Cursors

In this article, I want to tell you how to create and use server side cursors and how you can optimize a cursor performance.

Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, instead of the typical SQL commands that operate on all the rows in the set at one time. For example, you can use cursor to include a list of all user databases and make multiple operations against each database by passing each database name as a variable.

The server side cursors were first added in SQL Server 6.0 release and now supported in all editions of SQL Server 7.0 and SQL Server 2000.

Before using cursor, you first must declare the cursor. Once a cursor has been declared, you can open it and fetch from it. You can fetch row by row and make multiple operations on the currently active row in the cursor. When you have finished working with a cursor, you should close cursor and deallocate it to release SQL Server resources.

Declaring a Cursor

Before using cursor, you first must declare the cursor, i.e. define its scrolling behavior and the query used to build the result set on which the cursor operates. To declare cursor, you can use a syntax based on the SQL-92 standard and a syntax using a set of Transact-SQL extensions.

SQL-92 Syntax

This is SQL-92 Syntax:

DECLARE cursor_name [INSENSITIVE] [SCROLL] CURSOR

FOR select_statement

[FOR {READ ONLY | UPDATE [OF column_name [,...n]]}]

where

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

INSENSITIVE - specifies that cursor will use a temporary copy of the data instead of base tables. This cursor does not allow modifications and modifications made to base tables are not reflected in the data returned by fetches made to this cursor.

SCROLL - specifies that cursor can fetch data in all directions, not only sequentially until the end of the result set. If this argument is not specified, FETCH NEXT is the only fetch option supported.

select_statement - the standard select statement, cannot contain COMPUTE, COMPUTE BY, FOR BROWSE, and INTO keywords.

READ ONLY - specifies that cursor cannot be updated.

UPDATE [OF column_name [,...n]] - specifies that all cursor's columns can be updated (if OF column_name [,...n] is not specified), or only the columns listed in the OF column_name [,...n] list allow modifications.

Cursor Options Compatibility

Transact-SQL Extended Syntax

This is Transact-SQL Extended Syntax:

DECLARE cursor_name CURSOR

[LOCAL | GLOBAL]

[FORWARD_ONLY | SCROLL]

[STATIC | KEYSET | DYNAMIC | FAST_FORWARD]

[READ_ONLY | SCROLL_LOCKS | OPTIMISTIC]

[TYPE_WARNING]

FOR select_statement

[FOR UPDATE [OF column_name [,...n]]]

where

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

LOCAL - specifies that cursor can be available only in the batch, stored procedure, or trigger in which the cursor was created. The LOCAL cursor will be implicitly deallocated when the batch, stored procedure, or trigger terminates.

GLOBAL - specifies that cursor is global to the connection. The GLOBAL cursor will be implicitly deallocated at disconnect.

FORWARD_ONLY - specifies that cursor can only fetch data sequentially from the first to the last row. FETCH NEXT is the only fetch option supported.

STATIC - specifies that cursor will use a temporary copy of the data instead of base tables. This cursor does not allow modifications and modifications made to base tables are not reflected in the data returned by fetches made to this cursor.

KEYSET - specifies that cursor uses the set of keys that uniquely identify the cursor's rows (keyset), so that the membership and order of rows in the cursor are fixed when the cursor is opened. SQL Server uses a table in tempdb to store keyset. The KEYSET cursor allows updates nonkey values from being made through this cursor, but inserts made by other users are not visible. Updates nonkey values made by other users are visible as the owner scrolls around the cursor, but updates key values made by other users are not visible. If a row is deleted, an attempt to fetch the row returns an @@FETCH_STATUS of -2.

DYNAMIC - specifies that cursor reflects all data changes made to the base tables as you scroll around the cursor. FETCH ABSOLUTE option is not supported with DYNAMIC cursor.

FAST_FORWARD - specifies that cursor will be FORWARD_ONLY and READ_ONLY cursor. The FAST_FORWARD cursors produce the least amount of overhead on SQL Server.

READ ONLY - specifies that cursor cannot be updated.

SCROLL_LOCKS - specifies that cursor will lock the rows as they are read into the cursor to ensure that positioned updates or deletes made through the cursor will be succeed.

