Lab Objective After this Lab, you will be able to: Up to this point in this book you have seen cursors used to gather specific data from a single SELECT statement. In Chapter 14, "Packages," you learned how to bring a number of procedures into a large program called a package. A package may have one cursor that is used by a few procedures. In this case, each of the procedures that use the same cursor would have to declare, open, fetch, and close the cursor. In the current version of PL/SQL, cursors can be declared and manipulated like any other PL/SQL variable. This type of variable is called a cursor variable or a REF CURSOR. A cursor variable is just a reference or a handle to a static cursor. It permits a programmer to pass this reference to the same cursor among all the program's units that need access to the cursor. A cursor variable binds the cursor's SELECT statement dynamically at runtime. Explicit cursors are used to name a work area that holds the information of a multirow query. A cursor variable may be used to point to the area in memory where the result of a multirow query is stored. The cursor always refers to the same information in a work area, while a cursor variable can point to different work areas. Cursors are static, and cursor variables can be seen as dynamic because they are not tied to any one specific query. Cursor variables give you easy access to centralized data retrieval. You can use a cursor variable to pass the result set of a query between stored procedures and various clients. A query work area remains accessible as long as a cursor variable points to it. So you can freely pass a cursor variable from one scope to another. There are two types of cursor variables; one is called strong and the other is called weak. To execute a multirow query, the Oracle server opens a work area called a cursor to store processing information. To access the information, you either name the work area, or you use a cursor variable that points to the work area. A cursor always refers to the same work area, and a cursor variable can refer to different work areas. Hence, cursors and cursor variables are not interoperable. An explicit cursor is static and is associated with one SQL statement. A cursor variable can be associated with different statements at runtime. Primarily you use a cursor variable to pass a pointer to query results sets between PL/SQL stored subprograms and various clients, such as a client Oracle Developer Forms application. None of them owns the result set; they simply share a pointer to the query work area that stores the result set. You can declare a cursor variable on the client side, open and fetch from it on the server side, and then continue to fetch from it on the client side. Cursor variables differ from cursors the way constants differ from variables. A cursor is static; a cursor variable is dynamic. In PL/SQL a cursor variable has a REF CURSOR data type, where REF stands for reference and CURSOR stands for the class of the object. You will now learn the syntax for declaring and using a cursor variable. To create a cursor variable, you first need to define a REF CURSOR type and then declare a variable of that type. Before you declare the REF CURSOR of a strong type, you must first declare a record that has the data types of the result set of the SELECT statement that you plan to use (note that this is not necessary for a weak REF CURSOR). FOR EXAMPLE TYPE inst_city_type IS RECORD (first_name instructor.first_name%TYPE; last_name instructor.last_name%TYPE; city zipcode.city%TYPE; state zipcode.state%TYPE) Second, you must declare a composite data type for the cursor variable that is of the type REF CURSOR. The syntax is as follows: TYPE ref_type_name is REF CURSOR [RETURN return_type]; The ref_type_name is a type specified in subsequent declarations. The return type represents a record type for a strong cursor; a weak cursor does not have a specific return type but can handle any combination of data items in a SELECT statement. The REF CURSOR keyword indicates that the new type will be a pointer to the defined type. The return_type indicates the types of SELECT list that are eventually returned by the cursor variable. The return type must be a record type. FOR EXAMPLE TYPE inst_city_cur IS REF CURSOR RETURN inst_city_type; A cursor variable can be strong (restrictive) or weak (nonrestrictive). A strong cursor variable is a REF CURSOR type definition that specifies a return_type; a weak definition does not. PL/SQL enables you to associate a strong type with type-comparable queries only, while a weak type can be associated with any query. This makes a strong cursor variable less error prone but weak REF CURSORS types more flexible. These are the key steps for handling a cursor variable: -
Define and declare the cursor variable. Open the cursor variable. Associate a cursor variable with a multirow SELECT statement, execute the query, and identify the result set. An OPEN FOR statement can open the same cursor variable for different queries. You do not need to close a cursor variable before reopening it. Keep in mind that when you reopen a cursor variable for a different query, the previous query is lost. -
