plsql les02

Editor's Notes

• #3: Lesson Aim In this lesson, you learn how to create and invoke functions.
• #4: Overview of Stored Functions A function is a named PL/SQL block that can accept parameters, be invoked, and return a value. In general, you use a function to compute a value. Functions and procedures are structured alike. A function must return a value to the calling environment, whereas a procedure returns zero or more values to its calling environment. Like a procedure, a function has a header, a declarative section, an executable section, and an optional exception-handling section. A function must have a RETURN clause in the header and at least one RETURN statement in the executable section. Functions can be stored in the database as schema objects for repeated execution. A function that is stored in the database is referred to as a stored function. Functions can also be created on client-side applications. Functions promote reusability and maintainability. When validated, they can be used in any number of applications. If the processing requirements change, only the function needs to be updated. A function may also be called as part of a SQL expression or as part of a PL/SQL expression. In the context of a SQL expression, a function must obey specific rules to control side effects. In a PL/SQL expression, the function identifier acts like a variable whose value depends on the parameters passed to it.
• #5: Syntax for Creating Functions A function is a PL/SQL block that returns a value. A RETURN statement must be provided to return a value with a data type that is consistent with the function declaration. You create new functions with the CREATE FUNCTION statement, which may declare a list of parameters, must return one value, and must define the actions to be performed by the standard PL/SQL block. You should consider the following points about the CREATE FUNCTION statement: The REPLACE option indicates that if the function exists, it will be dropped and replaced with the new version that is created by the statement. The RETURN data type must not include a size specification. The PL/SQL block starts with a BEGIN after declaration of any local variables and ends with an END , optionally followed by the function_name . There must be at least one RETURN expression statement . You cannot reference host or bind variables in the PL/SQL block of a stored function. Note: Although the OUT and IN OUT parameter modes can be used with functions, it is not good programming practice to use them with functions. However, if you need to return more than one value from a function, consider returning the values in a composite data structure such as a PL/SQL record or a PL/SQL table.
• #6: How to Develop Stored Functions The diagram illustrates the basic steps involved in developing a stored function. To develop a stored function, perform the following steps: 1. Create a file by using your favorite text or code editor to edit the function syntax, and saving the code in a file typically with a .sql extension. 2. Load the function code from the file into the buffer by using i SQL*Plus as the PL/SQL development environment. 3. Execute the CREATE FUNCTION statement to compile and store the function in the database. 4. After successful compilation, invoke the function from a PL/SQL environment or application. Returning a Value Add a RETURN clause with the data type in the header of the function. Include one RETURN statement in the executable section. Multiple RETURN statements are allowed in a function (usually within an IF statement). Only one RETURN statement is executed, because after the value is returned, processing of the block ceases. Use the SHOW ERRORS or SHOW ERRORS FUNCTION function_name i SQL*Plus commands to view compilation errors.
• #7: Stored Function: Example The get_sal function is created with a single input parameter and returns the salary as a number. Execute the command as shown, or save it in a script file and run the script to create the get_sal function. The get_sal function returns the common programming practice of assigning a returning value to a local variable and uses a single RETURN statement in the executable section of the code to return the value stored in the local variable. If your function has an exception section, then it may also contain a RETURN statement. Invoke a function as part of a PL/SQL expression, because the function will return a value to the calling environment. The second code box uses the i SQL*Plus EXECUTE command to call the DBMS_OUTPUT.PUT_LINE procedure whose argument is the return value from the function get_sal . In this case, get_sal is invoked first to calculate the salary of the employee with ID 100. The salary value returned is supplied as the value of the DBMS_OUTPUT.PUT_LINE parameter, which displays the result (if you have executed a SET SERVEROUTPUT ON ). Note: A function must always return a value. The example does not return a value if a row is not found for a given id . Ideally, create an exception handler to return a value as well.
• #8: Ways to Execute Functions If functions are designed thoughtfully, they can be powerful constructs. Functions can be invoked in the following ways: As part of PL/SQL expressions: You can use host or local variables to hold the returned value from a function. The first example in the slide uses a host variable and the second example uses a local variable in an anonymous block. As a parameter to another subprogram: The third example in the slide demonstrates this usage. The get_sal function with all its arguments is nested in the parameter required by the DBMS_OUTPUT.PUT_LINE procedure. This comes from the concept of nesting functions as discussed in the Oracle Database 10g: SQL Fundamentals I course. As an expression in a SQL statement: The last example shows how a function can be used as a single-row function in a SQL statement. Note: The benefits and restrictions that apply to functions when used in a SQL statement are discussed in the next few pages.
• #9: Advantages of User-Defined Functions in SQL Statements SQL statements can reference PL/SQL user-defined functions anywhere a SQL expression is allowed. For example, a user-defined function can be used anywhere a built-in SQL function, such as UPPER() , can be placed. Advantages Permits calculations that are too complex, awkward, or unavailable with SQL Increases data independence by processing complex data analysis within the Oracle server, rather than by retrieving the data into an application Increases efficiency of queries by performing functions in the query rather than in the application Manipulates new types of data (for example, latitude and longitude) by encoding character strings and using functions to operate on the strings
• #10: Function in SQL Expressions: Example The example in the slide shows how to create a tax function to calculate income tax. The function accepts a NUMBER parameter and returns the calculated income tax based on a simple flat tax rate of 8%. In i SQL*Plus, the tax function is invoked as an expression in the SELECT clause along with the employee ID, last name, and salary for employees in a department with ID 100 . The return result from the tax function is displayed with the regular output from the query.
