Tài liệu Oracle PLSQL Language- P7 pdf

as a whole. 6.3.2.2 Vulnerability to data errors If an implicit SELECT statement returns more than one row, it raises the TOO_MANY_ROWS exception. When this happens, execution in the current block terminates and control is passed to the exception section. Unless you deliberately plan to handle this scenario, use of the implicit cursor is a declaration of faith. You are saying, "I trust that query to always return a single row!" It may well be that today, with the current data, the query will only return a single row. If the nature of the data ever changes, however, you may find that the SELECT statement which formerly identified a single row now returns several. Your program will raise an exception. Perhaps this is what you will want. On the other hand, perhaps the presence of additional records is inconsequential and should be ignored. With the implicit query, you cannot easily handle these different possibilities. With an explicit query, your program will be protected against changes in data and will continue to fetch rows without raising exceptions. 6.3.2.3 Diminished programmatic control The implicit cursor version of a SELECT statement is a black box. You pass the SQL statement to the SQL layer in the database and it returns (you hope) a single row. You can't get inside the separate operations of the cursor, such as the open and close stages. You can't examine the attributes of the cursor to see whether a row was found, for example, or if the cursor has already been opened. You can't easily apply traditional programming control constructs, such as an IF statement, to your data access. Sometimes you don't need this level of control. Sometimes you just think you don't need this level of control. I have found that if I am going to build programs in PL/SQL, I want as much control as I can possibly get. Always Use Explicit Cursors! My rule of thumb is always to use an explicit cursor for all SELECT statements in my applications, even if an implicit cursor might run a little bit faster and even if, by coding an explicit cursor, I have to write more code (declaration, open, fetch, close). By setting and following this clear-cut rule, I give myself one less thing to think about. I do not have to determine if a particular SELECT statement will return only one row and therefore be a candidate for an implicit cursor. I do not have to wonder about the conditions under which a single-row query might suddenly return more than one row, thus requiring a TOO_MANY_ROWS exception handler. I am guaranteed to get vastly improved programmatic control over that data access and more finely- tuned exception handling for the cursor. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 6.3.3 Explicit Cursors An explicit cursor is a SELECT statement that is explicitly defined in the declaration section of your code and, in the process, assigned a name. There is no such thing as an explicit cursor for UPDATE, DELETE, and INSERT statements. With explicit cursors, you have complete control over how to access information in the database. You decide when to OPEN the cursor, when to FETCH records from the cursor (and therefore from the table or tables in the SELECT statement of the cursor) how many records to fetch, and when to CLOSE the cursor. Information about the current state of your cursor is available through examination of the cursor attributes. This granularity of control makes the explicit cursor an invaluable tool for your development effort. Let's look at an example. The following anonymous block looks up the employee type description for an employee type code: 1 DECLARE 2 /* Explicit declaration of a cursor */ 3 CURSOR emptyp_cur IS 4 SELECT emptyp.type_desc 5 FROM employees emp, employee_type emptyp 6 WHERE emp.type_code = emptyp.type_code; 7 BEGIN 8 /* Check to see if cursor is already open. If not, open it. */ 9 IF NOT emptyp_cur%ISOPEN 10 THEN 11 OPEN emptyp_cur; 12 END IF; 13 14 /* Fetch row from cursor directly into an Oracle Forms item */ 15 FETCH emptyp_cur INTO :emp.type_desc; 16 17 /* Close the cursor */ 18 CLOSE emptyp_cur; 19 END; This PL/SQL block performs the following cursor actions: Action Line(s) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Declare the cursor 3 Open the cursor (if not already open) 9, 11 Fetch one or more rows from the cursor 15 Close the cursor 18 The next few sections examine each of these steps in more detail. For the remainder of this chapter, unless noted otherwise, the word "cursor" refers to the explicit cursor. Previous: 6.2 Cursors in PL/ SQL Oracle PL/SQL Programming, 2nd Edition Next: 6.4 Declaring Cursors 6.2 Cursors in PL/SQL Book Index 6.4 Declaring Cursors The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates. All rights reserved. