Collections

A collection is an ordered group of elements, all of the same type (for example, the grades for a class of students). Each element has a unique subscript that determines its position in the collection. PL/SQL offers three kinds of collections: associative arrays, nested tables, and varrays (short for variable-size arrays). Nested tables extend the functionality of associative arrays (formerly called "PL/SQL tables" or "index-by tables").

Collections work like the arrays found in most third-generation programming languages. Collections can have only one dimension. Most collections are indexed by integers, although associative arrays can also be indexed by strings. To model multi-dimensional arrays, you can declare collections whose items are other collections.

Nested tables and varrays can store instances of an object type and, conversely, can be attributes of an object type. Also, collections can be passed as parameters. So, you can use them to move columns of data into and out of database tables or between client-side applications and stored subprograms.

For more information, see "Defining Collection Types".

Syntax

Text description of the illustration collection_type_definition.gif


Text description of the illustration element_type.gif

Keyword and Parameter Description

element_type

This is any PL/SQL datatype except BINARY_INTEGER, BOOLEAN, LONG, LONG RAW, NATURAL, NATURALN, PLS_INTEGER, POSITIVE, POSITIVEN, REF CURSOR, SIGNTYPE, or STRING. Also, with varrays, element_type cannot be BLOB, CLOB, or an object type with BLOB or CLOB attributes.

INDEX BY type_name

This optional clause lets you define associative arrays, where you specify the subscript values to use rather than the system defining them in sequence.

type_name can be BINARY_INTEGER, PLS_INTEGER, or a string type such as VARCHAR2.

size_limit

This is a positive integer literal that specifies the maximum size of a varray, which is the maximum number of elements the varray can contain.

type_name

This identifies a user-defined collection type that was defined using the datatype specifier TABLE or VARRAY.

Usage Notes

Nested tables extend the functionality of index-by tables, so they differ in several ways. See "Choosing Between Nested Tables and Associative Arrays".

Every element reference includes the collection name and one or more subscripts enclosed in parentheses; the subscripts determine which element is processed. Except for associative arrays, which can have negative subscripts, collection subscripts have a fixed lower bound of 1. Subscripts for multilevel collections are evaluated in any order; if a subscript includes an expression that modifies the value of a different subscript, the result is undefined.

You can define all three collection types in the declarative part of any PL/SQL block, subprogram, or package. But, only nested table and varray types can be CREATEd and stored in an Oracle database.

Associative arrays and nested tables can be sparse (have non-consecutive subscripts), but varrays are always dense (have consecutive subscripts). Unlike nested tables, varrays retain their ordering and subscripts when stored in the database.

Initially, associative arrays are sparse. That enables you, for example, to store reference data in a temporary variable using a primary key (account numbers or employee numbers for example) as the index.

Collections follow the usual scoping and instantiation rules. In a package, collections are instantiated when you first reference the package and cease to exist when you end the database session. In a block or subprogram, local collections are instantiated when you enter the block or subprogram and cease to exist when you exit.

Until you initialize it, a nested table or varray is atomically null (that is, the collection itself is null, not its elements). To initialize a nested table or varray, you use a constructor, which is a system-defined function with the same name as the collection type. This function "constructs" a collection from the elements passed to it.

Because nested tables and varrays can be atomically null, they can be tested for nullity. However, they cannot be compared for equality or inequality. This restriction also applies to implicit comparisons. For example, collections cannot appear in a DISTINCT, GROUP BY, or ORDER BY list.

Collections can store instances of an object type and, conversely, can be attributes of an object type. Also, collections can be passed as parameters. So, you can use them to move columns of data into and out of database tables or between client-side applications and stored subprograms.

When calling a function that returns a collection, you use the following syntax to reference elements in the collection:

collection_name(parameter_list)(subscript)

With the Oracle Call Interface (OCI) or the Oracle Precompilers, you can bind host arrays to index-by tables declared as the formal parameters of a subprogram. That lets you pass host arrays to stored functions and procedures.

Examples

To specify the element type of a collection, you can use %TYPE or %ROWTYPE, as the following example shows:

DECLARE
   TYPE JobList IS VARRAY(10) OF emp.job%TYPE;  -- based on column
   CURSOR c1 IS SELECT * FROM dept;
   TYPE DeptFile IS TABLE OF c1%ROWTYPE;  -- based on cursor
   TYPE EmpFile IS VARRAY(150) OF emp%ROWTYPE;  -- based on database 
table

In the next example, you use a RECORD type to specify the element type:

DECLARE
   TYPE Entry IS RECORD (
      term    VARCHAR2(20), 
      meaning VARCHAR2(200)); 
   TYPE Glossary IS VARRAY(250) OF Entry;

In the example below, you declare an index-by table of records. Each element of the table stores a row from the emp database table.

DECLARE
   TYPE EmpTabTyp IS TABLE OF emp%ROWTYPE
      INDEX BY BINARY_INTEGER;
   emp_tab EmpTabTyp;
BEGIN
   /* Retrieve employee record. */
   SELECT * INTO emp_tab(7468) FROM emp WHERE empno = 7468;

When defining a VARRAY type, you must specify its maximum size. In the following example, you define a type that stores up to 366 dates:

DECLARE
   TYPE Calendar IS VARRAY(366) OF DATE;

Once you define a collection type, you can declare collections of that type, as the following SQL*Plus script shows:

CREATE TYPE Project AS OBJECT(
   project_no NUMBER(2), 
   title      VARCHAR2(35),
   cost       NUMBER(7,2))
/
CREATE TYPE ProjectList AS VARRAY(50) OF Project  -- VARRAY type
/
CREATE TABLE department (
   idnum    NUMBER(2),
   name     VARCHAR2(15),
   budget   NUMBER(11,2),
   projects ProjectList)  -- declare varray
/

The identifier projects represents an entire varray. Each element of projects will store a Project object.

In the following example, you declare a nested table as the formal parameter of a packaged procedure:

CREATE PACKAGE personnel AS
   TYPE Staff IS TABLE OF Employee;
   ...
   PROCEDURE award_bonuses (members IN Staff);

You can specify a collection type in the RETURN clause of a function spec, as the following example shows:

DECLARE
   TYPE SalesForce IS VARRAY(20) OF Salesperson;
   FUNCTION top_performers (n INTEGER) RETURN SalesForce IS ...

In the following example, you update the list of projects assigned to the Security Department:

DECLARE
   new_projects ProjectList :=
      ProjectList(Project(1, 'Issue New Employee Badges', 13500),
                  Project(2, 'Inspect Emergency Exits', 1900),
                  Project(3, 'Upgrade Alarm System', 3350),
                  Project(4, 'Analyze Local Crime Stats', 825));
BEGIN
   UPDATE department 
      SET projects = new_projects WHERE name = 'Security';

In the next example, you retrieve all the projects for the Accounting Department into a local varray:

DECLARE
   my_projects ProjectList;
BEGIN
   SELECT projects INTO my_projects FROM department 
      WHERE name = 'Accounting';

Related Topics

Collection Methods, Object Types, Records