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Collections are ideal for representing one-to-many relationships among data. Oracle offers you two main types of collections: varrays and nested tables. We ll look at these two types of collections in more detail in the following sections.

Varrays are stored in the database as RAW or BLOB objects. A varray is simply an ordered collection of data elements. Each element in the array is identified by an index, which is used to access that particular element. Here s how you declare a VARRAY type: SQL> CREATE TYPE prices AS VARRAY (10) OF NUMBER (12,2);

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A nested table consists of an ordered set of data elements. The ordered set can be of an object type or an Oracle built-in type. Here s a simple example: SQL> CREATE TYPE lineitem_table AS TABLE OF lineitem; To access the elements of a collection with SQL, you can use the TABLE operator, as shown in the following example. Here, history is a nested table and courses is the column you want to insert data into:

SQL> INSERT INTO TABLE(SELECT courses FROM department WHERE name = 'History') VALUES('Modern India');

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You can see that by determining how the script was invoked (using the soft link or using the name of the file), you avoid having to force all of the files you edit into the RCS. 13 contains further discussion of the technique of using the syntax of the invocation to select a script s behavior. You could upgrade this script by defining directories for which it is assumed that all files inside those directories should be checked into the RCS even if they weren t originally. (The /etc directory would be a good candidate for this treatment.) Files residing in other directories would be edited as usual, without any version control.

You can create not just types, but also type hierarchies, which consist of parent supertypes and child subtypes connected to the parent types by inheritance. Here s an example of how you can create a subtype from a supertype. First, create the supertype: SQL> CREATE TYPE person_t AS OBJECT ( name varchar2(80), social_sec_no number, hire_date date, member function age() RETURN number, member function print() RETURN varchar2) NOT FINAL; Next, create the subtype, which will inherit all the attributes and methods from its supertype: SQL> CREATE TYPE employee_t UNDER person_t (salary number, commission number, member function wages () RETURN number, OVERRIDING member function print () RETURN varchar2);

The CAST operator enables you to do two things. It lets you convert built-in data types and also convert a collection-type value into another collection-type value. Here s an example of using CAST with built-in data types: SQL> SELECT product_id, CAST(description AS VARCHAR2(30)) FROM product_desc;

Although SQL is easy to learn and has a lot of powerful features, it doesn t allow the procedural constructs of third-generation languages such as C. PL/SQL is Oracle s proprietary extension to SQL, and it provides you the functionality of a serious programming language. One of the big advantages of using PL/SQL is that you can use program units called procedures or packages in the database, thus increasing code reuse and performance.

A PL/SQL block is an executable program. A PL/SQL code block, whether encapsulated in a program unit such as a procedure or specified as a free-form anonymous block, consists of the following structures, with a total of four key statements, only two of which are mandatory: DECLARE: In this optional section, you declare the program variables and cursors. BEGIN: This mandatory statement indicates that SQL and PL/SQL statements will follow it. EXCEPTION: This optional statement specifies error handling. END: This mandatory statement indicates the end of the PL/SQL code block.