Generating Unique Identifiers in Oracle Databases

Oracle databases, prior to version 12c, didn’t offer a direct equivalent to the AUTO_INCREMENT feature found in other database systems like MySQL or PostgreSQL. However, it’s entirely possible to generate unique identifiers for your tables using a combination of sequences and triggers. Starting with Oracle 12c, the IDENTITY feature provides a more streamlined approach. This tutorial covers both methods, providing a comprehensive understanding of how to create auto-incrementing or uniquely generated IDs in Oracle.

Understanding the Need for Unique Identifiers

Unique identifiers, typically primary keys, are crucial for database integrity and efficient data management. They ensure each row in a table can be uniquely identified, preventing data duplication and enabling relationships between tables.

Method 1: Using Sequences and Triggers (Oracle 11g and Earlier)

This method simulates auto-incrementing behavior by leveraging Oracle sequences and triggers.

1. Create a Sequence: A sequence is a database object that generates a series of numbers.

   CREATE SEQUENCE my_sequence
   START WITH 1
   INCREMENT BY 1;
   

   • START WITH: Specifies the initial value of the sequence.
   • INCREMENT BY: Defines the increment between consecutive sequence values.

2. Create the Table: Define your table with a primary key column intended to hold the unique identifier.

   CREATE TABLE my_table (
       id NUMBER PRIMARY KEY,
       data VARCHAR2(255)
   );
   

3. Create a Trigger: The trigger automatically populates the id column with the next value from the sequence before each insert operation.

   CREATE OR REPLACE TRIGGER my_table_trg
   BEFORE INSERT ON my_table
   FOR EACH ROW
   BEGIN
       SELECT my_sequence.NEXTVAL INTO :NEW.id FROM DUAL;
   END;
   /
   

   • BEFORE INSERT: The trigger executes before each insert operation.
   • :NEW.id: Refers to the id column of the row being inserted.
   • my_sequence.NEXTVAL: Retrieves the next value from the sequence.
   • FROM DUAL: DUAL is a dummy table used for selecting pseudo-columns like NEXTVAL.

Example:

INSERT INTO my_table (data) VALUES ('Some data');
INSERT INTO my_table (data) VALUES ('More data');

The id column will automatically be populated with unique, incrementing values (1, 2, etc.).

Method 2: Using the IDENTITY Feature (Oracle 12c and Later)

Oracle 12c introduced the IDENTITY feature, simplifying the process of creating auto-incrementing columns.

1. Create the Table with IDENTITY: Define the table with a column designated as IDENTITY.

   CREATE TABLE my_table (
       id NUMBER GENERATED ALWAYS AS IDENTITY,
       data VARCHAR2(255)
   );
   

   • GENERATED ALWAYS AS IDENTITY: Specifies that the id column is an identity column, automatically generating values.

Alternatively, you can specify the start and increment values:

CREATE TABLE my_table (
    id NUMBER GENERATED ALWAYS AS IDENTITY (START WITH 1 INCREMENT BY 1),
    data VARCHAR2(255)
);

Another way to use the IDENTITY feature is to use a sequence:

CREATE SEQUENCE my_sequence START WITH 1;

CREATE TABLE my_table (
    id NUMBER DEFAULT my_sequence.NEXTVAL,
    data VARCHAR2(255)
);

ALTER TABLE my_table ADD CONSTRAINT my_table_pk PRIMARY KEY (id);

In this case, the sequence my_sequence generates the values for the id column.

Example:

INSERT INTO my_table (data) VALUES ('Some data');
INSERT INTO my_table (data) VALUES ('More data');

The id column will be automatically populated with unique, incrementing values. The database manages the generation of these values without the need for a separate trigger.

Choosing the Right Method

• Oracle 11g or Earlier: Use the sequence and trigger combination.
• Oracle 12c or Later: The IDENTITY feature is the preferred approach due to its simplicity and efficiency. The sequence and trigger method still works but is generally less efficient and more verbose.

Considerations

• Globally Unique Identifiers (GUIDs): If you need truly globally unique identifiers (e.g., across multiple databases or systems), consider using the SYS_GUID() function. However, GUIDs are less efficient for indexing and storage compared to numeric sequences.

• Performance: While the IDENTITY feature is generally efficient, monitor performance if you have very high insert rates. Consider adjusting sequence caching parameters if necessary.