4 cursors
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This document discusses cursors in Oracle PL/SQL. It defines a cursor as a handle or pointer to a context area that allows a program to control how SQL statements are processed. There are two types of cursors - implicit cursors that are automatically created, and explicit cursors that are declared by the programmer. The processing of an explicit cursor involves declaring it, opening it to allocate memory, fetching rows from it one at a time, and closing it to free memory. Cursors can be used to retrieve and process rows from a query.
4 cursors
- 2. CURSOR MANIPULATION • To process an SQL statement, ORACLE needs to create an area of memory known as the context area; this will have the information needed to process the statement. • This information includes the number of rows processed by the statement, a pointer to the parsed representation of the statement. • In a query, the active set refers to the rows that will be returned. Bordoloi and
- 3. CURSOR MANIPULATION • A cursor is a handle, or pointer, to the context area. • Through the cursor, a PL/SQL program can control the context area and what happens to it as the statement is processed. • Two important features about the cursor are 1. Cursors allow you to fetch and process rows returned by a SELECT statement, one row at a time. 2. A cursor is named so that it can be referenced. Bordoloi and
- 4. Types Of Cursors • There are two types of cursors: 1. An IMPLICIT cursor is automatically declared by Oracle every time an SQL statement is executed. The user will not be aware of this happening and will not be able to control or process the information in an implicit cursor. 2. An EXPLICIT cursor is defined by the program for any query that returns more than one row of data. That means the programmer has declared the cursor within the PL/SQL code block. Bordoloi and
- 5. IMPLICIT CURSOR • Any given PL/SQL block issues an implicit cursor whenever an SQL statement is executed, as long as an explicit cursor does not exist for that SQL statement. • A cursor is automatically associated with every DML (Data Manipulation) statement (UPDATE, DELETE, INSERT). • All UPDATE and DELETE statements have cursors that identify the set of rows that will be affected by the operation. • An INSERT statement needs a place to receive the data that is to be inserted in the database; the implicit cursor fulfills this need. • The most recently opened cursor is called the “SQL%” Cursor. Bordoloi and
- 6. The Processing Of An Implicit Cursor • The implicit cursor is used to process INSERT, UPDATE, DELETE, and SELECT INTO statements. • During the processing of an implicit cursor, Oracle automatically performs the OPEN, FETCH, and CLOSE operations. • An implicit cursor cannot tell you how many rows were affected by an update. SQL%ROWCOUNT returns numbers of rows updated. It can be used as follows: BEGIN UPDATE student SET first_name = 'B' WHERE first_name LIKE 'B%'; DBMS_OUTPUT.PUT_LINE(SQL%ROWCOUNT); Bordoloi and END;
- 7. EXPLICIT CURSOR • The only means of generating an explicit cursor is for the cursor to be named in the DECLARE section of the PL/SQL Block. • The advantages of declaring an explicit cursor over the indirect implicit cursor are that the explicit cursor gives more programmatic control to the programmer. • Implicit cursors are less efficient than explicit cursors and thus it is harder to trap data errors. Bordoloi and
- 8. EXPLICIT CURSOR • The process of working with an explicit cursor consists of the following steps: • DECLARING the cursor. This initializes the cursor into memory. • OPENING the cursor. The previously declared cursor can now be opened; memory is allotted. • FETCHING the cursor. The previously declared and opened cursor can now retrieve data; this is the process of fetching the cursor. • CLOSING the cursor. The previously declared, opened, and fetched cursor must now be closed to release memory allocation. Bordoloi and
- 9. DECLARING A CURSOR • Declaring a cursor defines the name of the cursor and associates it with a SELECT statement. • The first step is to Declare the Cursor with the following syntax: CURSOR c_cursor_name IS select statement • Cursor names follow the same rules of scope and visibility that apply to the PL/SQL identifiers. • Because the name of the cursor is a PL/SQL identifier, it must be declared before it is referenced. • Any valid select statement can be used to define a cursor, including joins and statements with the Bordoloi and MINUS clause. UNION or
