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1.17 Collections

There are three types of collections: associative arrays (formerly known as index-by tables or PL/SQL tables), nested tables, and VARRAYs.

Associative arrays

Single-dimension, unbounded collections of homogeneous elements available only in PL/SQL, not in the database. Associative arrays are initially sparse; they have nonconsecutive subscripts.

Nested tables

Single-dimension, unbounded collections of homogeneous elements available in both PL/SQL and the database as columns or tables. Nested tables are initially dense (they have consecutive subscripts), but they can become sparse through deletions.

VARRAYs

Variable-size arrays. Single-dimension, bounded collections of homogeneous elements available in both PL/SQL and the database. VARRAYs are never sparse. Unlike nested tables, their element order is preserved when you store and retrieve them from the database.

The following table compares these similar collection types:

   

Collection type

 

Characteristic

Associative array

Nested table

VARRAY

Dimensionality

Single

Single

Single

Usable in SQL?

No

Yes

Yes

Usable as a column datatype in a table?

No

Yes; data stored "out of line" (in a separate table)

Yes; data typically stored "in line" (in the same table)

Uninitialized state

Empty (cannot be NULL); elements are undefined

Atomically null; illegal to reference elements

Atomically null; illegal to reference elements

Initialization

Automatic, when declared

Via constructor, fetch, assignment

Via constructor, fetch, assignment

In PL/SQL, elements referenced by

BINARY_INTEGER (-2,147,483,647

.. 2,147,483,647) or character string (VARCHAR2); maximum length of VARCHAR2 is 30, minimum length is 1

Positive integer between 1 and 2,147483,647

Positive integer between 1 and 2,147483,647

Sparse?

Yes

Initially no; after deletions, yes

No

Bounded?

No

Can be extended

Yes

Can assign a value to any element at any time?

Yes

No; may need to EXTEND first

No; may need to EXTEND first, and cannot EXTEND past the upper bound

Means of extending

Assign value to element with a new subscript

Use built-in EXTEND or TRIM function to condense, with no predefined maximum

Use EXTEND or TRIM, but only up to declared maximum size.

Can be compared for equality?

No

No

No

Elements retain ordinal position and subscript when stored and retrieved from the database

N/A—can't be stored in database

No

Yes

1.17.1 Declaring a Collection

Collections are implemented as TYPEs. As with any programmer-defined type, you must first define the type; then you can declare instances of that type. The TYPE definition can be stored in the database or declared in the PL/SQL program. Each instance of the TYPE is a collection.

The syntax for declaring an associative array is:

TYPE type_name IS TABLE OF element_type [NOT NULL] 
   INDEX BY {BINARY_INTEGER | VARCHAR2 (size_limit)};

The syntax for a nested table is:

[CREATE [OR REPLACE]] TYPE type_name IS TABLE OF 
   element_type [NOT NULL];

The syntax for a VARRAY is:

[CREATE [OR REPLACE]] TYPE type_name IS VARRAY | 
   VARYING ARRAY (max_elements) OF element_type 
   [NOT NULL];

The CREATE keyword defines the statement to be DDL and indicates that this type will exist in the database. The optional OR REPLACE keywords are used to rebuild an existing type, preserving the privileges. type_name is any valid identifier that will be used later to declare the collection. max_elements is the maximum size of the VARRAY. element_type is the type of the collection's elements. All elements are of a single type, which can be most scalar datatypes, an object type, or a REF object type. If the elements are objects, the object type itself cannot have an attribute that is a collection. Explicitly disallowed collection datatypes are BOOLEAN, NCHAR, NCLOB, NVARCHAR2, REF CURSOR, TABLE, and VARRAY.

NOT NULL indicates that a collection of this type cannot have any null elements. However, the collection can be atomically null (uninitialized).

1.17.2 Initializing Collections

Initializing an associative array is trivial—simply declaring it also initializes it. Initializing a nested table or a VARRAY can be done in any of three ways: explicitly with a constructor, or implicitly with a fetch from the database or with a direct assignment of another collection variable.

