The role of unique in mysql

The UNIQUE keyword in MySQL creates a unique index, forcing the values ​​of specific columns or column groups in the table to be unique, preventing duplication, improving query speed, avoiding inserting duplicate data, and optimizing data storage.

The role of UNIQUE keyword in MySQL

The UNIQUE keyword is used to create a unique index in a MySQL table. It enforces that the value of a column or a group of columns in the table must be unique and cannot be repeated.

Function

• Ensure data integrity: UNIQUE index can prevent duplicate values ​​from appearing in the same column or column group, ensuring data accuracy.
• Speed ​​up query speed: For columns that contain unique indexes, MySQL can use index lookup instead of scanning the entire table, thus greatly improving query efficiency.
• Avoid inserting duplicate data: When trying to insert duplicate values, MySQL will return an error or conflict to prevent data duplication.
• Optimize data storage: UNIQUE index helps eliminate redundant data and optimize data storage space.

Using

To create a UNIQUE index, you can use the following syntax when creating or modifying a table:

<code>CREATE TABLE table_name (
  column_name1 UNIQUE,
  column_name2 UNIQUE,
  ...
);</code>

or

<code>ALTER TABLE table_name ADD UNIQUE (column_name1, column_name2, ...);</code>

Note

• UNIQUE index only allows one null value.
• If duplicate values ​​already exist in the table, you need to delete the duplicate values ​​before creating a UNIQUE index.
• UNIQUE indexes are similar to PRIMARY KEY indexes, but PRIMARY KEY indexes also enforce that column values ​​are not NULL.
• You can use the IGNORE DUPLICATE KEYS option to force MySQL to ignore duplicate values, but this will reduce data integrity.

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