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DECLARE CURSOR (Transact-SQL)

  • Artykuł

Defines the attributes of a Transact-SQL server cursor, such as its scrolling behavior and the query used to build the result set on which the cursor operates. DECLARE CURSOR accepts both a syntax based on the SQL-92 standard and a syntax using a set of Transact-SQL extensions.

Topic link iconTransact-SQL Syntax Conventions

Syntax

SQL 92 Syntax
DECLARE cursor_name [ INSENSITIVE ] [ SCROLL ] CURSOR 
     FOR select_statement 
     [ FOR { READ ONLY | UPDATE [ OF column_name [ ,...n ] ] } ]
[;]
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 ] ] ]
[;]

Arguments

  • cursor_name
    Is the name of the Transact-SQL server cursor defined. cursor_name must conform to the rules for identifiers. For more information about rules for identifiers, see Using Identifiers As Object Names.
  • INSENSITIVE
    Defines a cursor that makes a temporary copy of the data to be used by the cursor. All requests to the cursor are answered from this temporary table in tempdb; therefore, modifications made to base tables are not reflected in the data returned by fetches made to this cursor, and this cursor does not allow modifications. When SQL-92 syntax is used, if INSENSITIVE is omitted, committed deletes and updates made to the underlying tables (by any user) are reflected in subsequent fetches.
  • SCROLL
    Specifies that all fetch options (FIRST, LAST, PRIOR, NEXT, RELATIVE, ABSOLUTE) are available. If SCROLL is not specified in an SQL-92 DECLARE CURSOR, NEXT is the only fetch option supported. SCROLL cannot be specified if FAST_FORWARD is also specified.

  • select_statement
    Is a standard SELECT statement that defines the result set of the cursor. The keywords COMPUTE, COMPUTE BY, FOR BROWSE, and INTO are not allowed within select_statement of a cursor declaration.

    Microsoft SQL Server implicitly converts the cursor to another type if clauses in select_statement conflict with the functionality of the requested cursor type. For more information, see Using Implicit Cursor Conversions.

  • READ ONLY
    Prevents updates made through this cursor. The cursor cannot be referenced in a WHERE CURRENT OF clause in an UPDATE or DELETE statement. This option overrides the default capability of a cursor to be updated.
  • UPDATE [OF column_name [,...n]]
    Defines updatable columns within the cursor. If OF column_name [,...n] is specified, only the columns listed allow modifications. If UPDATE is specified without a column list, all columns can be updated.
  • cursor_name
    Is the name of the Transact-SQL server cursor defined. cursor_name must conform to the rules for identifiers. For more information about rules for identifiers, see Using Identifiers As Object Names.
  • LOCAL
    Specifies that the scope of the cursor is local to the batch, stored procedure, or trigger in which the cursor was created. The cursor name is only valid within this scope. The cursor can be referenced by local cursor variables in the batch, stored procedure, or trigger, or a stored procedure OUTPUT parameter. An OUTPUT parameter is used to pass the local cursor back to the calling batch, stored procedure, or trigger, which can assign the parameter to a cursor variable to reference the cursor after the stored procedure terminates. The cursor is implicitly deallocated when the batch, stored procedure, or trigger terminates, unless the cursor was passed back in an OUTPUT parameter. If it is passed back in an OUTPUT parameter, the cursor is deallocated when the last variable referencing it is deallocated or goes out of scope.

  • GLOBAL
    Specifies that the scope of the cursor is global to the connection. The cursor name can be referenced in any stored procedure or batch executed by the connection. The cursor is only implicitly deallocated at disconnect.

    Note

    If neither GLOBAL or LOCAL is specified, the default is controlled by the setting of the default to local cursor database option. In SQL Server version 7.0, this option defaults to FALSE to match earlier versions of SQL Server, in which all cursors were global. The default of this option may change in future versions of SQL Server. For more information, see Setting Database Options.

  • FORWARD_ONLY
    Specifies that the cursor can only be scrolled from the first to the last row. FETCH NEXT is the only supported fetch option. If FORWARD_ONLY is specified without the STATIC, KEYSET, or DYNAMIC keywords, the cursor operates as a DYNAMIC cursor. When neither FORWARD_ONLY nor SCROLL is specified, FORWARD_ONLY is the default, unless the keywords STATIC, KEYSET, or DYNAMIC are specified. STATIC, KEYSET, and DYNAMIC cursors default to SCROLL. Unlike database APIs such as ODBC and ADO, FORWARD_ONLY is supported with STATIC, KEYSET, and DYNAMIC Transact-SQL cursors.
  • STATIC
    Defines a cursor that makes a temporary copy of the data to be used by the cursor. All requests to the cursor are answered from this temporary table in tempdb; therefore, modifications made to base tables are not reflected in the data returned by fetches made to this cursor, and this cursor does not allow modifications.