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Previous: 6.1 Transaction Management Chapter 6 Database Interaction and Cursors Next: 6.3 Implicit and Explicit Cursors 6.2 Cursors in PL/SQL When you execute a SQL statement from PL/SQL, the Oracle RDBMS assigns a private work area for that statement. This work area contains information about the SQL statement and the set of data returned or affected by that statement. The PL/SQL cursor is a mechanism by which you can name that work area and manipulate the information within it. In its simplest form, you can think of a cursor as a pointer into a table in the database. For example, the following cursor declaration associates the entire employee table with the cursor named employee_cur: CURSOR employee_cur IS SELECT * FROM employee; Once I have declared the cursor, I can open it: OPEN employee_cur; And then I can fetch rows from it: FETCH employee_cur INTO employee_rec; and, finally, I can close the cursor: CLOSE employee_cur; In this case, each record fetched from this cursor represents an entire record in the employee table. You can, however, associate any valid SELECT statement with a cursor. In the next example I have a join of three tables in my cursor declaration: DECLARE CURSOR joke_feedback_cur IS Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. SELECT J.name, R.laugh_volume, C.name FROM joke J, response R, comedian C WHERE J.joke_id = R.joke_id AND J.joker_id = C.joker_id; BEGIN END; Here, the cursor does not act as a pointer into any actual table in the database. Instead, the cursor is a pointer into the virtual table represented by the SELECT statement (SELECT is called a virtual table because the data it produces has the same structure as a table rows and columns but it exists only for the duration of the execution of the SQL statement). If the triple-join returns 20 rows, each row containing the three columns in the preceding example, then the cursor functions as a pointer into those 20 rows. 6.2.1 Types of Cursors You have lots of options in PL/SQL for executing SQL, and all of them occur as some type of cursor. Generally, there are two types of SQL that you can execute in PL/SQL: static and dynamic. SQL is static if the content of the SQL statement is determined at compile time. A SQL statement is dynamic if it is constructed at runtime and then executed. Dynamic SQL is made possible in PL/SQL only through the use of the DBMS_SQL built-in package (see Appendix C, Built-In Packages). All other forms of SQL executed inside a PL/SQL program represent static SQL; these forms of cursors are the focus of the remainder of this chapter. Even within the category of static SQL, we have further differentiation. With the advent of PL/SQL Release 2.3, you can choose between two distinct types of cursor objects: Static cursor objects These are the really static cursors of PL/SQL. The SQL is determined at compile time, and the cursor always refers to one SQL statement, which is known at compile time. The examples shown earlier in this chapter are static cursors. Unless otherwise noted, any reference to "static cursor" refers to this sub-category of static (as opposed to dynamic) cursors. Cursor variables You can declare a variable which references a cursor object in the database. Your variable may refer to different SQL statements at different times (but that SQL is defined at compile time, not run time). The cursor variable is one of the newest enhancements to PL/SQL and will be unfamiliar to most programmers. Cursor variables act as references to cursor objects. As a true variable, a cursor Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. variable can change its value as your program executes. The variable can refer to different cursor objects (queries) at different times. You can also pass a cursor variable as a parameter to a procedure or function. Cursor variables are discussed later in this chapter. Static PL/SQL cursors have been available since PL/SQL Version 1. The static version of cursors "hardcodes" a link between the cursor name and a SELECT statement. The static cursor itself comes in two flavors: implicit and explicit. PL/SQL declares and manages an implicit cursor every time you execute a SQL DML statement, such as an INSERT or a SELECT that returns a single row. You, the programmer, define your own explicit cursors in your code. You must use an explicit cursor when you need to retrieve more than one row of data at a time through a SELECT statement. You can then use the cursor to fetch these rows one at a time. The set of rows returned by the query associated with an explicit cursor is called the active set or result set of the cursor. The row to which the explicit cursor points is called the current row of the result set. The bulk of this chapter is devoted to the management of static, explicit cursors. All information about cursor variables is localized in Section 6.12, "Cursor Variables". Any references to PL/SQL cursors and cursor characteristics outside of that section will pertain to static cursors. 6.2.2 Cursor Operations Regardless of the type of cursor, PL/SQL performs the same operations to execute a SQL statement from within your program: PARSE The first step in processing an SQL statement is to parse it to make sure it is valid and to determine the execution plan (using either the rule-based or cost-based optimizer). BIND When you bind, you associate values from your program (host variables) with placeholders inside your SQL statement. For static SQL, the SQL engine itself performs these binds. When you use dynamic SQL, you explicitly request a binding of variable values. OPEN When you open a cursor, the bind variables are used to determine the result set for the SQL statement. The pointer to the active or current row is set to the first row. Sometimes you will not explicitly open a cursor; instead the PL/SQL engine will perform this operation for you (as with implicit cursors). EXECUTE In the execute phase, the statement is run within the SQL engine. