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What is Collection?

A Collection is an ordered group of elements of particular data types. It can be a collection of simple data type or complex data type (like user-defined or record types).

In the collection, each element is identified by a term called "subscript." Each item in the collection is assigned with a unique subscript. The data in that collection can be manipulated or fetched by referring to that unique subscript.

Collections are most useful things when a large data of the same type need to be processed or manipulated. Collections can be populated and manipulated as whole using 'BULK' option in Oracle.

In this tutorial, you will learn-

Collections are classified based on the structure, subscript, and storage as shown below.

  • Index-by-tables (also known as Associative Array)
  • Nested tables
  • Varrays

At any point, data in the collection can be referred by three terms Collection name, Subscript, Field/Column name as "<collection_name>(<subscript>).<column_name>". You are going to learn about these above-mentioned collection categories further in the below section.

Varrays

Varray is a collection method in which the size of the array is fixed. The array size cannot be exceeded than its fixed value. The subscript of the Varray is of a numeric value. Following are the attributes of Varrays.

  • Upper limit size is fixed
  • Populated sequentially starting with the subscript '1'
  • This collection type is always dense, i.e. we cannot delete any array elements. Varray can be deleted as a whole, or it can be trimmed from the end.
  • Since it always is dense in nature, it has very less flexibility.
  • It is more appropriate to use when the array size is known and to perform similar activities on all the array elements.
  • The subscript and sequence always remain stable, i.e. the subscript and count of the collection is always same.
  • They need to be initialized before using them in programs. Any operation (except EXISTS operation) on an uninitialized collection will throw an error.
  • It can be created as a database object, which is visible throughout the database or inside the subprogram, which can be used only in that subprogram.

The below figure will explain the memory allocation of Varray (dense) diagrammatically.

Subscript 1 2 3 4 5 6 7
Value Xyz Dfv Sde Cxs Vbc Nhu Qwe

Syntax for VARRAY:

TYPE <type_name> IS VARRAY (<SIZE>) OF <DATA_TYPE>;
  • In the above syntax, type_name is declared as VARRAY of the type 'DATA_TYPE' for the given size limit. The data type can be either simple or complex type.

Nested Tables

A Nested table is a collection in which the size of the array is not fixed. It has the numeric subscript type. Below are more descriptions about nested table type.

  • The Nested table has no upper size limit.
  • Since the upper size limit is not fixed, the collection, memory needs to be extended each time before we use it. We can extend the collection using 'EXTEND' keyword.
  • Populated sequentially starting with the subscript '1'.
  • This collection type can be of both dense and sparse, i.e. we can create the collection as a dense, and we can also delete the individual array element randomly, which make it as sparse.
  • It gives more flexibility regarding deleting the array element.
  • It is stored in the system generated database table and can be used in the select query to fetch the values.
  • The subscript and sequence are not stable, i.e. the subscript and the count of the array element can vary.
  • They need to be initialized before using them in programs. Any operation (except EXISTS operation) on the uninitialized collection will throw an error.
  • It can be created as a database object, which is visible throughout the database or inside the subprogram, which can be used only in that subprogram.

The below figure will explain the memory allocation of Nested Table (dense and sparse) diagrammatically. The black colored element space denotes the empty element in a collection i.e. sparse.

Subscript 1 2 3 4 5 6 7
Value (dense) Xyz Dfv Sde Cxs Vbc Nhu Qwe
Value(sparse) Qwe Asd Afg Asd Wer

Syntax for Nested Table:

TYPE <tvpe name> IS TABLE OF <DATA TYPE>;
  • In the above syntax, type_name is declared as Nested table collection of the type 'DATA_TYPE'. The data type can be either simple or complex type.

Index-by-table

Index-by-table is a collection in which the array size is not fixed. Unlike the other collection types, in the index-by-table collection the subscript can consist be defined by the user. Following are the attributes of index-by-table.

  • The subscript can of integer or strings. At the time of creating the collection, the subscript type should be mentioned.
  • These collections are not stored sequentially.
  • They are always sparse in nature.
  • The array size is not fixed.
  • They cannot be stored in the database column. They shall be created and used in any program in that particular session.
  • They give more flexibility in terms of maintaining subscript.
  • The subscripts can be of negative subscript sequence also.
  • They are more appropriate to use for relatively smaller collective values in which the collection can be initialized and used within the same subprograms.
  • They need not be initialized before start using them.
  • It cannot be created as a database object. It can only be created inside the subprogram, which can be used only in that subprogram.
  • BULK COLLECT cannot be used in this collection type as the subscript should be given explicitly for each record in the collection.

The below figure will explain the memory allocation of Nested Table (sparse) diagrammatically. The black colored element space denotes the empty element in a collection i.e. sparse.

Subscript (varchar) FIRST SECOND THIRD FOURTH FIFTH SIXTH SEVENTH
Value(sparse) Qwe Asd Afg Asd Wer

Syntax for Index-by-Table

TYPE <type_name> IS TABLE OF <DATA_TYPE> INDEX BY YARCHAR2 (10);
  • In the above syntax, type_name is declared as an index-by-table collection of the type 'DATA_TYPE'. The data type can be either simple or complex type. The subsciprt/index variable is given as VARCHAR2 type with maximum size as 10.