The constructor is a built-in function with the same name as the collection. It constructs the collection from the elements passed to it. The first example shows how you can create a nested table of colors and explicitly initialize it to three elements with a constructor:

DECLARE
   TYPE colors_tab_t IS TABLE OF VARCHAR2(30);

   colors_tab_t('RED','GREEN','BLUE');
BEGIN

The next example shows how you can create the nested table of colors and implicitly initialize it with a fetch from the database:

-- Create the nested table to exist in the database.
CREATE TYPE colors_tab_t IS TABLE OF VARCHAR2(32);

-- Create table with nested table type as column.
CREATE TABLE color_models 
(model_type   VARCHAR2(12)
,colors       color_tab_t)
NESTED TABLE colors STORE AS 
   color_model_colors_tab;

-- Add some data to the table.
INSERT INTO color_models 
   VALUES('RGB',color_tab_t('RED','GREEN','BLUE'));
INSERT INTO color_models 
   VALUES('CYMK',color_tab_t('CYAN','YELLOW',
      'MAGENTA' 'BLACK'));

-- Initialize a collection of colors from the table.
DECLARE
   basic_colors colors_tab_t;
BEGIN
   SELECT colors INTO basic_colors
     FROM color_models
    WHERE model_type = 'RGB';
...
END;

The third example shows how you can implicitly initialize the table via an assignment from an existing collection:

DECLARE
   basic_colors Color_tab_t := 
      Color_tab_t ('RED','GREEN','BLUE');

   my_colors Color_tab_t;
BEGIN
   my_colors := basic_colors;
   my_colors(2) := 'MUSTARD';

1.17.3 Adding and Removing Elements

Elements in an associative array can be added simply by referencing new subscripts. To add elements to nested tables or VARRAYs, you must first enlarge the collection with the EXTEND function, and then you can assign a value to a new element using one of the methods described in the previous section.

Use the DELETE function to remove an element in a nested table regardless of its position. The TRIM function can also be used to remove elements, but only from the end of a collection. To avoid unexpected results, do not use both DELETE and TRIM on the same collection.

1.17.4 Collection Pseudo-Functions

There are several pseudo-functions defined for collections: CAST, MULTISET, and TABLE.

CAST

Maps a collection of one type to a collection of another type.

SELECT column_value
FROM TABLE(SELECT CAST(colors AS color_tab_t)
           FROM color_models_a
          WHERE model_type ='RGB');
MULTISET

Maps a database table to a collection. With MULTISET and CAST, you can retrieve rows from a database table as a collection-typed column.

SELECT b.genus ,b.species,
      CAST(MULTISET(SELECT bh.country
                      FROM bird_habitats bh
                     WHERE bh.genus = b.genus
                       AND bh.species = b.species)
          AS country_tab_t)
FROM birds b;
TABLE

Maps a collection to a database table (the inverse of MULTISET).

SELECT *
  FROM color_models c
 WHERE 'RED' IN (SELECT * FROM TABLE(c.colors));

You can use TABLE( ) to unnest a transient collection:

DECLARE
   birthdays Birthdate_t :=
      Birthdate_t('24-SEP-1984', '19-JUN-1993');
BEGIN
   FOR the_rec IN
      (SELECT COLUMN_VALUE
         FROM TABLE(CAST(birthdays AS Birthdate_t)))

1.17.5 Collection Methods

There are a number of built-in functions (methods) defined for all collections. These methods are called with dot notation:

collection_name.method_name[(parameters)]

The methods are listed in the following table:

Collection method

Description

COUNT function

Returns the current number of elements in the collection.

DELETE [( i [ , j ] )] procedure

Removes element i or elements i through j from a nested table or associative array. When called with no parameters, removes all elements in the collection. Reduces the COUNT if the element is not already DELETEd. Does not apply to VARRAYs.

EXISTS ( i ) function

Returns TRUE or FALSE to indicate whether element i exists. If the collection is an uninitialized nested table or VARRAY, returns FALSE.

EXTEND [( n [ , i ] )] procedure

Appends n elements to a collection, initializing them to the value of element i. n is optional and defaults to 1.

FIRST function

Returns the lowest index in use. Returns NULL when applied to empty initialized collections.

LAST function

Returns the greatest index in use. Returns NULL when applied to empty initialized collections.

LIMIT function

Returns the maximum number of allowed elements in a VARRAY. Returns NULL for associative arrays and nested tables.

PRIOR ( i ) function

Returns the index immediately before element i. Returns NULL if i is less than or equal to FIRST.

NEXT ( i ) function

Returns the index immediately after element i. Returns NULL if i is greater than or equal to COUNT.

