PL/SQL高级-Creating Packages (00)

Objectives

After completing this lesson,you should be able to do the following

• Describe packages and list their components
• Create a package to group together related variables,cursors,constants,exceptions,procedures,and fuctions.
• Designate a package construct as either public or private
• Invoke a package construct
• Describe the use of a bodiless package

Lesson Agenda

• Identifying the benefits and the components of packages
• Working with packages
  • -Creating the package specification and body
  • -Invoking the package subprograms
  • -Removing a package
  • -Displaying the package information

What Are PL/SQL Packages?

• A package is a schema object that groups logically related PL/SQL types,variables,and subprograms.
• Packages usually have two parts:
  • -A specification(spec)
  • -A body
• The specification is the interface to the package.It declare the types,variables,constants,exceptions,cursors,and subprograms that can be referenced from outside the package.
• The body defines the queries for the cursors and the code for the subprograms.
• Enable the Oracle server to read multiple objects into memory at once.

Advantages of Using Packages

• Modularity:Encapsulating related constructs
• Easier maintenance:Keepling logically related functionality together
• Easier apllication design:Coding and compiling the specification and body separately
• Hiding information:
  • Only the declarations  in the package sepcification are visible and accessible to applications.
  • Private constructs in the package body are hidden and inaccessible.
  • All coding is hidden in the package body
• Added functionality:Persistency of public variables and cursors
• Better performance:
  • -The entire package is loaded into memory when the package is first referenced.
  • -The is only one copy in memory for all users
  • -The dependency hierarchy is simplified.
• Overloading:Multiple subprograms of the same name
• The package is a powerful and important element of the PL/SQL language.It should be the cornerstone of any application development project.
• Always construct your application around packages;avoid standlone procedure and fuctions.Even if tody you think that only one procedure is needed for a certain area of functionality,in the future you will almost certainly have two,then three,and then a dozen.At which point,you will find yourself saying,"Gee,I should really collect those together in a package!" That`s fine,except that now you have to go back to all the invocations of those unpackaged procedures and functions and add in the package prefix.So start wit a package and save yourself the trouble.