- 10. RECORD TYPES • A record is a composite data structure, which means that it is composed of more than one element. • Records are very much like a row of a database table, but each element of the record does not stand on its own. • PL/SQL supports three kinds of records: 1. Table based 2. Cursor_based, 3. Programmer-defined. Bordoloi and
- 11. RECORD TYPES • A table-based record is one whose structure is drawn from the list of columns in the table. • A cursor-based record is one whose structure matches the elements of a predefined cursor. • To create a table-based or cursor_based record use the %ROWTYPE attribute. <record_name> <table_name or cursor_name>%ROWTYPE Bordoloi and
- 12. Example DECLARE vr_student student%ROWTYPE; BEGIN SELECT * INTO vr_student FROM student WHERE student_id = 156; DBMS_OUTPUT.PUT_LINE (vr_student.first_name||‘ ’||vr_student.last_name|| ‘ has an ID of 156’); EXCEPTION WHEN no_data_found THEN RAISE_APPLICATION_ERROR(-2001,‘The Student’||‘is not in the database’); END; Bordoloi and
- 13. OPENING A CURSOR • The next step in controlling an explicit cursor is to open it. When the Open cursor statement is processed, the following four actions will take place automatically: 1. The variables (including bind variables) in the WHERE clause are examined. 2. Based on the values of the variables, the active set is determined and the PL/SQL engine executes the query for that cursor. Variables are examined at cursor open time only. 3. The PL/SQL engine identifies the active set of data—the rows from all involved tables that meet the WHERE clause criteria. 4. The active set pointer is set to the first row. Bordoloi and
- 14. OPENING A CURSOR • The syntax for opening a cursor is: OPEN cursor_name; Bordoloi and
- 15. FETCHING ROWS IN A CURSOR • After the cursor has been declared and opened, you can then retrieve data from the cursor. • The process of getting the data from the cursor is referred to as fetching the cursor. • There are two methods of fetching a cursor, done with the following command: FETCH cursor_name INTO PL/SQL variables; or FETCH cursor_name INTO PL/SQL record; Bordoloi and
- 16. FETCHING ROWS IN A CURSOR • When the cursor is fetched the following occurs: 1. The fetch command is used to retrieve one row at a time from the active set. This is generally done inside a loop. The values of each row in the active set can then be stored into the corresponding variables or PL/SQL record one at a time, performing operations on each one successively. 2. After each FETCH, the active set pointer is moved forward to the next row. Thus, each fetch will return successive rows of the active set, until the entire set is returned. The last FETCH will not assign values to the output variables; they will still contain their prior values. Bordoloi and
- 17. Example DECLARE CURSOR c_zip IS SELECT * FROM zipcode; vr_zip c_zip%ROWTYPE; BEGIN OPEN c_zipcode; LOOP FETCH c_zip INTO vr_zip; EXIT WHEN c_zip%NOTFOUND; DBMS_OUTPUT.PUT_LINE(vr_zip.zipcode|| ‘ ’||vr_zip.city||‘ ’||vr_zip.state); END LOOP; Bordoloi and ...
- 18. CLOSING A CURSOR • Once all of the rows in the cursor have been processed (retrieved), the cursor should be closed. • This tells the PL/SQL engine that the program is finished with the cursor, and the resources associated with it can be freed. • The syntax for closing the cursor is: CLOSE cursor_name; • Once a cursor is closed, it is no longer valid to fetch from it. • Likewise, it is not possible to close an already closed cursor (either one will result in an Oracle error). Bordoloi and
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- 20. USING CURSOR FOR LOOPS AND NESTING CURSORS • When using the cursor FOR LOOP, the process of opening, fetching, and closing are implicitly handled. • This makes the blocks much simpler to code and easier to maintain. • The cursor FOR LOOP specifies a sequence of statements to be repeated once for each row returned by the cursor. • Use the cursor FOR LOOP if you need to FETCH and PROCESS each and every record from a cursor. Bordoloi and