Fetch rows from the result set. Retrieve rows from the result set one at a time. Note that the return type of the cursor variable must be compatible with the variable named in the INTO clause of the FETCH statement. The FETCH statement retrieves rows from the result set one at a time. PL/SQL verifies that the return type of the cursor variable is compatible with the INTO clause of the FETCH statement. For each query column value returned, there must be a type type-comparable variable in the INTO clause. Also, the number of query column values must equal the number of variables. In case of a mismatch in number or type, the error occurs at compiletime for strongly typed cursor variables and at runtime for weakly typed cursor variables. -
Close the cursor variable. The next example is a complete example showing the use of a cursor variable in a package. FOR EXAMPLE -- csh15_18a.sql CREATE OR REPLACE PACKAGE course_pkg AS TYPE course_rec_typ IS RECORD (first_name student.first_name%TYPE, last_name student.last_name%TYPE, course_no course.course_no%TYPE, description course.description%TYPE, section_no section.section_no%TYPE ); TYPE course_cur IS REF CURSOR RETURN course_rec_typ; PROCEDURE get_course_list (p_student_id NUMBER , p_instructor_id NUMBER , course_list_cv IN OUT course_cur); END course_pkg; CREATE OR REPLACE PACKAGE BODY course_pkg AS PROCEDURE get_course_list (p_student_id NUMBER , p_instructor_id NUMBER , course_list_cv IN OUT course_cur) IS BEGIN IF p_student_id IS NULL AND p_instructor_id IS NULL THEN OPEN course_list_cv FOR SELECT 'Please choose a student-' First_name, 'instructor combination' Last_name, NULL course_no, NULL description, NULL section_no FROM dual; ELSIF p_student_id IS NULL THEN OPEN course_list_cv FOR SELECT s.first_name first_name, s.last_name last_name, c.course_no course_no, c.description description, se.section_no section_no FROM instructor i, student s, section se, course c, enrollment e WHERE i.instructor_id = p_instructor_id AND i.instructor_id = se.instructor_id AND se.course_no = c.course_no AND e.student_id = s.student_id AND e.section_id = se.section_id ORDER BY c.course_no, se.section_no; ELSIF p_instructor_id IS NULL THEN OPEN course_list_cv FOR SELECT i.first_name first_name, i.last_name last_name, c.course_no course_no, c.description description, se.section_no section_no FROM instructor i, student s, section se, course c, enrollment e WHERE s.student_id = p_student_id AND i.instructor_id = se.instructor_id AND se.course_no = c.course_no AND e.student_id = s.student_id AND e.section_id = se.section_id ORDER BY c.course_no, se.section_no; END IF; END get_course_list; END course_pkg; You can pass query results sets between PL/SQL stored subprograms and various clients. This works because PL/SQL and its clients share a pointer to the query work area identifying the result set. This can be done in a client program like SQL*Plus by defining a host variable with a data type of REFCURSOR to hold the query result generated from a REF CURSOR in a stored program. In order to see what is being stored in the SQL*Plus variable, use the SQL*Plus PRINT command. Optionally you can have the SQL*Plus command SET AUTOPRINT ON to display the query results automatically. Lab 15.2 Exercises 15.2.1 Make Use of Cursor Variables a) | Take a look at the previous example and explain why the package has two different TYPE declarations. Also explain how the procedure get_course_list is making use of the cursor variable. | b) | Create a SQL*Plus variable that is a cursor variable type. | c) | Execute the procedure course_pkg.get_course_list, with three different types of variable combinations to show the three possible results sets. After you execute the procedure, display the values of the SQL*Plus variable you declared in question (a). | d) | Create another package called student_info_pkg that has a single procedure called get_student_info. The get_student_info package will have three parameters. The first one is the student_id and the second is a number called p_choice; the last is a weak cursor variable. The p_choice indicates what information will be delivered about the student. The p_choice indicates what information will deliver about the student. If it is 1, then return the information about the student from the STUDENT table. If it is 2, then list all the courses the student is enrolled in with the student names of the fellow students enrolled in the same section as the student with the student_id that was passed in. If it is 3, then return the instructor name for that student, with the information about the courses that the student is enrolled in. | e) | Run the get_student_info procedure in SQL*Plus and display the results. | Lab 15.2 Exercise Answers 15.2.1 Answersa) | Take a look at the previous example and explain why the package has two different TYPE declarations. Also explain how the procedure get_course_list is making use of the cursor variable. | A1: | Answer: In script ch15_18a there are two declarations