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CHAPTER 8 ORACLE OBJECTS: AN OBJECTIVE ANALYSIS
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Using Varrays to Store Data
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In this section, we ll walk through the various steps involved in creating and populating a table with a varray column. While examining some of the techniques involved in populating and retrieving data from a table that contains a varray column, you ll likely start to appreciate the increased code complexity that using objects and collections in tables introduces. Using varrays in tables involves the following steps: 1. Create the varray type. 2. Create the table containing a column of the varray type. 3. Perform Data Manipulation Language (DML) statements on the table containing a column of the varray type. Let s look at each of these steps separately.
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Creating a Varray Type
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The first step is to create the varray type itself. The following code creates a varray of varchar2 variables called varray_of_varchars that can store up to ten varchar2(50) entries: benchmark@ORA10G> create or replace type varray_of_varchars as 2 varray(10) of varchar2(50); 3 / Type created.
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Creating a Table Containing a Column of the Varray Type
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The next step is to create the table with the varray column. In the following code, we create a table, dep_email_addresses, that stores up to ten e-mail addresses of a department in the column email_addresses of type varray_of_varchars: benchmark@ORA10G> create table dep_email_addresses 2 ( 3 dep_no number, 4 email_addresses varray_of_varchars 5 ); Table created.
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Performing DML on a Table Containing a Column of the Varray Type
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We insert some values into the table dep_email_addresses next. Notice how we initialize the varray column using the Oracle-supplied constructor: benchmark@ORA10G> insert into dep_email_addresses( dep_no, email_addresses) 2 values( 10, varray_of_varchars('king@mycompany.com', 3 'joe@mycompany.com', 'john@mycompany.com') ); 1 row created.
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CHAPTER 8 ORACLE OBJECTS: AN OBJECTIVE ANALYSIS
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We query the values we inserted so far to get one row we inserted. The result shows the column email_addresses as a single entity representing all the e-mail addresses nested within: benchmark@ORA10G> select * from dep_email_addresses; DEP_NO EMAIL_ADDRESSES ------ ---------------------------------------------------------------10 VARRAY_OF_VARCHARS('king@mycompany.com', 'joe@mycompany.com', 'john@mycompany.com') To unnest the individual e-mail address values tucked away in the varray column into separate rows, we can use the table clause along with the keyword column_value in our query as follows. Essentially, we have to do a join of the table dep_email_addresses with our varray column cast as a table using the table clause. As you can see, what would be a simple select joining a parent/child table looks more complicated when we use objects to store data in tables: benchmark@ORA10G> select c.dep_no, a.column_value 2 from dep_email_addresses c, table(c.email_addresses) a; DEP_NO -----10 10 10 COLUMN_VALUE -------------------------------------------------king@mycompany.com joe@mycompany.com john@mycompany.com
3 rows selected. Next, we insert another row into dep_email_addresses. This row is obtained as the result of a query from the same table (this shows how to use the syntax for insert into <table_name> <select from>-type statements with tables containing columns of the collection type). The code is even more complicated in this case. The innermost select uses the syntax to unnest the varray column values, as you saw in the previous example. The keyword multiset is used to specify that the subquery can return more than a row (without it, Oracle will give an error if the subquery returns more than one row). The cast function casts a value from one type to another. We use it here to cast the resulting query rows into our varray type: benchmark@ORA10G> insert into dep_email_addresses 2 select 20, 3 cast 4 ( 5 multiset 6 ( 7 select a.column_value 8 from dep_email_addresses c, TABLE(c.email_addresses) a 9 ) 10 as varray_of_varchars 11 ) 12 from dep_email_addresses 13 where dep_no = 10; 1 row created.
CHAPTER 8 ORACLE OBJECTS: AN OBJECTIVE ANALYSIS
To update a value in the varray column, you have to replace the entire varray with a new varray value you cannot selectively replace or add an element. Thus for a simple update, you will have to use procedural code. This is a huge limitation that should more or less convince you that using varrays in tables is not a good idea in general. Not only does the code complexity increase, but also the performance of an update decreases since you are now forced to use procedural code instead of SQL. As an example of a procedural update, see the following code for the procedure add_email_address. The procedure adds a new a e-mail address to a given department number in the dep_email_addresses table: benchmark@ORA10G> create or replace procedure add_email_address( 2 p_dep_no in number, p_email_address in varchar2) 3 is 4 l_prev_email_addresses dep_email_addresses.email_addresses%type; 5 begin 6 First, we select the old varray column value into a varray variable: 8 9 10 11 12 select c.email_addresses into l_prev_email_addresses from dep_email_addresses c where dep_no = p_dep_no; Next, we extend the varray variable to store the new e-mail address: 14 l_prev_email_addresses.extend(1); We then store the new e-mail address as the last element of the varray variable: 16 17 l_prev_email_addresses( l_prev_email_addresses.count) := p_email_address; Finally, we replace the varray column in the table with the new varray variable value: 20 21 22 23 24 update dep_email_addresses d set d.email_addresses = l_prev_email_addresses where dep_no = p_dep_no; end add_email_address; /
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