- 21. Example DECLARE CURSOR c_student IS SELECT student_id, last_name, first_name FROM student WHERE student_id < 110; BEGIN FOR r_student IN c_student LOOP INSERT INTO table_log VALUES(r_student.last_name); END LOOP; END; Bordoloi and
- 22. PROCESSING NESTED CURSORS • Cursors can be nested inside each other. • It is just a loop inside a loop, much like nested loops. • If you had one parent cursor and two child cursors, then each time the parent cursor makes a single loop, it will loop through each child cursor once and then begin a second round. • In the following example, you will encounter a nested cursor with a single child cursor. Bordoloi and
- 23. Example • There are two cursors 1 DECLARE in this example. 2 v_zip zipcode.zip%TYPE; • The first is a cursor of 3 CURSOR c_zip IS the zipcodes and the 4 SELECT zip, city, state second cursor is a list 5 FROM zipcode of students. 6 WHERE state = 'CT'; • The variable v_zip is 7 CURSOR c_student IS initialized in line 14 8 SELECT first_name, to be the zipcode of last_name the current record of 9 FROM student the c_zip cursor. 10 WHERE zip = v_zip; • The c_student cursor 11 BEGIN ties in c_zip cursor by 12 FOR r_zip IN c_zip means of this 13 Bordoloi and LOOP
- 24. Contd. Thus, when the cursor is 14 v_zip := r_zip.zip; processed in lines 18–22, it 15 DBMS_OUTPUT.PUT_LINE is retrieving students that (CHR(10)); have the zipcode of the 16 DBMS_OUTPUT.PUT_LINE current record for the parent ('Students living in '|| cursor. 17 r_zip.city); The parent cursor is processed 18 FOR r_student in c_student from lines 12–23. Each 19 LOOP iteration of the parent 20 DBMS_OUTPUT.PUT_LINE cursor will only execute the (r_student.first_name|| DBMS_OUTPUT in lines 21 ' '||r_student.last_name); 16 and 17 once. 22 END LOOP; 23 END LOOP; The DBMS_OUTPUT in line 20 24* END; will be executed once for each iteration of the child loop, producing a line of Bordoloi and output for each student.
- 25. CURSORS WITH PARAMETERS • A cursor can be declared with parameters. • This enables a cursor to generate a specific result set, which is, on the one hand, more narrow, but on the other hand, reusable. • A cursor of all the data from the zipcode table may be very useful, but it would be more useful for certain data processing if it held information for only one state. CURSOR c_zip (p_state IN zipcode.state%TYPE) IS SELECT zip, city, state FROM zipcode WHERE state = p_state; Bordoloi and
- 26. CURSORS WITH PARAMETERS • Cursor parameters make the cursor more reusable. • Cursor parameters can be assigned default values. • The scope of the cursor parameters is local to the cursor. • The mode of the parameters can only be IN. • When a cursor has been declared as taking a parameter, it must be called with a value for that parameter. • The c_zip cursor that was just declared is called as follows: OPEN c_zip (parameter_value) Bordoloi and
- 27. USING A FOR UPDATE CURSOR • The CURSOR FOR UPDATE clause is only used with a cursor when you want to update tables in the database. • Generally, when you execute a SELECT statement, you are not locking any rows. • The purpose of using the FOR UPDATE clause is to lock the rows of the tables that you want to update, so that another user cannot perform an update until you perform your update and release the lock. • The next COMMIT or ROLLBACK statement releases the lock. Bordoloi and
- 28. USING A FOR UPDATE CURSOR • The syntax is simply to add FOR UPDATE to the end of the cursor definition. • If there are multiple items being selected, but you only want to lock one of them, then end the cursor definition with the following syntax: FOR UPDATE OF <item_name> Bordoloi and
- 29. WHERE CURRENT OF CLAUSE • Use WHERE CURRENT OF when you want to update the most recently fetched row. • WHERE CURRENT OF can only be used with a FOR UPDATE OF cursor. • The advantage of the WHERE CURRENT OF clause is that it enables you to eliminate the WHERE clause in the UPDATE statement. Bordoloi and
- 30. Example DECLARE CURSOR c_stud_zip IS SELECT s.studid_id, z.city FROM student s, zipcode z WHERE z.city = 'Brooklyn' AND s.szip = z.zip FOR UPDATE OF sphone; BEGIN FOR r_stud_zip IN c_stud_zip LOOP DBMS_OUTPUT.PUT_LINE(r_stud_zip.studid); UPDATE student SET sphone = '718'||SUBSTR(sphone,4) WHERE CURRENT OF c_stud_zip; END LOOP; END; Bordoloi and
- 31. END Bordoloi and