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. FETCH If you are performing a query, the FETCH command retrieves the next row from the cursor's result set. Each time you fetch, PL/SQL moves the pointer forward in the result set. When working with explicit cursors, remember that if there are no more rows to retrieve, then FETCH does nothing (it does not raise an error). CLOSE The CLOSE statement closes the cursor and releases all memory used by the cursor. Once closed, the cursor no longer has a result set. Sometimes you will not explicitly close a cursor; instead the PL/SQL engine will perform this operation for you (as with implicit cursors). Figure 6.1 shows how some of these different operations are used to fetch information from the database into your PL/SQL program. Figure 6.1: Using cursor operations to fetch database information into your program Previous: 6.1 Transaction Management Oracle PL/SQL Programming, 2nd Edition Next: 6.3 Implicit and Explicit Cursors 6.1 Transaction Management Book Index 6.3 Implicit and Explicit Cursors The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates. All rights reserved. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Previous: 5.2 Sequential Control Statements Chapter 6 Next: 6.2 Cursors in PL/ SQL 6. Database Interaction and Cursors Contents: Transaction Management Cursors in PL/SQL Implicit and Explicit Cursors Declaring Cursors Opening Cursors Fetching from Cursors Column Aliases in Cursors Closing Cursors Cursor Attributes Cursor Parameters SELECT FOR UPDATE in Cursors Cursor Variables Working with Cursors PL/SQL is tightly integrated with the Oracle database via the SQL language. From within PL/SQL, you can execute any DML (data manipulation language) statements, including INSERTs, UPDATEs, DELETEs, and, of course, queries. You can also join multiple SQL statements together logically as a transaction, so that they are either saved ("committed" in SQL parlance) together or rejected in their entirety (rolled back). This chapter examines the SQL statements available inside PL/SQL to manage transactions. It then moves on to cursors, which give you a way to fetch and process database information in your PL/SQL program. 6.1 Transaction Management The Oracle RDBMS provides a very robust transaction model, as you might expect for a relational database. You (or more precisely, your application code) determine what constitutes a transaction, the logical unit of work that must be either saved together with a COMMIT statement or rolled back together with a ROLLBACK statement. A transaction begins implicitly with the first SQL statement issued since the last COMMIT or ROLLBACK (or with the start of a session). Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. PL/SQL provides the following statements for transaction management: COMMIT Saves all outstanding changes since the last COMMIT or ROLLBACK and releases all locks. ROLLBACK Erases all outstanding changes since the last COMMIT or ROLLBACK and releases all locks. ROLLBACK TO SAVEPOINT Erases all changes made since the specified savepoint was established. SAVEPOINT Establishes a savepoint, which then allows you to perform partial ROLLBACKs. SET TRANSACTION Allows you to begin a read-only or read-write session, establish an isolation level, or assign the current transaction to a specified rollback segment. LOCK TABLE Allows you to lock an entire database table in the specified mode. This overrides the default row-level locking usually applied to a table. These statements are explained in more detail in the following sections. 6.1.1 The COMMIT Statement When you COMMIT, you make permanent any changes made by your session to the database in the current transaction. Once you commit, your changes will be visible to other Oracle sessions or users. The syntax for the COMMIT statement is: COMMIT [WORK] [COMMENT text]; The WORK keyword is optional and can be used to improve readability. The COMMENT keyword specifies a comment which is then associated with the current transaction. The text must be a quoted literal and can be no more than 50 characters in length. The COMMENT text is usually employed with distributed transactions. This text can be handy for examining and resolving in-doubt transactions within a two-phase commit framework. It is stored in the data dictionary along with the transaction ID. Note that COMMIT releases any row and table locks issued in your session, such as with a SELECT FOR UPDATE statement. It also erases any savepoints issued since the last COMMIT or Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ROLLBACK. Once you COMMIT your changes, you cannot roll them back with a ROLLBACK statement. The following statements are all valid uses of the COMMIT: COMMIT; COMMIT WORK; COMMIT COMMENT 'maintaining account balance'. 