TRIM [( n )] procedure

Removes n elements at the end of the collection with the largest index. n is optional and defaults to 1. If n is NULL, TRIM does nothing. Associative arrays cannot be TRIMmed.

The EXISTS function returns a BOOLEAN, and all other functions and procedures return BINARY_INTEGER except for collections indexed by VARCHAR2, which can return character strings. All parameters are of the BINARY_INTEGER type. Only EXISTS can be used on uninitialized nested tables or VARRAYs. Other methods applied to these atomically null collections will raise the COLLECTION_IS_NULL exception.

DELETE and TRIM both remove elements from a nested table, but TRIM also removes the placeholder, while DELETE does not. This behavior may be confusing, because TRIM can remove previously DELETEd elements.

Here is an example of some collection methods in use with an associative array:

DECLARE
   TYPE population_type IS
      TABLE OF NUMBER INDEX BY VARCHAR2(64);
   continent_population population_type;
   howmany NUMBER;
   limit VARCHAR2(64);
BEGIN
   continent_population('Australia') := 30000000;
   -- Create new entry
   continent_population('Antarctica') := 1000;
   -- Replace old value
   continent_population('Antarctica') := 1001;
   limit := continent_population.FIRST;
   DBMS_OUTPUT.PUT_LINE (limit);
   DBMS_OUTPUT.PUT_LINE (continent_population(limit));
   limit := continent_population.LAST;
   DBMS_OUTPUT.PUT_LINE (limit);
   DBMS_OUTPUT.PUT_LINE (continent_population(limit));
END;
/

This example produces the following output:

Antarctica
1001
Australia
30000000

Here is an example of some collection methods in use with a nested table:

DECLARE
   TYPE colors_tab_t IS TABLE OF VARCHAR2(30);
   my_list colors_tab_t := 
      colors_tab_t('RED','GREEN','BLUE');
   element BINARY_INTEGER;
BEGIN
   DBMS_OUTPUT.PUT_LINE('my_list has '
      ||my_list.COUNT||' elements');
   my_list.DELETE(2); -- delete element two
   DBMS_OUTPUT.PUT_LINE('my_list has '
      ||my_list.COUNT||' elements');

   FOR element IN my_list.FIRST..my_list.LAST
   LOOP
      IF my_list.EXISTS(element) 
      THEN
         DBMS_OUTPUT.PUT_LINE(my_list(element) 
            || ' Prior= '||my_list.PRIOR(element)
            || ' Next= ' ||my_list.NEXT(element));
      ELSE
         DBMS_OUTPUT.PUT_LINE('Element '|| element 
            ||' deleted. Prior= '||my_
               list.PRIOR(element)
            || ' Next= '||my_list.NEXT(element));
      END IF;
   END LOOP;
END;

This example produces the output:

my_list has 3 elements
my_list has 2 elements
RED Prior=  Next= 3
Element 2 deleted. Prior= 1 Next= 3
BLUE Prior= 1 Next=

1.17.6 Collections and Privileges

As with other TYPEs in the database, you need the EXECUTE privilege on that TYPE in order to use a collection type created by another schema (user account) in the database.

Note that Oracle9i Release 2 made it possible to use synonyms for user-defined TYPE names.

1.17.7 Nested Collections (Oracle9i)

Nested collections are collections contained in members that are collections themselves. Nesting collections is a powerful way to implement object-oriented programming constructs within PL/SQL programs. For example:

CREATE TYPE books IS TABLE OF VARCHAR2(64);
CREATE TYPE our_books IS TABLE OF books;

1.17.8 Bulk Binds

You can use collections to improve the performance of SQL operations executed iteratively by using bulk binds. Bulk binds reduce the number of context switches between the PL/SQL engine and the database engine. Two PL/SQL language constructs implement bulk binds: FORALL and BULK COLLECT INTO.

