How to create a unique index in Oracle?

To create a unique index in Oracle, you use the CREATE UNIQUE INDEX statement, which ensures that all values in the specified column(s) are distinct across all rows in a table.

Understanding Unique Indexes in Oracle

A unique index is a powerful tool in Oracle for maintaining data integrity and optimizing query performance.

What it does: It enforces the uniqueness of values in one or more columns within a table. This means that no two rows can have the same value (for a single-column index) or the same combination of values (for a composite index) in the indexed columns.
Why use it:
- Data Integrity: Guarantees that specific data points remain unique, preventing duplicate entries that could corrupt your data.
- Performance: Speeds up data retrieval for queries that filter or sort by the indexed columns. It also helps Oracle quickly locate rows during UPDATE and DELETE operations.
- Constraint Enforcement: When you define a PRIMARY KEY or UNIQUE constraint on a table, Oracle automatically creates a unique index behind the scenes to enforce that constraint. This is often the preferred method for declaring uniqueness.

Basic Syntax for Creating a Unique Index

The fundamental command for creating a unique index in Oracle is straightforward:

CREATE UNIQUE INDEX index_name
ON table_name (column1, column2, ..., columnN);

Let's break down the components of this statement:

CREATE UNIQUE INDEX: These keywords signal to Oracle your intention to create a unique index. The UNIQUE keyword is crucial as it defines that the indexed columns must contain unique combinations of values.
index_name: This is a user-defined name for your index. It should be descriptive and follow Oracle's naming conventions (e.g., UX_TableName_ColumnName).
ON table_name: Specifies the name of the table on which the index will be created.
(column1, column2, ..., columnN): A comma-separated list of one or more columns from table_name that you want to include in the index.
- For a single-column unique index, you list just one column.
- For a composite unique index, you list multiple columns. In this case, the combination of values across all listed columns must be unique for each row.

Example: Single-Column Unique Index

Suppose you have a table Customers and want to ensure that each customer's CustomerID is unique.

Create a sample table (if needed):

CREATE TABLE Customers (
    CustomerID NUMBER(10),
    FirstName VARCHAR2(50),
    LastName VARCHAR2(50),
    Email VARCHAR2(100)
);

Create the unique index:
```
CREATE UNIQUE INDEX UX_Customers_CustomerID
ON Customers (CustomerID);
```
This statement creates an index named UX_Customers_CustomerID on the CustomerID column. If you try to insert a customer with an CustomerID that already exists, Oracle will raise an ORA-00001: unique constraint violated error.

Example: Composite Unique Index

Consider a scenario where you want to ensure that for a ProjectTasks table, no two tasks for the same ProjectID can have the same TaskName. The TaskName alone might not be unique across all projects, but the combination of ProjectID and TaskName must be.

Create a sample table:

CREATE TABLE ProjectTasks (
    TaskID NUMBER(10) PRIMARY KEY,
    ProjectID NUMBER(10),
    TaskName VARCHAR2(100),
    Status VARCHAR2(20)
);

Create the composite unique index:
```
CREATE UNIQUE INDEX UX_ProjectTasks_ProjTask
ON ProjectTasks (ProjectID, TaskName);
```
With this index, a task with (ProjectID=101, TaskName='Design UI') is unique. You can also have (ProjectID=102, TaskName='Design UI') because the combination is different. However, you cannot have another entry (ProjectID=101, TaskName='Design UI').

Important Considerations

NULL Values: Oracle's unique indexes and unique constraints allow multiple NULL values in the indexed column(s). This is because NULL is considered an unknown value, and thus, two NULLs are not considered equal. If you need to ensure no NULLs and all unique values, you must also define a NOT NULL constraint on the column.
Unique Constraints vs. Unique Indexes: While you can explicitly create a unique index, it's often better practice to define a UNIQUE constraint. When you define a UNIQUE constraint, Oracle automatically creates an underlying unique index to enforce it. This approach clearly expresses the intent for data integrity.
```
-- Example: Adding a UNIQUE constraint, which implicitly creates a unique index
ALTER TABLE Customers
ADD CONSTRAINT UQ_Customers_Email UNIQUE (Email);
```
This adds a constraint UQ_Customers_Email to the Email column, making sure all email addresses are unique.
Existing Data: If the table already contains duplicate values in the columns you specify for the unique index, the CREATE UNIQUE INDEX statement will fail. You must clean up the duplicate data before the index can be created successfully.
Performance Overhead: While unique indexes significantly improve read performance, they introduce a slight overhead during INSERT, UPDATE, and DELETE operations because Oracle must maintain the index structure. This trade-off is typically worthwhile for the data integrity and query performance benefits.

Managing Unique Indexes

Viewing Unique Indexes: You can query Oracle's data dictionary views to see information about your unique indexes.
```
SELECT index_name, table_name, uniqueness, status
FROM USER_INDEXES
WHERE table_name = 'CUSTOMERS';
```
Dropping a Unique Index: If an index is no longer needed, you can drop it.
```
DROP INDEX UX_Customers_CustomerID;
```
Note: If the unique index was created implicitly by a UNIQUE or PRIMARY KEY constraint, you should drop the constraint instead of the index directly. Dropping the constraint will automatically drop its associated index.
```
ALTER TABLE Customers
DROP CONSTRAINT UQ_Customers_Email;
```

Best Practices

Prioritize Constraints: For enforcing business rules, use PRIMARY KEY or UNIQUE constraints. They offer better semantic clarity and Oracle handles the underlying index creation.
Descriptive Naming: Adopt a consistent naming convention (e.g., UX_TableName_ColumnName) for your indexes to improve readability and maintenance.
Validate Data: Always check for existing duplicate data before attempting to create a unique index on an existing table to avoid errors.

For more comprehensive details on indexing in Oracle, refer to the official Oracle Database SQL Language Reference.