How to use unique in oracle

The UNIQUE constraint in Oracle ensures that specific columns or column combinations in the table have unique values ​​to prevent duplicate data insertion. It is implemented through the following rules: 1. In an insert or update operation, the value of a specified column or column combination cannot be repeated with an existing value; 2. Indexes are allowed to be created to improve query efficiency.

Usage of UNIQUE constraint in Oracle

UNIQUE constraint is a database constraint used to ensure that a certain A column or combination of columns with unique values. When you apply a UNIQUE constraint to a column, the database enforces the following rule:

• No duplicate values ​​are allowed in that column or combination of columns.
• For an insert or update of the same row, if the value of the column or column combination conflicts with an existing row, the operation will fail.

Usage:

To define a UNIQUE constraint, you can use the following syntax:

<code>ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_list);</code>

Where:

• table_name is the name of the table to which the constraint applies.
• constraint_name is the name of the constraint to be created.
• column_list is a list of columns or column combinations to be defined as unique.

Function:

UNIQUE constraints have the following functions:

• Maintain the integrity and uniqueness of the data.
• Prevent duplicate data from entering the table.
• Improve query efficiency because indexes can be built on columns with UNIQUE constraints.
• Helps identify and remove duplicate data.

Example:

To create a UNIQUE constraint for the "customer_id" column in the "Customers" table, you can use the following command:

<code>ALTER TABLE Customers ADD CONSTRAINT customer_id_unique UNIQUE (customer_id);</code>

Note:

UNIQUE constraints are different from PRIMARY KEY constraints. PRIMARY KEY constraints enforce uniqueness and non-null values, while UNIQUE constraints only enforce uniqueness.

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