OPTIMISTIC - specifies that cursor does not lock rows as they are read into the cursor. So, the positioned updates or deletes made through the cursor will not succeed if the row has been updated outside the cursor since this row was read into the cursor.

TYPE_WARNING - specifies that if the cursor will be implicitly converted from the requested type to another, a warning message will be sent to the client.

select_statement - the standard select statement, cannot contain COMPUTE, COMPUTE BY, FOR BROWSE, and INTO keywords.

UPDATE [OF column_name [,...n]] - specifies that all cursor's columns can be updated (if OF column_name [,...n] is not specified), or only the columns listed in the OF column_name [,...n] list allow modifications.

Cursor Options Compatibility

Opening a Cursor

Once a cursor has been declared, you must open it to fetch data from it. To open a cursor, you can use the following syntax:

OPEN { { [GLOBAL] cursor_name } | cursor_variable_name}

where

GLOBAL - if this argument was not specified and both a global and a local cursor exist with the same name, the local cursor will be opened; otherwise, the global cursor will be opened.

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

cursor_variable_name - the name of a cursor variable that references a cursor.

After a cursor is opening, you can determine the number of rows that were found by the cursor. To get this number, you can use @@CURSOR_ROWS scalar function.

Fetching a Cursor

Once a cursor has been opened, you can fetch from it row by row and make multiple operations on the currently active row in the cursor. To fetch from a cursor, you can use the following syntax:

FETCH

        [    [    NEXT | PRIOR | FIRST | LAST

                | ABSOLUTE {n | @nvar}

                | RELATIVE {n | @nvar}

            ]

            FROM

        ]

{ { [GLOBAL] cursor_name } | @cursor_variable_name}

[INTO @variable_name[,...n] ]

where

NEXT - the default cursor fetch option. FETCH NEXT returns the next row after the current row.

PRIOR - returns the prior row before the current row.

FIRST - returns the first row in the cursor.

LAST - returns the last row in the cursor.

ABSOLUTE {n | @nvar} - returns the nth row in the cursor. If a positive number was specified, the rows are counted from the top of the data set; if 0 was specified, no rows are returned; if a negative number was specified, the number of rows will be counted from the bottom of the data set.

RELATIVE {n | @nvar} - returns the nth row in the cursor relative to the current row. If a positive number was specified, returns the nth row beyond the current row; if a negative number was specified, returns the nth row prior the current row; if 0 was specified, returns the current row.

GLOBAL - if this argument was not specified and both a global and a local cursor exist with the same name, the local cursor will be fetched; otherwise, the global cursor will be fetched.

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

cursor_variable_name - the name of a cursor variable that references a cursor.

INTO @variable_name[,...n] - allows data returned from the cursor to be held in temporary variables. The type of variables must match the type of columns in the cursor select list or support implicit conversion. The number of variables must match the number of columns in the cursor select list.

Closing a Cursor

When you have finished working with a cursor, you can close it to release any resources and locks that SQL Server may have used while the cursor was open.
To close a cursor, you can use the following syntax:

CLOSE { { [GLOBAL] cursor_name } | cursor_variable_name }

where

GLOBAL - if this argument was not specified and both a global and a local cursor exist with the same name, the local cursor will be closed; otherwise, the global cursor will be closed.

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

cursor_variable_name - the name of a cursor variable that references a cursor.

Note. If you have closed a cursor, but have not deallocated it, you can open it again when needed.

Deallocating a Cursor

When you have finished working with a cursor and want to completely release SQL Server resources that were used by a cursor, you can deallocate a cursor.
To deallocate a cursor, you can use the following syntax:

DEALLOCATE { { [GLOBAL] cursor_name } | @cursor_variable_name}

where

GLOBAL - if this argument was not specified and both a global and a local cursor exist with the same name, the local cursor will be deallocated; otherwise, the global cursor will be deallocated.

cursor_name - the name of the server side cursor, must contain from 1 to 128 characters.

cursor_variable_name - the name of a cursor variable that references a cursor.

Note. Deallocating a cursor completely removes all cursor references. So, after a cursor is deallocated, it no longer can be opened.

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