Structured Query Language (SQL) - Cultural View of Technology

Cursor (databases) 30
Positioned update/delete statements
Cursors can not only be used to fetch data from the DBMS into an application but also to identify a row in a table to
be updated or deleted. The SQL:2003 standard defines positioned update and positioned delete SQL statements for
that purpose. Such statements do not use a regular WHERE clause with predicates. Instead, a cursor identifies the
row. The cursor must be opened and positioned on a row already using the FETCH statement.
UPDATE table_name
SET ...
WHERE CURRENT OF cursor_name
DELETE
FROM table_name
WHERE CURRENT OF cursor_name
The cursor must operate on an updatable result set in order to successfully execute a positioned update or delete
statement. Otherwise, the DBMS would not know how to apply the data changes to the underlying tables referred to
in the cursor.
Cursors in distributed transactions
Using cursors in distributed transactions (X/Open XA Environments), which are controlled using a transaction
monitor, is no different than cursors in non-distributed transactions.
One has to pay attention when using holdable cursors, however. Connections can be used by different applications.
Thus, once a transaction has been ended and committed, a subsequent transaction (running in a different application)
could inherit existing holdable cursors. Therefore, an application developer has to be aware of that situation.
Cursors in XQuery
The XQuery language allows cursors to be created using the subsequence() function.
The format is:
let $displayed-sequence := subsequence($result, $start, $item-count)
Where $result is the result of the initial XQuery, $start is the item number to start and $item-count is the number of
items to return.
Equivalently this can also be done using a predicate:
let $displayed-sequence := $result[$start to $end]
Where $end is the end sequence.
For complete examples see the XQuery Wikibook [1] .