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<blockquote>
<p>
<b><font size="+1">16.5 How do I匔reate and use the nested table collection type?</font></b>
<p>
<B>Problem</B>
<p>
I want to model master-detail relationships within my database using collection objects, but for some of these relationships I don抰 know how many detail elements there will be, and for others there will be a large number of detail elements. I know that VARRAYs are not helpful in these situations. How do I implement nested tables in my database and do I insert and extract data from them?
<p>
<B>Technique</B>
<p>
Chapter 9 included a nested table definition to help explain unused segment space. This How-To revisits and more closely examines this definition. Nested table definitions start with an object table type definition and conclude by defining a relational table with an embedded object type (see How-To 16.3 for more information about object tables). Nested tables are similar to VARRAYs in that DML commands on them involve some modifications to traditional syntax; the THE keyword is an important new operator for manipulating data residing in nested tables.
<p>
<B>Steps</B>
<p>
<B>1.</B>	The script CHP16_14.SQL contains DDL to define a nested table. Start SQL*Plus, connect as the WAITE user, and use the START command to load and execute this script. The script and its output appear in <A HREF="image/16oht14.gif" TARGET="_blank">Figure 16.14</A>. <p>
The first CREATE statement in the script defines an object type, EMP_TYPE16_5, which acts as a template for the employee information the database must store. The second CREATE statement defines an object table type comprised of objects of the EMP_TYPE16_5 type. The final and most interesting CREATE command defines a relational table with an embedded column called EMPS to contain a nested table.
<p>
<B>2.</B>	Load and execute the script CHP16_15.SQL to insert two rows into the DEPT16_5 table as shown in <A HREF="image/16oht15.gif" TARGET="_blank">Figure 16.15</A>.
<p>
The insert statements use nested constructor methods. As in earlier examples, Oracle8 defines these constructors for us. The outer constructor, EMP_TABLE16_5, instantiates an object of the nested table type. The inner constructor, EMP_TYPE16_5 instantiates an employee object. In this case, the inner constructor is instantiating an employee object to store in the nested object table called EMPS.
<p>
<B>3.</B>	Query the rows of the nested table EMPS by using the START command to load and execute the script CHP16_16.SQL. The script and its output appear in <A HREF="image/16oht16.gif" TARGET="_blank">Figure 16.16</A>.
<p>
The query in <A HREF="image/16oht15.gif" TARGET="_blank">Figure 16.15</A> returns the employee numbers and names for the staff members in the janitorial department. The THE keyword prefixing the subquery beginning in line 2 indicates to Oracle that the subquery抯 result is a nested table and not a scalar value.
<p>
<B>How It Works</B>
<p>
The DDL statements in Step 1 set up the nested table structure by creating an employee object type and an object table type. The CREATE TABLE statement in Step 1 uses these definitions to create a relational table with a column containing a nested table reference. Step 2 uses constructors to populate the relational table and the nested table. Step 3 uses subquery syntax and the THE operator, new in Oracle8, to query the nested table.
<p>
<B>Comments</B>
<p>
Nested tables are stored in different data segments than the data from the main table (this is called out-of-line storage). Unlike VARRAYs, the amount of data that can reside in a nested table is limited only by available logical and physical space. Nested tables do accommodate SELECT statements in the form appearing in Step 3.
<p>
Subqueries prefaced by the THE keyword are sometimes called flattened subqueries to indicate that they return multiple rows from the nested table for a single row in the main table. Although Oracle8 selects this level of SELECT functionality, it is necessary to write a PL/SQL procedure to fashion a query returning columns from the main table and columns from the nested table or to create a subquery returning more than a single column value from the nested table.
<p>
Under Oracle8 version 8.02 beta for Windows NT, if you apply the familiar SELECT * construct to a table containing a nested table, you will generate a SQL*Plus internal error.

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