Constructor and Initialization Concept in Collections

Constructors are the in-built function provided by the oracle that has the same name as of the object or collections. They are executed first whenever object or collections are getting referred for the first time in a session. Below are the important details of constructor in collection context:

  • For collections, these constructors should be called explicitly to initialize it.
  • Both Varray and Nested tables need to be initialized through these constructors before getting referred into the program.
  • Constructor implicitly extends the memory allocation for a collection (except Varray), hence constructor can also assign the variables to the collections.
  • Assigning values to the collection through constructors will never make the collection sparse.

Collection Methods

Oracle provides many functions to manipulate and to work with the collections. These functions are very much useful in the program to determine and to modify the different attribute of the collections. The Following table will give the different functions and their description.

Method Description SYNTAX
EXISTS (n) This method will return Boolean results. It will return 'TRUE' if the nth element exists in that collection, else it will return FALSE. Only EXISTS functions can be used in uninitialized collection <collection_name>.EXISTS(element_position)
COUNT Gives the total count of the elements present in a collection <collection_name>.COUNT
LIMIT It returns the maximum size of the collection. For Varray, it will return the fixed size that has been defined. For Nested table and Index-by-table, it gives NULL <collection_name>.LIMIT
FIRST Returns the value of the first index variable(subscript) of the collections <collection_name>.FIRST
LAST Returns the value of the last index variable(subscript) of the collections <collection_name>.LAST
PRIOR (n) Returns precedes index variable in a collection of the nth element. If there is no precedes index value NULL is returned <collection_name>.PRIOR(n)
NEXT (n) Returns succeeds index variable in a collection of the nth element. If there is no succeeds index value NULL is returned <collection_name>.NEXT(n)
EXTEND Extends one element in a collection at the end <collection_name>.EXTEND
EXTEND (n) Extends n elements at the end of a collection <collection_name>.EXTEND(n)
EXTEND (n,i) Extends n copies of the ith element at the end of the collection <collection_name>.EXTEND(n,i)
TRIM Removes one element from the end of the collection <collection_name>.TRIM
TRIM (n) Removes n elements from the end of collection <collection_name>.TRIM (n)
DELETE Deletes all the elements from the collection. Makes the collection empty <collection_name>.DELETE
DELETE (n) Deletes the nth element from the collection. If the nth element is NULL, then this will do nothing <collection_name>.DELETE(n)
DELETE (m,n) Deletes the element in the range mth to nth in the collection <collection_name>.DELETE(m,n)

Example1: Record Type at Subprogram level

In this example, we are going to see how to populate the collection using 'BULK COLLECT' and how to refer the collection data.

Complex Data Types in PL/SQL

DECLARE
TYPE emp_det IS RECORD
(
EMP_NO NUMBER,
EMP_NAME VARCHAR2(150),
MANAGER NUMBER,
SALARY NUMBER
);
TYPE emp_det_tbl IS TABLE OF emp_det; guru99_emp_rec emp_det_tbl:= emp_det_tbl(); 
BEGIN
INSERT INTO emp (emp_no,emp_name, salary, manager) VALUES (1000,’AAA’,25000,1000);
INSERT INTO emp (emp_no,emp_name, salary, manager) VALUES (1001,'XXX’,10000,1000);
INSERT INTO emp (emp_no, emp_name, salary, manager) VALUES (1002,'YYY',15000,1000);
INSERT INTO emp (emp_no,emp_name,salary, manager) VALUES (1003,’ZZZ’,'7500,1000);
COMMIT:
SELECT emp no,emp_name,manager,salary BULK COLLECT INTO guru99_emp_rec
FROM emp;
dbms_output.put_line (‘Employee Detail');
FOR i IN guru99_emp_rec.FIRST..guru99_emp_rec.LAST
LOOP
dbms_output.put_line (‘Employee Number: '||guru99_emp_rec(i).emp_no); 
dbms_output.put_line (‘Employee Name: '||guru99_emp_rec(i).emp_name); 
dbms_output.put_line (‘Employee Salary:'|| guru99_emp_rec(i).salary); 
dbms_output.put_line(‘Employee Manager Number:'||guru99_emp_rec(i).manager);
dbms_output.put_line('--------------------------------');
END LOOP;
END;
/

Code Explanation:

  • Code line 2-8: Record type 'emp_det' is declared with columns emp_no, emp_name, salary and manager of data type NUMBER, VARCHAR2, NUMBER, NUMBER.
  • Code line 9: Creating the collection 'emp_det_tbl' of record type element 'emp_det'
  • Code line 10: Declaring the variable 'guru99_emp_rec' as 'emp_det_tbl' type and initialized with null constructor.
  • Code line 12-15: Inserting the sample data into the 'emp' table.
  • Code line 16: Committing the insert transaction.
  • Code line 17: Fetching the records from 'emp' table and populating the collection variable as a bulk using the command "BULK COLLECT". Now the variable 'guru99_emp_rec' contains all the record that are present in the table 'emp'.
  • Code line 19-26: Setting the 'FOR' loop using to print all the records in the collection one-by-one. The collection method FIRST and LAST is used as lower and higher limit of the loop.

Output: As you can see in the above screenshot when the above code is executed you will get the following output

Employee Detail
Employee Number: 1000
Employee Name: AAA
Employee Salary: 25000
Employee Manager Number: 1000
----------------------------------------------
Employee Number: 1001
Employee Name: XXX
Employee Salary: 10000
Employee Manager Number: 1000
----------------------------------------------
Employee Number: 1002
Employee Name: YYY
Employee Salary: 15000
Employee Manager Number: 1000
----------------------------------------------
Employee Number: 1003
Employee Name: ZZZ
Employee Salary: 7500
Employee Manager Number: 1000
----------------------------------------------

 

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