• #11: Locations to Call User-Defined Functions A PL/SQL user-defined function can be called from any SQL expression where a built-in single-row function can be called. Example: SELECT employee_id, tax(salary) FROM employees WHERE tax(salary) > (SELECT MAX( tax(salary) ) FROM employees WHERE department_id = 30) ORDER BY tax(salary) DESC; …
• #12: Restrictions on Calling Functions from SQL Expressions The user-defined PL/SQL functions that are callable from SQL expressions must meet the following requirements: The function must be stored in the database. The function parameters must be input only and valid SQL data types. The functions must return data types that are valid SQL data types. They cannot be PL/SQL-specific data types such as BOOLEAN , RECORD , or TABLE . The same restriction applies to the parameters of the function. The following restrictions apply when calling a function in a SQL statement: Parameters must use positional notation. Named notation is not supported. You must own or have the EXECUTE privilege on the function. Other restrictions on a user-defined function include the following: It cannot be called from the CHECK constraint clause of a CREATE TABLE or ALTER TABLE statement. It cannot be used to specify a default value for a column. Note: Only stored functions are callable from SQL statements. Stored procedures cannot be called unless invoked from a function that meets the preceding requirements.
• #13: Controlling Side Effects When Calling Functions from SQL Expressions To execute a SQL statement that calls a stored function, the Oracle server must know whether the function is free of specific side effects. The side effects are unacceptable changes to database tables. Additional restrictions apply when a function is called in expressions of SQL statements. In particular, when a function is called from : A SELECT statement or a parallel UPDATE or DELETE statement, the function cannot modify any database tables An UPDATE or DELETE statement, the function cannot query or modify any database tables modified by that statement A SELECT , INSERT , UPDATE , or DELETE statement, the function cannot execute directly or indirectly through another subprogram or SQL transaction control statements such as: A COMMIT or ROLLBACK statement A session control statement (such as SET ROLE ) A system control statement (such as ALTER SYSTEM ) Any DDL statements (such as CREATE ), because they are followed by an automatic commit
• #14: Restrictions on Calling Functions from SQL: Example The dml_call_sql function in the slide contains an INSERT statement that inserts a new record into the EMPLOYEES table and returns the input salary value incremented by 100. This function is invoked in the UPDATE statement that modifies the salary of employee 170 to the amount returned from the function. The UPDATE statement fails with an error indicating that the table is mutating (that is, changes are already in progress in the same table). In the following example, the query_call_sql function queries the SALARY column of the EMPLOYEES table: CREATE OR REPLACE FUNCTION query_call_sql(a NUMBER) RETURN NUMBER IS s NUMBER; BEGIN SELECT salary INTO s FROM employees WHERE employee_id = 170; RETURN (s + a); END; When invoked from the following UPDATE statement, it returns the error message similar to the error message shown in the slide: UPDATE employees SET salary = query_call_sql(100) WHERE employee_id = 170;
• #15: Removing Functions When a stored function is no longer required, you can use a SQL statement in i SQL*Plus to drop it. To remove a stored function by using i SQL*Plus, execute the DROP FUNCTION SQL command. CREATE OR REPLACE Versus DROP and CREATE The REPLACE clause in the CREATE OR REPLACE syntax is equivalent to dropping a function and re-creating it. When you use the CREATE OR REPLACE syntax, the privileges granted on this object to other users remain the same. When you DROP a function and then re-create it, all the privileges granted on this function are automatically revoked.
• #16: Viewing Functions in the Data Dictionary The source code for PL/SQL functions is stored in the data dictionary tables. The source code is accessible for PL/SQL functions that are successfully or unsuccessfully compiled. To view the PL/SQL function code stored in the data dictionary, execute a SELECT statement on the following tables where the TYPE column value is 'FUNCTION' : The USER_SOURCE table to display the PL/SQL code that you own The ALL_SOURCE table to display the PL/SQL code to which you have been granted EXECUTE right by the owner of that subprogram code The first query example shows how to display the source code for all the functions in your schema. The second query, which uses the USER_OBJECTS data dictionary view, lists the names of all functions that you own.
• #17: How Procedures and Functions Differ You create a procedure to store a series of actions for later execution. A procedure can contain zero or more parameters that can be transferred to and from the calling environment, but a procedure does not have to return a value. A procedure can call a function to assist with its actions. Note: A procedure containing a single OUT parameter would be better rewritten as a function returning the value. You create a function when you want to compute a value that must be returned to the calling environment. A function can contain zero or more parameters that are transferred from the calling environment. Functions typically return only a single value, and the value is returned through a RETURN statement. The functions used in SQL statements should not use OUT or IN OUT mode parameters. Although a function using output parameters can be used in a PL/SQL procedure or block, it cannot be used in SQL statements.
• #18: Summary A function is a named PL/SQL block that must return a value. Generally, you create a function to compute and return a value, and you create a procedure to perform an action. A function can be created or dropped. A function is invoked as a part of an expression.
• #19: Practice 2: Overview If you encounter compilation errors when using i SQL*Plus, use the SHOW ERRORS command. If you correct any compilation errors in i SQL*Plus, do so in the original script file, not in the buffer, and then rerun the new version of the file. This will save a new version of the program unit to the data dictionary.