6.1.2 The ROLLBACK Statement When you ROLLBACK, you undo some or all changes made by your session to the database in the current transaction. Why would you want to erase changes? From an ad hoc SQL standpoint, the ROLLBACK gives you a way to erase mistakes you might have made, as in: DELETE FROM orders; "No, no! I meant to delete only the orders before May 1995!" No problem, just issue ROLLBACK. From an application coding standpoint, ROLLBACK is important because it allows you to clean up or restart from a "clean state" when a problem occurs. The syntax for the ROLLBACK statement is: ROLLBACK [WORK] [TO [SAVEPOINT] savepoint_name]; There are two basic ways to use ROLLBACK: without parameters or with the TO clause to indicate a savepoint at which the ROLLBACK should stop. The parameterless ROLLBACK undoes all outstanding changes in your transaction. The ROLLBACK TO version allows you to undo all changes and release all acquired locks which were issued since the savepoint identified by savepoint_name was marked (see the next section on the SAVEPOINT statement for more information on how to mark a savepoint in your application). The savepoint_name is an undeclared Oracle identifier. It cannot be a literal (enclosed in quotes) or variable name. All of the following uses of ROLLBACK are valid: ROLLBACK; ROLLBACK WORK; ROLLBACK TO begin_cleanup; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... transaction) Previous: 5.2 Sequential Control Statements 5.2 Sequential Control Statements Oracle PL/SQL Programming, 2nd Edition Book Index Next: 6.2 Cursors in PL/ SQL 6.2 Cursors in PL/SQL The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates All rights reserved Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Previous: 5.1 Conditional Control Statements Chapter... 5.1 Conditional Control Statements 5.1 Conditional Control Statements Oracle PL/SQL Programming, 2nd Edition Book Index Next: 6 Database Interaction and Cursors 6 Database Interaction and Cursors The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates All rights reserved Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Previous: 4.7 Tips for Creating and Using... Creating and Using Variables 4.7 Tips for Creating and Using Variables Oracle PL/SQL Programming, 2nd Edition Next: 5.2 Sequential Control Statements Book Index 5.2 Sequential Control Statements The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates All rights reserved Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Previous: 4.6 ProgrammerDefined Subtypes... purchase PDF Split-Merge on www.verypdf.com to remove this watermark where table_reference_list is a list of one or more table references (identifying either a local table/ view or a remote entity through a database link), and lock_mode is the mode of the lock, which can be one of the following: ROW SHARE ROW EXCLUSIVE SHARE UPDATE SHARE SHARE ROW EXCLUSIVE EXCLUSIVE If you specify the NOWAIT keyword, Oracle. .. more visible You can also issue an EXIT statement for a particular labeled loop from within another (enclosed) loop Finally, you can GOTO that loop label, even though it was Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark "designed" for a loop (Chapter 7 describes the details of loop processing.) When PL/SQL encounters a GOTO statement, it immediately shifts control to the... END LOOP; END; FUNCTION new_formula (molecule_in IN NUMBER) RETURN VARCHAR2 IS BEGIN construct formula for molecule RETURN formula_string; /* Illegal! */ END; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 5.2.1.2 Target labels and scope of GOTO The target label must be in the same scope as the GOTO statement In the context of the GOTO statement, each of the... THEN GOTO old_status; /* Crosses IF clause boundary! */ ELSIF status = 'OLD' THEN GOTO new_status; /* Crosses IF clause boundary! */ END IF; Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark q Don't jump into the middle of a loop You cannot jump into the middle of a loop with a GOTO This code produces an error: FOR month_num IN 1 12 LOOP ... || The label and GOTO must be in the same section! */ GOTO out_of_here; EXCEPTION WHEN OTHERS THEN /* Out of scope! */ NULL; END; 5.2.2 The NULL Statement Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Usually when you write a statement in a program, you want it to do something There are cases, however, when you want to tell PL/SQL to do absolutely nothing,... possibility and I really want nothing to happen." IF :report.selection = 'DETAIL' THEN exec_detail_report; ELSE NULL; END IF; 5.2.2.2 Nullifying the effect of a raised exception Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark The optional exception section of a program contains one or more exception handlers These handlers trap and handle errors that have been raised in your program... NULL statement is very different in each of these statements The addition of an ELSE NULL clause to the IF statement has an impact only on the readability of the code The Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark IF statement executes in precisely the same way it would have without the ELSE NULL clause When you include an exception handler with the NULL executable . Cursors The Oracle Library Navigation Copyright (c) 2000 O'Reilly & Associates. All rights reserved. Please purchase PDF Split-Merge on www.verypdf.com. the following cursor actions: Action Line(s) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. Declare the cursor 3 Open the