  • KEYSET
    Specifies that the membership and order of rows in the cursor are fixed when the cursor is opened. The set of keys that uniquely identify the rows is built into a table in tempdb known as the keyset.

    Note

    If the query references at least one table without a unique index, the keyset cursor is converted to a static cursor.

    Changes to nonkey values in the base tables, either made by the cursor owner or committed by other users, are visible as the owner scrolls around the cursor. Inserts made by other users are not visible (inserts cannot be made through a Transact-SQL server cursor). If a row is deleted, an attempt to fetch the row returns an @@FETCH_STATUS of -2. Updates of key values from outside the cursor resemble a delete of the old row followed by an insert of the new row. The row with the new values is not visible, and attempts to fetch the row with the old values return an @@FETCH_STATUS of -2. The new values are visible if the update is done through the cursor by specifying the WHERE CURRENT OF clause.

  • DYNAMIC
    Defines a cursor that reflects all data changes made to the rows in its result set as you scroll around the cursor. The data values, order, and membership of the rows can change on each fetch. The ABSOLUTE fetch option is not supported with dynamic cursors.

  • FAST_FORWARD
    Specifies a FORWARD_ONLY, READ_ONLY cursor with performance optimizations enabled. FAST_FORWARD cannot be specified if SCROLL or FOR_UPDATE is also specified.

    Note

    In SQL Server 2000, FAST_FORWARD and FORWARD_ONLY cursor options are mutually exclusive. If both are specified, an error is raised. In SQL Server 2005, both keywords can be used in the same DECLARE CURSOR statement.

  • READ_ONLY
    Prevents updates made through this cursor. The cursor cannot be referenced in a WHERE CURRENT OF clause in an UPDATE or DELETE statement. This option overrides the default capability of a cursor to be updated.
  • SCROLL_LOCKS
    Specifies that positioned updates or deletes made through the cursor are guaranteed to succeed. Microsoft SQL Server locks the rows as they are read into the cursor to ensure their availability for later modifications. SCROLL_LOCKS cannot be specified if FAST_FORWARD or STATIC is also specified.
  • OPTIMISTIC
    Specifies that positioned updates or deletes made through the cursor do not succeed if the row has been updated since it was read into the cursor. SQL Server does not lock rows as they are read into the cursor. It instead uses comparisons of timestamp column values, or a checksum value if the table has no timestamp column, to determine whether the row was modified after it was read into the cursor. If the row was modified, the attempted positioned update or delete fails. OPTIMISTIC cannot be specified if FAST_FORWARD is also specified.
  • TYPE_WARNING
    Specifies that a warning message is sent to the client if the cursor is implicitly converted from the requested type to another.

  • select_statement
    Is a standard SELECT statement that defines the result set of the cursor. The keywords COMPUTE, COMPUTE BY, FOR BROWSE, and INTO are not allowed within select_statement of a cursor declaration.

    Note

    You can use a query hint within a cursor declaration; however, if you also use the FOR UPDATE OF clause, specify OPTION (query_hint) after FOR UPDATE OF.

    SQL Server implicitly converts the cursor to another type if clauses in select_statement conflict with the functionality of the requested cursor type. For more information, see Implicit Cursor Conversions.

  • FOR UPDATE [OF column_name [,...n]]
    Defines updatable columns within the cursor. If OF column_name [,...n] is supplied, only the columns listed allow modifications. If UPDATE is specified without a column list, all columns can be updated, unless the READ_ONLY concurrency option was specified.

Remarks

DECLARE CURSOR defines the attributes of a Transact-SQL server cursor, such as its scrolling behavior and the query used to build the result set on which the cursor operates. The OPEN statement populates the result set, and FETCH returns a row from the result set. The CLOSE statement releases the current result set associated with the cursor. The DEALLOCATE statement releases the resources used by the cursor.

The first form of the DECLARE CURSOR statement uses the SQL-92 syntax for declaring cursor behaviors. The second form of DECLARE CURSOR uses Transact-SQL extensions that allow you to define cursors using the same cursor types used in the database API cursor functions of ODBC or ADO.

