13436 cursors

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CursorsCursorsIn Oracle

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What is a cursorWhat is a cursor

y For every SQL statement execution

certain area in memory is allocated.

y PL/SQL allow you to name this area.

y This private SQL area is called context

area or cursor.

y A cursor acts as a handle or pointer into

the context area.

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What is a cursorWhat is a cursor� When you declare a cursor, you get a

pointer variable, which does not point anything.

� When the cursor is opened, memory isallocated and the cursor structure iscreated.

� The cursor variable now points the cursor.

� When the cursor is closed the memory

allocated for the cursor is released.� Cursors allow the programmer to retrieve

data from a table and perform actions onthat data one row at a time.

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Types of CursorsTypes of Cursors

y Implicit Cursors: For SQL queries

returning single row

y Explicit Cursors: are used in queries

that return multiple rows.

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Implicit CursorsImplicit Cursors

y Implicit cursors are simple SELECT

statements

y They are written in the BEGIN block

(executable section) of the PL/SQL.

y Implicit cursors are easy to code

y They retrieve exactly one row.

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ExampleExample

SELECT ename, sal INTO ena, esa FROM EMP whereEMPNO = 7844;

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Explicit CursorsExplicit Cursors

y Explicit cursors are used in queries that

return multiple rows.

y The set of rows fetched by a query is

called active set.

y The size of the active set meets the

search criteria in the select statement.

y Explicit cursor is declared in theDECLARE section of PL/SQL program.

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SyntaxSyntax

� CURSOR <cursor-name> IS <select statement>

� Sample Code:

DECLARE

CURSOR emp_cur IS SELECT ename FROMEMP;

BEGIN

--------- - --------------- - ------

END ;

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Cont..Cont..

y Processing multiple rows is similar to file

processing.

y For processing a file you need to open it,

process records and then close.

y Similarly user-defined explicit cursor

needs to be opened, before reading the

rows, after which it is closed.

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Opening a CursorOpening a Cursor

y Syntax: OPEN <cursor-name>;

y Example : OPEN emp_cur;

When a cursor is opened the active set is

determined, the rows satisfying the where

clause in the select statement are added

to the active set. A pointer is established

and points to the first row in the activeset.

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Fetching From the CursorFetching From the Cursor

y Syntax: FETCH <cursor-name> INTO

<variables>;

y Example: FETCH emp_cur INTO ena;

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Closing a CursorClosing a Cursor

y CLOSE <cursor-name>;

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Explicit Cursor AttributesExplicit Cursor Attributes

� Every cursor defined by the user has 4 attributes.

The attributes are:

� %NOTFOUND: It is a Boolean attribute, which evaluates to true, if the last fetch failed. i.e. when

there are no rows left in the cursor to fetch.� %FOUND: Boolean variable, which evaluates to

true if the last fetch, succeeded.

� %ROWCOUNT: It·s a numeric attribute, which returns number of rows fetched by the cursor so

far.� %ISOPEN: A Boolean variable, which evaluates to

true if the cursor is opened otherwise to false.

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Example: Fetch for Each rowExample: Fetch for Each row

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Fetch using LOOPFetch using LOOP

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