sandeep EMP_SALARY 25000 EMP_CITY Alwar
EMP_ID: 4 EMP_NAME Sanjeev EMP_SALARY 24000 EMP_CITY Alwar
EMP_ID: 5 EMP_NAME Neeraj EMP_SALARY 28000 EMP_CITY Alwar
EMP_ID: 6 EMP_NAME Naru EMP_SALARY 20000 EMP_CITY Alwar
EMP_ID: 7 EMP_NAME Omi EMP_SALARY 23000 EMP_CITY Alwar

This is a simple example of a cursor that prints the value of a table.

Example 2 (SCROLL)

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
LOCAL SCROLL FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
FETCH RELATIVE 3 FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH ABSOLUTE 3 FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH FIRST FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH LAST FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH PRIOR FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR

Output:

EMP_ID: 4 EMP_NAME Sanjeev EMP_SALARY 24000 EMP_CITY Alwar
EMP_ID: 3 EMP_NAME Sandeep EMP_SALARY 25000 EMP_CITY Alwar
EMP_ID: 1 EMP_NAME Pankaj EMP_SALARY 25000 EMP_CITY Alwar
EMP_ID: 7 EMP_NAME Omi EMP_SALARY 23000 EMP_CITY Alwar
EMP_ID: 6 EMP_NAME Naru EMP_SALARY 20000 EMP_CITY Alwar
EMP_ID: 7 EMP_NAME Omi EMP_SALARY 23000 EMP_CITY Alwar

In this example we will use SCROLL to fetch the data. This example contains all six modes of SCROLL (FIRST, LAST, PRIOR, NEXT, RELATIVE and ABSOLUTE).

Example 3 (STATIC CURSOR)

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
STATIC FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
If @EMP_ID%2=0
BEGIN
PRINT 'EMP_ID: ' + CONVERT(NVARCHAR(MAX),@EMP_ID)+ ' EMP_NAME '+@EMP_NAME +' EMP_SALARY ' +CONVERT(NVARCHAR(MAX),@EMP_SALARY) + ' EMP_CITY ' +@EMP_CITY
END
FETCH FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR

Output

EMP_ID: 2 EMP_NAME Rahul EMP_SALARY 26000 EMP_CITY Alwar
EMP_ID: 4 EMP_NAME Sanjeev EMP_SALARY 24000 EMP_CITY Alwar
EMP_ID: 6 EMP_NAME Naru EMP_SALARY 20000 EMP_CITY Alwar

Example 4

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
STATIC FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
If @EMP_ID%2=0
BEGIN
UPDATE Employee SET Emp_Salary=15000 WHERE CURRENT OF EMP_CURSOR
END
FETCH FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR

Output

Msg 16929, Level 16, State 1, Line 16
The cursor is READ ONLY.
The statement has been terminated.
Msg 16929, Level 16, State 1, Line 16
The cursor is READ ONLY.
The statement has been terminated.
Msg 16929, Level 16, State 1, Line 16
The cursor is READ ONLY.
The statement has been terminated.

When executing this cursor, we will get an error because the static cursor does not allow modifications in data.

Example 5 (DYNAMIC CURSOR)

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
DYNAMIC FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
If @EMP_ID%2=0
BEGIN
UPDATE Employee SET Emp_Salary=15000 WHERE CURRENT OF EMP_CURSOR
END
FETCH FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR
SELECT * FROM Employee

Output

Output

Example 6

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR CURSOR
FAST_FORWARD FOR
SELECT * FROM Employee
OPEN EMP_CURSOR
FETCH NEXT FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
If @EMP_ID%2=0
BEGIN
UPDATE Employee SET Emp_Salary=15000 WHERE CURRENT OF EMP_CURSOR
END
FETCH FROM EMP_CURSOR INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
CLOSE EMP_CURSOR
DEALLOCATE EMP_CURSOR

Output

A FAST_FORWARD cursor also reads as a static cursor. We cannot modify data in a FAST_FORWARD cursor.

Example 7

SET NOCOUNT ON
DECLARE @EMP_ID INT
DECLARE @EMP_NAME NVARCHAR(MAX)
DECLARE @EMP_SALARY INT
DECLARE @EMP_CITY NVARCHAR(MAX)
DECLARE EMP_CURSOR1 CURSOR
KEYSET scroll
FOR
SELECT EMP_ID ,EMP_NAME,EMP_SALARY,EMP_CITY FROM Employee order by Emp_Id
OPEN EMP_CURSOR1
IF @@CURSOR_ROWS > 0
BEGIN
FETCH NEXT FROM EMP_CURSOR1 INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
WHILE @@FETCH_STATUS = 0
BEGIN
If @EMP_ID%2=0
UPDATE Employee SET EMP_NAME='PANKAJ KUMAR CHOUDHARY' WHERE CURRENT OF EMP_CURSOR1
FETCH NEXT FROM EMP_CURSOR1 INTO @EMP_ID ,@EMP_NAME,@EMP_SALARY,@EMP_CITY
END
END
CLOSE EMP_CURSOR1
DEALLOCATE EMP_CURSOR1
SET NOCOUNT OFF
SELECT * FROM Employee

Output

result

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