of a TYPE in the package header. The first is for the record type course_rec_type. This record type is declared to define the result set of the SELECT statements that will be used for the cursor variable. When data items in a record do not match a single table, it is necessary to create a record type. The second TYPE declaration is for the cursor variable also known as REF CURSOR. The variable has the name, course_cur, and it is declared as a strong cursor, meaning that it can only be used for a single record type. The record type is, course_rec_type. The procedure get_course_list in the course_pkg is made so that it can return a cursor variable that holds three different result sets. Each of the result sets is of the same record type. The first type is for when both IN parameters of student ID and instructor ID are null. This will produce a result set that is a message, 'Please choose a student-instructor combination.' The next way the procedure runs is if the instructor_id is passed in but the student_id is null (note that the logic of the procedure is a reverse negative; saying in the second clause of the IF statement p_student_id IS NULL, means when the instructor_id is passed in). This will run a SELECT statement to populate the cursor variable that holds a list of all the courses this instructor teaches and the students enrolled in these classes. The last way this can run is for a student_id and no instructor_id. This will produce a result set of all the courses the student is enrolled in and the instructors for each section. Also note that while the cursor variable is opened it is never closed. | b) | Create a SQL*Plus variable that is a cursor variable type. | A2: | SQL> VARIABLE course_cv REFCURSOR | c) | Execute the procedure course_pkg.get_course_list, with three different types of variable combinations to show the three possible results sets. After you execute the procedure, display the values of the SQL*Plus variable you declared in question (a). | A3: | Answer: There are three ways to execute this procedure. The first way would be to pass a student ID and not an instructor ID. SQL> exec course_pkg.get_course_list(102, NULL, :course_cv); PL/SQL procedure successfully completed. SQL> print course_cv FIRST_NAME LAST_NAME COURSE_NO DESCRIPTION SECTION_NO ---------- ---------- ---------- ---------------------- ---------- Charles Lowry 25 Intro to Programming 2 Nina Schorin 25 Intro to Programming 5 The next method would be to pass an instructor ID and not a student ID. SQL> exec course_pkg.get_course_list(NULL, 102, :course_cv); PL/SQL procedure successfully completed. SQL> print course_cv FIRST_NAME LAST_NAME COURSE_NO DESCRIPTION SECTION_NO ------------ ------------ --------- ------------------------ ---------- Jeff Runyan 10 DP Overview 2 Dawn Dennis 25 Intro to Programming 4 May Jodoin 25 Intro to Programming 4 Jim Joas 25 Intro to Programming 4 Arun Griffen 25 Intro to Programming 4 Alfred Hutheesing 25 Intro to Programming 4 Lula Oates 100 Hands-On Windows 1 Regina Bose 100 Hands-On Windows 1 Jenny Goldsmith 100 Hands-On Windows 1 Roger Snow 100 Hands-On Windows 1 Rommel Frost 100 Hands-On Windows 1 Debra Boyce 100 Hands-On Windows 1 Janet Jung 120 Intro to Java Programming 4 John Smith 124 Advanced Java Programming 1 Charles Caro 124 Advanced Java Programming 1 Sharon Thompson 124 Advanced Java Programming 1 Evan Fielding 124 Advanced Java Programming 1 Ronald Tangaribuan 124 Advanced Java Programming 1 N Kuehn 146 Java for C/C++ Programmers 2 Derrick Baltazar 146 Java for C/C++ Programmers 2 Angela Torres 240 Intro to the Basic Language 2 The last method would be not to pass either the student ID or the instructor ID. SQL> exec course_pkg.get_course_list(NULL, NULL, :course_cv); PL/SQL procedure successfully completed. SQL> print course_cv FIRST_NAME LAST_NAME C DESCRIPTION S ----------------------- ------------------------- - --------------- Please choose a student- instructor combination | d) | Create another package called student_info_pkg that has a single procedure called get_student_info. The get_student_info package will have three parameters. The first one is the student_id and the second is a number called p_choice; the last is a weak cursor variable. The p_choice indicates what information will be delivered about the student. The p_choice indicates what information will deliver about the student. If it is 1, then return the information about the student from the STUDENT table. If it is 2, then list all the courses the student is enrolled in with the student names of the fellow students enrolled in the same section as the student with the student_id that was passed in. If it is 3, then return the instructor name for that student, with the information about the courses that the student is enrolled in. | A4: | CREATE OR REPLACE PACKAGE student_info_pkg AS TYPE student_details IS REF CURSOR; PROCEDURE get_student_info (p_student_id NUMBER , p_choice NUMBER , details_cv IN OUT student_details); END student_info_pkg; CREATE OR REPLACE PACKAGE BODY student_info_pkg AS PROCEDURE get_student_info (p_student_id NUMBER , p_choice NUMBER , details_cv IN OUT student_details) IS BEGIN IF p_choice = 1 THEN OPEN details_cv FOR SELECT s.first_name first_name, s.last_name last_name, s.street_address address, z.city city, z.state state, z.zip zip FROM student s, zipcode z WHERE s.student_id = p_student_id AND z.zip = s.zip; ELSIF p_choice = 2 THEN OPEN details_cv FOR SELECT c.course_no course_no, c.description description, se.section_no section_no, s.first_name first_name, s.last_name last_name FROM student s, section se, course c, enrollment e WHERE se.course_no = c.course_no AND e.student_id = s.student_id AND e.section_id = se.section_id AND se.section_id in (SELECT e.section_id FROM student s, enrollment e WHERE s.student_id = p_student_id AND s.student_id = e.student_id) ORDER BY c.course_no; ELSIF p_choice = 3 THEN OPEN details_cv FOR SELECT i.first_name first_name, i.last_name last_name, c.course_no course_no, c.description description, se.section_no section_no FROM instructor i, student s, section se, course c, enrollment e WHERE s.student_id = p_student_id AND i.instructor_id = se.instructor_id AND se.course_no = c.course_no AND e.student_id = s.student_id AND e.section_id = se.section_id ORDER BY c.course_no, se.section_no; END IF; END get_student_info; END student_info_pkg; | e) | Run the get_student_info procedure in SQL*Plus and display the results. | A5: | SQL> VARIABLE student_cv REFCURSOR SQL> execute student_info_pkg.GET_STUDENT_INFO (102, 1, :student_cv); PL/SQL procedure successfully completed. SQL> print student_cv FIRST_ LAST_NAM ADDRESS CITY ST ZIP ------ -------- ------------------ --------------- -- ----- Fred Crocitto 101-09 120th St. Richmond Hill NY 11419 SQL> execute student_info_pkg.GET_STUDENT_INFO (102, 2, :student_cv); PL/SQL procedure successfully completed. SQL> print student_cv COURSE_NO DESCRIPTION SECTION_NO FIRST_NAME LAST_NAME ---------- ------------------ ---------- ---------- ----------- 25 Intro to Programming 2 Fred Crocitto 25 Intro to Programming 2 Judy Sethi 25 Intro to Programming 2 Jenny Goldsmith 25 Intro to Programming 2 Barbara Robichaud 25 Intro to Programming 2 Jeffrey Citron 25 Intro to Programming 2 George Kocka 25 Intro to Programming 5 Fred Crocitto 25 Intro to Programming 5 Hazel Lasseter 25 Intro to Programming 5 James Miller 25 Intro to Programming 5 Regina Gates 25 Intro to Programming 5 Arlyne Sheppard 25 Intro to Programming 5 Thomas Edwards 25 Intro to Programming 5 Sylvia Perrin 25 Intro to Programming 5 M. Diokno 25 Intro to Programming 5 Edgar Moffat 25 Intro to Programming 5 Bessie Heedles 25 Intro to Programming 5 Walter Boremmann 25 Intro to Programming 5 Lorrane Velasco SQL> execute student_info_pkg.GET_STUDENT_INFO (214, 3, :student_cv); PL/SQL procedure successfully completed. SQL> print student_cv FIRST_NAME LAST_NAME COURSE_NO DESCRIPTION SECTION_NO ---------- ------------ ---------- --------------------------- Marilyn Frantzen 120 Intro to Java Programming 1 Fernand Hanks 122 Intermediate Java Programming 5 Gary Pertez 130 Intro to Unix 2 Marilyn Frantzen 145 Internet Protocols 1 Rules for Using Cursor Variables You cannot use cursor variables with remote subprograms on another server. Do not use FOR UPDATE with OPEN FOR in processing a cursor variable. You cannot use comparison operators to test cursor variables. A cursor variable cannot be assigned a null value. A REF CURSOR types cannot be used in a CREATE TABLE or VIEW statements. A stored procedure that uses a cursor variable can only be used as a query block data source; it cannot be used for a DML block data source. Using a ref cursor is ideal for queries that are dependent only on variations in SQL statements and not PL/SQL.
| Lab 15.2 Self-Review Questions In order to test your progress, you should be able to answer the following questions. Answers appear in Appendix A, Section 15.2. 1) | The main benefit of using parameters with cursors is that it makes the cursor reusable. _____ True _____ False
| 2) | Which of the following are acceptable types of parameters to be used with cursors? _____ IN _____ OUT _____ %ROWTYPE _____ IN OUT
| 3) | By adding the keywords FOR UPDATE at the end of a cursor, you are _____ simply alerting the DBA that you are updating a table. _____ freeing up rollback segments for the update. _____ locking the indicated rows for an update. _____ creating a bind variable.
| 4) | Adding the keywords WHERE CURRENT OF to a FOR UPDATE cursor causes which of the following to take place? _____ The DBA gets annoyed. _____ Rows are locked and unlocked one at a time. _____ The update occurs for the current record in the cursor. _____ The scope of the cursor is increased.
| 5) | The principal difference between a FOR UPDATE cursor without a WHERE CURRENT OF clause and one with a WHERE CURRENT OF clause is that _____ without the clause the update needs to have a WHERE clause. _____ rows are only locked with the extra clause present. _____ only the items specified in the WHERE CURRENT OF clause are locked. _____ processing will only occur for the current row of the cursor.
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