What does unique key mean in mysql

The unique key in MySQL is a constraint that ensures the uniqueness of a column value or column combination. Duplication of values ​​is allowed but a specific combination of values ​​can only appear once. It is mainly used to ensure data integrity and improve queries. Efficiency and enforcing business rules.

#What is a unique key in MySQL?

unique key is a constraint used in MySQL to ensure the uniqueness of one or more columns in a table. It allows values ​​in a column to appear repeatedly, but only once for each specific combination of values.

The role of unique key

unique key is mainly used for the following purposes:

• Ensure the integrity and uniqueness of data : Unique key ensures that there will be no duplicate records in the data in the table, thus ensuring the accuracy and reliability of the data.
• Improve query efficiency: Unique key can create indexes to speed up queries based on unique columns. Indexes allow MySQL to quickly locate specific values ​​instead of scanning the entire table.
• Enforce business rules: A unique key can enforce business rules, such as ensuring the uniqueness of a customer ID or order number.

Creation of unique key

In MySQL, use the ALTER TABLE statement to create a unique key:

<code>ALTER TABLE table_name ADD UNIQUE (column_name);</code>

For example, the following statement is in the name Create a unique index in the orders table, based on the order_id column:

<code>ALTER TABLE orders ADD UNIQUE (order_id);</code>

The difference between unique key and primary key

• The primary key is a unique key, and it is also used to identify each row of the table.
• unique key is a constraint that guarantees the uniqueness of a column, but it does not have to be used to identify rows.

Limitations of unique key

• unique key allows values ​​in a column to appear repeatedly, but each specific combination of values ​​can only appear once.
• If you update data on a unique key column and the new value is the same as the existing value, the update will fail.
• If you insert duplicate values ​​on a unique key column, the insertion will fail.

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