Cursors

For viewing and updating

Cursors

How to step through the returned set of rows

Selected set

• When a Select statement is issued, the returned set is a set of rows.

• To step through, or process the rows in the set, it is necessary to be able to look at each one in turn.

• A CURSOR allows the user to iterate through the returned set.

Managing explicit cursors

• The cursor can be declared in any part of a PL/SQL block, subprogram or package.

• Within the program body, the cursor can be OPENed, FETCHed and CLOSEd.

• A cursor cannot be assigned a value or set of values – it must get its contents from a select statement.

Declaring a cursor

CURSOR cursor_name [(parameter[, parameter]…)]

[RETURN return_type] IS select_statement;

• Where – return_type is a record or row in a database table– Parameter is defined as:

cursor_parameter)name [IN] datatype [{:= | DEFAULT} expression]

Example cursor declaration

DECLARECURSOR lateOrd IS

SELECT supplierorderno, (delivereddate – supplierOrderDate) from sorder where (delivereddate – supplierOrderDate) > 5 or delivereddate is null;

• This cursor will contain a set of rows of supplier orders that either took more than 5 days to deliver, or have not been delivered.

• The scope of the cursor is the scope of the block in which it is declared.

Declaring the cursor with parameters

DECLARE nodays integer; CURSOR lateOrd IS SELECT supplierorderno, (delivereddate – supplierOrderDate) from sorder where (delivereddate – supplierOrderDate) > nodays or delivereddate is null;

Opening and closing the cursor

DECLARE CURSOR lateOrd IS SELECT ……BEGIN OPEN lateOrd;… CLOSE lateOrd;END;This opens the cursor, but doesn’t fill it.If it is a parameterised cursor, it could be: OPEN lateOrd(3);

Fetching the cursorDECLARE CURSOR lateOrd IS SELECT

supplierorderno, (delivereddate - supplierOrderDate) from sorder where (delivereddate - supplierOrderDate) > 5 or delivereddate is null; LocOrdNo sOrder.SupplierORDERNO%Type; DaysLate Integer; BEGINOPEN lateOrd;LOOPFETCH lateOrd into LocOrdNo, DaysLate;EXIT WHEN lateOrd%NOTFOUND;dbms_output.put_line('Order number '||LocOrdNo||' was delivered '||DaysLate||' days

late.');End Loop;Close LateOrd;END;

Fetching

• Fetch retrieves the rows one at a time from the result set.

• The named exceptions relating to cursors are:– %found, %notfound, %rowcount and

%isopen.– These do not automatically raise an

exception, so you must test for them.

Cursor loop

Open LateOrd;LOOP FETCH lateOrd INTO LocOrdNo, DaysLate; dbms_output.put_line (lateOrd%rowcount||’. Order number ’||LocOrdNo||’ was ‘||DaysLate||’ days late.’);

END LOOP;Dbms_output.put_line (‘Altogether, there were || LateOrd%rowcount||’ late orders’);

Close LateOrd;• This numbers the late orders.

Other Cursor qualities

• The implicit For loop can open the cursor, fetch each item individually and then close it (see next slide).

• Instead of having to itemise the columns in the table, we can declare a cursor row type:

• Declare• cursor s1 is …

s1_rec s1%rowtype;

Updating in a shared database

• Selecting places no lock on a row.

• To lock a row, select it with a row share lock, and then update it.

• A row share lock allows others to:– Read the locked row, but only return the ‘pre-

locked’ state– Read any other row– Lock any other row.

Row-share lock

• A row-share lock allows for– Read consistency.

• The session placing the lock may update the locked row.

• N.B. The session placing the lock MUST RELEASE the lock.– This is done by COMMIT or ROLLBACK.

Cursors for update• I want to up the cost price of all stock items that are

supplied by supplier number 101 by 10%.Declare cursor s1 is select * from stock where supplier_Id = 501 for

update; s1_rec s1%rowtype;Begin for s1_rec in s1 loop update stock set unitcostprice = unitcostprice * 1.1 where current of s1; end loop; commit;End;