PL/SQL package is nothing but a logical grouping of a related subprogram (procedure/function) into a single element. Package is compiled and stored as a database objects that can be used later.
In this tutorial, you will learn-
PL/SQL package basically have two components.
- Package Specification
- Package Body
Package specification consists of a declaration of all the public variables, cursors, objects, procedures, functions, and exception.
Below are few characteristics of Package specification
- The elements which are all declared in the specification can be accessed from outside of the package. Such element are known as public element.
- The package specification is a standalone element that means it can exist alone without package body.
- Whenever a package is referred an instance of the package is created for that particular session.
- After the instance is created for a session, all the package elements that are initiated in that instance are valid till the end of the session.
- The above syntax shows the creation of package specification.
It consists of definition of all the elements that are present in the package specification. It can also have definition of elements that are not declared in the specification, these elements are called private elements and can be called only from inside the package.
Below are characteristics of package body.
- It should contain definitions for all the subprograms/cursors that have been declared in the specification.
- It can also have more subprograms or other elements that are not declared in specification. These are called private elements.
- It is a dependable object, and it depends on package specification.
- The state of the package body becomes 'Invalid' every time when the specification is compiled. Therefore, it needs to be recompiled each time after the compilation of specification.
- The private elements should be defined first before they are used in the package body.
- The first part of the package is the global declaration part. This includes variables, cursors and private elements (forward declaration) that is visible to the entire package.
- The last part of the package is Package initialization part that execute one time whenever a package is referred first time in the session.
- The above syntax shows the creation of package body.
Now we are going to see how to refer package elements in the program.
Once the elements are declared and defined in the package, we need to refer the elements in order to use them.
All the public elements of the package can be referred by calling the package name followed by the element name separated by period i.e. '<package_name>.<element_name>'.
The public variable of the package can also be used in the same way to assign and fetch values from them i.e. '<package_name>.<variable_name>'.
In PL/SQL whenever a package is referred/called in a session a new instance will be created for that package.
Oracle provides a facility to initialize package elements or to perform any activity at the time of this instance creation through 'Package Initialization'.
This is nothing but an execution block that is written in the package body after defining all the package elements. This block will be executed whenever a package is referred for the first time in the session.
- The above syntax shows the definition of package initialization in the package body.
Forward declaration/reference in the package is nothing but declaring the private elements separately and defining it in the later part of the package body.
Private elements can be referred only if it is already declared in the package body. For this reason forward declaration is used. But it is rather unusual to use because in most of the time private elements are declared and defined in the first part of the package body.
Forward declaration is an option provided by oracle, it is not mandatory and using and not using is up to programmer's requirement.
- The above syntax shows forward declaration. The private elements are declared separately in the forward part of the package, and they have been defined in the later part.
Unlike other elements one needs to be careful in using cursors inside the package.
If the cursor is defined in the package specification or in global part of package body, then the cursor once opened will persist till the end of the session.
So one should always use the cursor attributes '%ISOPEN' to verify the state of the cursor before referring it.
Overloading is the concept of having many subprograms with the same name. These subprograms will be differing from each other by number of parameters or types of parameters or return type i.e. subprogram with same name but with different number of parameters, different type of parameters or different retype are considered as overloading.
This is useful when many subprogram needs to do the same task, but the way of calling each of them should be different. In this case, the subprogram name will be kept same for all and the parameters will be changed as per calling statement.
Example 1: In this example, we are going to create a package to get and set the values of employee's information in 'emp' table. The get_record function will return the record type output for the given employee number, and set_record procedure will insert the record type record into the emp table.
Step 1) Package Specification Creation
- Code line 1-5: Creating the package specification for guru99_get_set with one procedure and one function. These two are now public elements of this package.
Step 2) Package contains Package body, where all procedures and functions actual definition will be defined. In this step, Package Body is created.
- Code line 7: Creating the package body.
- Code line 9-16: Defining the element 'set_record' that is declared in the specification. This is same as defining the standalone procedure in PL/SQL.
- Code line 17-24: Defining the element 'get_record'. It is same as defining the standalone function.
- Code line 25-26: Defining the package initialization part.
Step 3) Creating an anonymous block to insert and display the records by referring to the above created package.
- Code line 34-37: Populating the data for record type variable in an anonymous block to call 'set_record' element of the package.
- Code line 38: Call has been made to 'set_record' of guru99_get_set package. Now the package is instantiated and it will persists till the end of the session.
- The package initialization part is executed since this is the first call to the package.
- The record in inserted by the 'set_record' element into the table.
- Code line 41: Calling the 'get_record' element in order to display the details of the inserted employee.
- The package is referred for the second time during the 'get_record' call to the package. But the initialization part is not executed this time as the package is already initialized in this session.
- Code line 42-45: Printing the employee details.
Since the package is the logical grouping of related things it has some dependencies. Following are the dependency that are to be taken care.
- Specification is a standalone object.
- Package body is dependent on specification.
- Package body can be compiled separately. Whenever specification is compiled, the body needs to be recompiled as it will become invalid.
- The subprogram in package body that are dependent on a private element should be defined only after the private element declaration.
- The database objects that are referred in the specification and body needs to be in valid status at the time of package compilation.
Once the package information is created, the package information such as package source, subprogram details and overload details are available in the oracle data definition tables.
Below table gives the data definition table and the package information that is available in the table.
|ALL_OBJECT||Gives the details of the package like object_id, creation_date, last_ddl_time, etc. It will contain the objects created by all users.||SELECT * FROM all_objects where object_name ='<package_name>'|
|USER_OBJECT||Gives the details of the package like object_id, creation_date, last_ddl_time, etc. It will contain the objects created by the current user.||SELECT * FROM user_objects where object_name ='<package_name>'|
|ALL_SOURCE||Gives the source of the objects created by all users.||SELECT * FROM all_source where name='<package_name>'|
|USER_SOURCE||Gives the source of the objects created by the current user.||SELECT * FROM user_source where name='<package_name>'|
|ALL_PROCEDURES||Gives the subprogram details like object_id, overload details, etc created by all users.||SELECT * FROM all_procedures Where object_name='<package_name>'|
|USER_PROCEDURES||Gives the subprogram details like object_id, overload details, etc. created by the current user.||SELECT * FROM user_procedures Where object_name='<package_name>'|
UTL File is the separate utility package provided by oracle to perform special tasks. This is mainly used for read and write the operating system files from PL/SQL packages or from subprograms. It got the separate functions to put the information and to get the information from files. It also allow to read/write in native character set.
Programmer can use this to write operating system files of any type and the file will be written directly in the database server. The name and directory path will be mentioned at the time writing.
We have now learnt the packages in PL/SQL, and you should be now able to work in the following.
- PL/SQL packages and its Components
- Characteristics of packages
- Referring and overloading package elements
- Managing dependencies in packages
- Viewing package information
- What is UTL File