A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
cursor is a control structure that enables traversal over the records in a database. Cursors facilitate subsequent processing in conjunction with the traversal, such as retrieval, addition and removal of database records. The database cursor characteristic of traversal makes cursors akin to the programming language concept of iterator
Iterator
In computer programming, an iterator is an object that enables a programmer to traverse a container. Various types of iterators are often provided via a container's interface...
.
Cursors are used by database programmers to process individual rows returned by database system
Database system
A database system is a term that is typically used to encapsulate the constructs of a data model, database Management system and database....
queries. Cursors enable manipulation of whole result set
Result set
An SQL result set is a set of rows from a database, as well as meta-information about the query such as the column names, and the types and sizes of each column. Depending on the database system, the number of rows in the result set may or may not be known. Usually, this number is not known up...
Procedural programming can sometimes be used as a synonym for imperative programming , but can also refer to a programming paradigm, derived from structured programming, based upon the concept of the procedure call...
languages lack. In this scenario, a cursor enables the rows in a result-set to be processed sequentially.
In SQL procedures, a cursor make it possible to define a result set (a set of data rows) and perform complex logic on a row by row basis. By using the same mechanics, an SQL procedure can also define a result set and return it directly to the caller of the SQL procedure or to a client application.
A cursor can be viewed as a pointer to one row in a set of rows. The cursor can only reference one row at a time, but can move to other rows of the result set as needed.
To use cursors in SQL procedures, you need to do the following:
Declare a cursor that defines a result set.
Open the cursor to establish the result set.
Fetch the data into local variables as needed from the cursor, one row at a time.
Close the cursor when done
To work with cursors you must use the following SQL statements:
SQL:2003 is the fifth revision of the SQL database query language. The latest revision of the standard is SQL:2008.-Summary:The SQL:2003 standard makes minor modifications to all parts of SQL:1999 , and officially introduces a few new features such as:* XML-related features * Window functions* the...
standard defines how to use cursors in applications in embedded SQL. Not all application bindings for relational database systems adhere to that standard, and some (such as CLI
Call Level Interface
The Call Level Interface is a software standard defined in ISO/IEC 9075-3:2003. The Call Level Interface defines how a program should send SQL queries to the database management system and how the returned recordsets should be handled by the application in a consistent way...
Java DataBase Connectivity, commonly referred to as JDBC, is an API for the Java programming language that defines how a client may access a database. It provides methods for querying and updating data in a database. JDBC is oriented towards relational databases...
A database management system is a software package with computer programs that control the creation, maintenance, and use of a database. It allows organizations to conveniently develop databases for various applications by database administrators and other specialists. A database is an integrated...
by using a DECLARE ... CURSOR statement and assigning the cursor a (compulsory) name:
DECLARE cursor_name CURSOR FOR SELECT ... FROM ...
Before code can access the data, it must open the cursor with the OPEN statement. Directly following a successful opening, the cursor is positioned before the first row in the result set.
OPEN cursor_name
Programs position cursors on a specific row in the result set with the FETCH statement. A fetch operation transfers the data of the row into the application.
FETCH cursor_name INTO ...
Once an application has processed all available rows or the fetch operation is to be positioned on a non-existing row (compare scrollable cursors below), the DBMS returns a SQLSTATE '02000' (usually accompanied by an SQLCODE +100) to indicate the end of the result set.
The final step involves closing the cursor using the CLOSE statement:
CLOSE cursor_name
After closing a cursor, a program can open it again, which implies that the DBMS re-evaluates the same query or a different query and builds a new result-set.
Scrollable cursors
Programmers may declare cursors as scrollable or not scrollable. The scrollability indicates the direction in which a cursor can move.
With a non-scrollable cursor, also known as forward-only, one can FETCH each row at most once, and the cursor automatically moves to the immediately following row. A fetch operation after the last row has been retrieved positions the cursor after the last row and returns SQLSTATE 02000 (SQLCODE +100).
A program may position a scrollable cursor anywhere in the result set using the FETCH SQL statement. The keyword SCROLL must be specified when declaring the cursor. The default is NO SCROLL, although different language bindings like JDBC may apply different default.
DECLARE cursor_namesensitivitySCROLL CURSOR FOR SELECT ... FROM ...
The target position for a scrollable cursor can be specified relative to the current cursor position or absolute from the beginning of the result set.
FETCH [ NEXT | PRIOR | FIRST | LAST ] FROM cursor_name
FETCH ABSOLUTE n FROM cursor_name
FETCH RELATIVE n FROM cursor_name
Scrollable cursors can potentially access the same row in the result set multiple times. Thus, data modifications (insert, update, delete operations) from other transactions could have an impact on the result set. A cursor can be SENSITIVE or INSENSITIVE to such data modifications. A sensitive cursor picks up data modifications impacting the result set of the cursor, and an insensitive cursor does not. Additionally, a cursor may be ASENSITIVE, in which case the DBMS tries to apply sensitivity as much as possible.
"WITH HOLD"
Cursors are usually closed automatically at the end of a transaction, i.e. when a COMMIT or ROLLBACK (or an implicit termination of the transaction) occurs. That behavior can be changed if the cursor is declared using the WITH HOLD clause. (The default is WITHOUT HOLD.) A holdable cursor is kept open over COMMIT and closed upon ROLLBACK. (Some DBMS deviate from this standard behavior and also keep holdable cursors open over ROLLBACK.)
DECLARE cursor_name CURSOR WITH HOLD FOR SELECT ... FROM ...
When a COMMIT occurs, a holdable cursor is positioned before the next row. Thus, a positioned UPDATE or positioned DELETE statement will only succeed after a FETCH operation occurred first in the transaction.
Note that JDBC defines cursors as holdable per default. This is done because JDBC also activates auto-commit per default. Due to the usual overhead associated with auto-commit and holdable cursors, both features should be explicitly deactivated at the connection level.
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 already positioned on a row by means of FETCH statement.
UPDATE table_name
SET ...
WHERE CURRENT OFcursor_name
DELETE
FROM table_name
WHERE CURRENT OFcursor_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.
- Features :XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents....
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]
In telecommunications, the round-trip delay time or round-trip time is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received...
each time. This uses much more network bandwidth than would ordinarily be needed for the execution of a single SQL statement like DELETE. Repeated network round trips can severely impact the speed of the operation using the cursor. Some DBMSs try to reduce this impact by using block fetch. Block fetch implies that multiple rows are sent together from the server to the client. The client stores a whole block of rows in a local buffer and retrieves the rows from there until that buffer is exhausted.
Cursors allocate resources on the server, for instance lock
Lock (database)
A lock is used when multiple users need to access a database concurrently. This prevents data from being corrupted or invalidated when multiple users try to write to the database. Any single user can only modify those database records to which they have applied a lock that gives them exclusive...
s, packages, processes, temporary storage, etc. For example, Microsoft SQL Server
Microsoft SQL Server
Microsoft SQL Server is a relational database server, developed by Microsoft: It is a software product whose primary function is to store and retrieve data as requested by other software applications, be it those on the same computer or those running on another computer across a network...
implements cursors by creating a temporary table and populating it with the query's result-set. If a cursor is not properly closed (deallocated), the resources will not be freed until the SQL session (connection) itself is closed. This wasting of resources on the server can not only lead to performance degradations but also to failures.
