What does unique mean in mysql

In MySQL, the UNIQUE constraint is used to ensure that the values ​​of a specific column in a table are unique. To create a UNIQUE constraint, you add the constraint when you create the table or by using the ALTER TABLE statement. It can improve data integrity, create indexes, and serve as an alternative to primary keys, but does not allow duplicate non-null values. The UNIQUE constraint differs from the PRIMARY KEY constraint in that it allows NULL values, can have multiple unique columns, and does not identify rows in the table.

UNIQUE constraint

In MySQL, the UNIQUE constraint is a table constraint that is used to ensure that the The values ​​in a given column are unique. It is very similar to the PRIMARY KEY and FOREIGN KEY constraints, but it allows multiple NULL values ​​with the same value to appear in the table.

How to use UNIQUE constraints?

To create a UNIQUE constraint on a column in a table, you can add the constraint when you create the table or use the ALTER TABLE statement.

<code>CREATE TABLE my_table (
  id INT NOT NULL AUTO_INCREMENT,
  name VARCHAR(255) UNIQUE
);

ALTER TABLE my_table ADD UNIQUE INDEX (email);</code>

Benefits of UNIQUE constraints

UNIQUE constraints have the following benefits:

• Data integrity:It ensures There will be no duplicate values ​​in the same column in the table, thus improving data integrity.
• Index creation: MySQL automatically creates indexes for columns with UNIQUE constraints, thereby improving query performance.
• Alternatives to Primary Keys: The UNIQUE constraint can be used as an alternative to primary keys because it still enforces uniqueness but allows NULL values ​​in the table.

The difference between UNIQUE constraints and PRIMARY KEY constraints

• UNIQUE constraints allow NULL values, while PRIMARY KEY constraints do not.
• There can be multiple columns with UNIQUE constraints in a table, but there can only be one PRIMARY KEY column.
• PRIMARY KEY constraints also identify rows in the table, while UNIQUE constraints do not.

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