What does unique mean in mysql

The unique constraint in MySQL ensures that the value in a column or column group is unique and prevents duplicate values. It is implemented by creating an index, which can enhance data integrity, query optimization, and data consistency.

The meaning of unique in MySQL

unique is a constraint in MySQL , used to ensure that values ​​in a column or set of columns in a table are unique. This means that no duplicate values ​​are allowed in the column or column group.

How it works:

• When a unique constraint is created on a table, MySQL creates an index on the column or column group.
• Indexes allow MySQL to quickly find and retrieve unique values.
• If the inserted or updated data violates the unique constraint, MySQL will raise an error and reject the operation.

Advantages:

• Data integrity: unique constraints ensure data integrity and prevent duplicate values ​​from entering the table.
• Query Optimization: By creating indexes, unique constraints can optimize query performance, especially when searches involving unique columns.
• Data Consistency: Unique constraints help maintain data consistency because it allows the establishment of referential integrity relationships between different tables or applications.

Example:

In the following example, we create a unique constraint on table Student to ensure that The value in the StudentID column is unique:

<code class="sql">CREATE TABLE Student (
  StudentID INT PRIMARY KEY,
  Name VARCHAR(255),
  Age INT,
  UNIQUE (StudentID)
);</code>

Note:

• unique constraint is similar to the primary key, but the primary key also forces the column to be non null.
• Unique constraints can be applied to multiple columns at the same time to form a composite index.
• Unique constraints do not apply to NULL values ​​because NULL values ​​are treated as the same value.
• Unique constraints can be used simultaneously with other constraints (such as nullable).

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