![DECLARE CURSOR statement [T-SQL]](arrow-right.gif){kind=icon}
Declares a cursor. Cursors are the primary means for manipulating the results of queries.
DECLARE cursor-name
[ SCROLL
| NO SCROLL
| DYNAMIC SCROLL
]
CURSOR FOR
{ select-statement
| statement-name
[ FOR {READ ONLY | UPDATE [ OF column-name-list ] } ]
| USING variable-name }
identifier
identifier | host-variable
identifiers
identifier
The following example illustrates how to declare a scroll cursor in Embedded SQL:
EXEC SQL DECLARE cur_employee SCROLL CURSOR FOR SELECT * FROM employee ;
The following example illustrates how to declare a cursor for a prepared statement in Embedded SQL:
EXEC SQL PREPARE employee_statement FROM 'SELECT emp_lname FROM employee' ; EXEC SQL DECLARE cur_employee CURSOR FOR employee_statement ;
The following example illustrates the use of cursors in a stored procedure:
BEGIN
DECLARE cur_employee CURSOR FOR
SELECT emp_lname
FROM employee ;
DECLARE name CHAR(40) ;
OPEN cur_employee;
LOOP
FETCH NEXT cur_employee INTO name ;
...
END LOOP
CLOSE cur_employee;
END
The DECLARE CURSOR statement declares a cursor with the specified name for a SELECT statement or a CALL statement.
SCROLL A cursor declared as SCROLL supports the NEXT, PRIOR, FIRST, LAST, ABSOLUTE, and RELATIVE options of the FETCH statement. A SCROLL cursor allows you to fetch an arbitrary row in the result set while the cursor is open.
NO SCROLL A cursor declared as NO SCROLL is restricted to moving forward through the result set using only the FETCH NEXT and FETCH ABSOLUTE (0) seek operations.
Since rows cannot be returned to once the cursor leaves the row, there are no sensitivity restrictions on the cursor. Consequently, when a NO SCROLL cursor is requested, Sybase IQ supplies the most efficient kind of cursor, which is an asensitive cursor.
DYNAMIC SCROLL A cursor declared as DYNAMIC SCROLL supports the NEXT, PRIOR, FIRST, LAST, ABSOLUTE, and RELATIVE options of the FETCH statement. A DYNAMIC SCROLL cursor allows you to fetch an arbitrary row in the result set while the cursor is open.
FOR statement-name Statements are named using the PREPARE statement. Cursors can be declared only for a prepared SELECT or CALL.
FOR READ ONLY A cursor declared FOR READ ONLY may not be used in a positioned UPDATE or a positioned DELETE operation.
FOR UPDATE You can update the cursor result set of a cursor declared FOR UPDATE. Only asensitive behavior is supported for updatable cursors; any other sensitivity is ignored.
When the cursor is opened, exclusive table locks are taken on all tables that are opened for update. Stand-alone LOAD TABLE, UPDATE, INSERT, DELETE, and TRUNCATE statements are not allowed on tables that are opened for update in the same transaction, since Sybase IQ permits only one statement to modify a table at a time. You can open only one updatable cursor on a specific table at a time.
Updatable cursors are allowed to scroll, except over Open Client.
READ ONLY is the default value of the FOR clause.
OF column-name-list The list of columns from the cursor result set (specified by the select-statement) defined as updatable.
USING variable-name You can declare a cursor on a variable in stored procedures and user-defined functions. The variable is a string containing a SELECT statement for the cursor. The variable must be available when the DECLARE is processed, and so must be one of the following:
A parameter to the procedure. For example:
create function get_row_count(in qry varchar)
returns int
begin
declare crsr cursor using qry;
declare rowcnt int;
set rowcnt = 0;
open crsr;
lp: loop
fetch crsr;
if SQLCODE <> 0 then leave lp end if;
set rowcnt = rowcnt + 1;
end loop;
return rowcnt;
end
Nested inside another BEGIN…END after the variable has been assigned a value. For example:
create procedure get_table_name( in id_value int, out tabname char(128) ) begin declare qry varchar; set qry = 'select table_name from SYS.SYSTABLE ' || 'where table_id=' || string(id_value); begin declare crsr cursor using qry; open crsr; fetch crsr into tabname; close crsr; end end
Statements are named using the PREPARE statement. Cursors can be declared only for a prepared SELECT or CALL.
Sybase IQ support of updatable cursors is similar to Adaptive Server Anywhere support of updatable cursors. For a full discussion of cursor types and working with cursors, see the Adaptive Server Anywhere Programming Guide. This section contains information important to the use of updatable cursors in Sybase IQ.
Sybase IQ supports one type of cursor sensitivity, which is defined in terms of which changes to underlying data are visible. All Sybase IQ cursors are asensitive, which means that changes may be reflected in the membership, order, or values of the result set seen through the cursor, or may not be reflected at all.
With an asensitive cursor, changes effected by positioned UPDATE and positioned DELETE statements are visible in the cursor result set, except where client side caching prevents seeing these changes. Inserted rows are not visible.
Rows that are updated so that they no longer meet the requirements of the WHERE clause of the open cursor are still visible.
When using cursors there is always a trade-off between efficiency and consistency. Asensitive cursors provide efficient performance at the expense of consistency.
Sybase IQ supports updatable cursors on single tables.
Supported query specifications for updatable cursors in Sybase IQ are as follows:
Expressions in the select list against columns that are not functionally dependent on columns being updated
Arbitrary subqueries with asensitive behavior, that is, changes to data referenced by subqueries are not visible in the cursor result set
ORDER BY clause; the ORDER BY columns may be updated, but the result set does not re-order
Columns that meet these requirements:
No CAST on a column
Base columns of a base table in the SELECT clause
There are no expressions or functions on that column in the SELECT clause and it is not duplicated in the select list (for example, SELECT c1, c1).
Base columns of a base table restricted to those listed in the FOR UPDATE OF column-name-list clause, if the clause is specified.
Sybase IQ does not permit updatable cursors on queries that contain any operator that precludes a one-to-one mapping of result set rows to rows in a base table, specifically:
SELECT DISTINCT
an operator that has a UNION
an operator that has a GROUP BY
an operator that has a SET function (single group or extended GROUP BY)
an operator that has an OLAP function, with the exception of RANK()
See the description of the UPDATE (positioned) statement [ESQL] [SP] for information on the columns and expressions allowed in the SET clause for the update of a row in the result set of a cursor.
Sybase IQ supports inserts only on updatable cursors where all nonnullable, nonidentity columns are both selected and updatable.
In Sybase IQ, COMMIT and ROLLBACK are not allowed inside an open updatable cursor, even if the cursor is opened as a hold cursor. IQ does support ROLLBACK TO SAVEPOINT inside an updatable cursor.
Any failure that occurs after the cursor is open results in a rollback of all operations that have been performed through this open cursor.
A declared cursor is read-only and not updatable in cases where:
The data extraction facility is enabled with the TEMP_EXTRACT_NAME1 option set to a pathname
As a join index, or within a join index
ANSI_CLOSE_CURSORS_ON_ROLLBACK is set OFF
CHAINED is set OFF
The statement is INSERT SELECT or SELECT INTO
More than one table is included
No updatable columns exist
If IQ fails to set an updatable cursor when requested, see the .iqmsg file for an related information.
Sybase IQ updatable cursors differ from ANSI SQL/3 standard behavior as follows:
Hold cursor update close on commit.
Sybase IQ locks tables when the cursor is open.
All updates, deletes, and insert operations are applied when the cursor is closed, in the following order: deletes first, then updates, then inserts.
None.
None.