The syntax for the FORALL statement is:

FORALL bulk_index IN lower_bound..upper_bound   [SAVE EXCEPTIONS]
   sql_statement; 

bulk_index can be used only in the sql_statement and only as a collection index (subscript). When PL/SQL processes this statement, the whole collection, instead of each individual collection element, is sent to the database server for processing. To delete all the accounts in the collection inactives from the table ledger, do this:

FORALL i IN inactives.FIRST..inactives.LAST
   DELETE FROM ledger WHERE acct_no = inactives(i);

The default is for Oracle to stop after the first exception encountered. Use the keywords SAVE EXCEPTIONS to tell Oracle that processing should continue after encountering exceptions. The cursor attribute %BULK_EXCEPTIONS stores a collection of records containing the errors. These records have two fields, EXCEPTION_INDEX and EXCEPTION_CODE, which contain the FOR ALL iteration during which the exception was raised, as well as the SQLCODE for the exception. If no exceptions are raised, the SQL%BULK_EXCEPTION.COUNT method returns 0. For example:

DECLARE
  TYPE NameList IS TABLE OF VARCHAR2(32);
  name_tab NameList := NameList('Pribyl'
          ,'Dawes','Feuerstein','Gennick'
          ,'Pribyl','Beresniewicz','Dawes','Dye');
  error_count NUMBER;
  bulk_errors EXCEPTION;
  PRAGMA exception_init(bulk_errors, -24381);
BEGIN
  FORALL indx IN name_tab.FIRST..name_tab.LAST SAVE EXCEPTIONS
    INSERT INTO authors (name) VALUES (name_tab(indx));
    -- authors has pk index on name
  EXCEPTION
    WHEN others THEN
      error_count := SQL%BULK_EXCEPTIONS.COUNT;
      DBMS_OUTPUT.PUT_LINE('Number of errors is ' || 
             error_count);
      FOR indx IN 1..error_count LOOP
        DBMS_OUTPUT.PUT_LINE('Error ' || indx || ' 
             occurred during '||'iteration ' || 
              SQL%BULK_EXCEPTIONS(indx).ERROR_INDEX);
        DBMS_OUTPUT.PUT_LINE('Error is ' || 
          SQLERRM(-SQL%BULK_EXCEPTIONS(indx).ERROR_CODE));
      END LOOP;
END;
/

Number of errors is 2
Error 1 occurred during iteration 5
Error is ORA-00001: unique constraint (.) violated
Error 2 occurred during iteration 7
Error is ORA-00001: unique constraint (.) violated

The syntax for the BULK COLLECT INTO clause is:

BULK COLLECT INTO collection_name_list;

where collection_name_list is a comma-delimited list of collections, one for each column in the SELECT. Collections of records cannot be a target of a BULK COLLECT INTO clause. However, Oracle does support retrieving a set of typed objects and "bulk collecting" them into a collection of objects.

The BULK COLLECT INTO clause can be used in SELECT INTO, FETCH INTO, or RETURNING INTO statements. For example:

DECLARE
   TYPE vendor_name_tab IS TABLE OF  
      vendors.name%TYPE;
   TYPE vendor_term_tab IS TABLE OF 
      vendors.terms%TYPE;
   v_names vendor_name_tab;
   v_terms vendor_term_tab;
BEGIN
   SELECT name, terms
     BULK COLLECT INTO v_names, v_terms
     FROM vendors
    WHERE terms < 30;
   ...
END;

The next function deletes products in an input list of categories, and the SQL RETURNING clause returns a list of deleted products:

FUNCTION cascade_category_delete (categorylist clist_t)
RETURN prodlist_t
IS
   prodlist prodlist_t;
BEGIN
   FORALL aprod IN categorylist.FIRST..categorylist.LAST
      DELETE FROM product WHERE product_id IN               
         categorylist(aprod)
      RETURNING product_id BULK COLLECT INTO prodlist;
   RETURN prodlist;
END;

You can use the SQL%BULK_ROWCOUNT cursor attribute for bulk bind operations. It is like an associative array containing the number of rows affected by the executions of the bulk bound statements. The nth element of SQL%BULK_ROWCOUNT contains the number of rows affected by the nth execution of the SQL statement. For example:

FORALL i IN inactives.FIRST..inactives.LAST
   DELETE FROM ledger WHERE acct_no = inactives(i);
FOR counter IN inactives.FIRST..inactives.LAST 
LOOP
   IF SQL%BULK_ROWCOUNT(counter) = 0
   THEN
      DBMS_OUTPUT.PUT_LINE('No rows deleted for '||
         counter);
   END IF;
END LOOP;

You cannot pass SQL%BULK_ROWCOUNT as a parameter to another program, or use an aggregate assignment to another collection. %ROWCOUNT contains a summation of all %BULK_ROWCOUNT elements. %FOUND and %NOTFOUND reflect only the last execution of the SQL statement.

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