You cannot mix the two forms. If you specify the SCROLL or INSENSITIVE keywords before the CURSOR keyword, you cannot use any keywords between the CURSOR and FOR select_statement keywords. If you specify any keywords between the CURSOR and FOR select_statement keywords, you cannot specify SCROLL or INSENSITIVE before the CURSOR keyword.

If a DECLARE CURSOR using Transact-SQL syntax does not specify READ_ONLY, OPTIMISTIC, or SCROLL_LOCKS, the default is as follows:

  • If the SELECT statement does not support updates (insufficient permissions, accessing remote tables that do not support updates, and so on), the cursor is READ_ONLY.
  • STATIC and FAST_FORWARD cursors default to READ_ONLY.
  • DYNAMIC and KEYSET cursors default to OPTIMISTIC.

Cursor names can be referenced only by other Transact-SQL statements. They cannot be referenced by database API functions. For example, after declaring a cursor, the cursor name cannot be referenced from OLE DB, ODBC or ADO functions or methods. The cursor rows cannot be fetched using the fetch functions or methods of the APIs; the rows can be fetched only by Transact-SQL FETCH statements.

After a cursor has been declared, these system stored procedures can be used to determine the characteristics of the cursor.

System stored procedures Description

sp_cursor_list

Returns a list of cursors currently visible on the connection and their attributes.

sp_describe_cursor

Describes the attributes of a cursor, such as whether it is a forward-only or scrolling cursor.

sp_describe_cursor_columns

Describes the attributes of the columns in the cursor result set.

sp_describe_cursor_tables

Describes the base tables accessed by the cursor.

Variables may be used as part of the select_statement that declares a cursor. Cursor variable values do not change after a cursor is declared. In SQL Server version 6.5 and earlier, variable values are refreshed every time a cursor is reopened.

Permissions

DECLARE CURSOR permissions default to any user that has SELECT permissions on the views, tables, and columns used in the cursor.

Examples

A. Use simple cursor and syntax

The result set generated at the opening of this cursor includes all rows and all columns in the table. This cursor can be updated, and all updates and deletes are represented in fetches made against this cursor. FETCH NEXT is the only fetch available because the SCROLL option has not been specified.

DECLARE vend_cursor CURSOR
FOR SELECT * FROM Purchasing.Vendor
OPEN vend_cursor
FETCH NEXT FROM vend_cursor

B. Use nested cursors to produce report output

This example shows how cursors can be nested to produce complex reports. The inner cursor is declared for each vendor.

SET NOCOUNT ON

DECLARE @vendor_id int, @vendor_name nvarchar(50),
@message varchar(80), @product nvarchar(50)

PRINT '-------- Vendor Products Report --------'

DECLARE vendor_cursor CURSOR FOR 
SELECT VendorID, Name
FROM Purchasing.Vendor
WHERE PreferredVendorStatus = 1
ORDER BY VendorID

OPEN vendor_cursor

FETCH NEXT FROM vendor_cursor 
INTO @vendor_id, @vendor_name

WHILE @@FETCH_STATUS = 0
BEGIN
PRINT ' '
SELECT @message = '----- Products From Vendor: ' + 
@vendor_name

PRINT @message

-- Declare an inner cursor based   
-- on vendor_id from the outer cursor.

DECLARE product_cursor CURSOR FOR 
SELECT v.Name
FROM Purchasing.ProductVendor pv, Production.Product v
WHERE pv.ProductID = v.ProductID AND
pv.VendorID = @vendor_id-- Variable value from the outer cursor

OPEN product_cursor
FETCH NEXT FROM product_cursor INTO @product

IF @@FETCH_STATUS <> 0 
PRINT '         <<None>>'     

WHILE @@FETCH_STATUS = 0
BEGIN

SELECT @message = '         ' + @product
PRINT @message
FETCH NEXT FROM product_cursor INTO @product

END

CLOSE product_cursor
DEALLOCATE product_cursor

-- Get the next vendor.
FETCH NEXT FROM vendor_cursor 
INTO @vendor_id, @vendor_name
END 
CLOSE vendor_cursor
DEALLOCATE vendor_cursor

See Also

Reference

@@FETCH_STATUS (Transact-SQL)
CLOSE (Transact-SQL)
Cursors (Transact-SQL)
DEALLOCATE (Transact-SQL)
FETCH (Transact-SQL)
SELECT (Transact-SQL)
sp_configure (Transact-SQL)

Help and Information

Getting SQL Server 2005 Assistance

Change History

Release History

5 December 2005
New content:
  • Added a note about using a query hint within a cursor declaration.