The difference between primary key constraints and unique constraints in mysql

The difference between primary key constraints and unique constraints in MySQL is that: the primary key contains up to multiple columns, does not allow empty or duplicate values, identifies each record, and can be used as a foreign key; the unique constraint can contain any number of columns, allowing Null values ​​are allowed but duplicate values ​​are not allowed to prevent duplication of specific column combinations.

The difference between primary key constraints and unique constraints in MySQL

Primary key constraints and unique constraints are used for maintenance in MySQL Important types of constraints for database integrity, there are key differences between them:

Definition:

• Primary key constraints: Specify one or more constraints in the table A column whose value uniquely identifies each record in the table.
• Unique constraints: Specify one or more columns in the table whose values ​​are unique in the table, but null values ​​are allowed.

Main differences:

1. Number of columns:

• Primary key constraints can only contain at most One or more columns.
• Unique constraints can contain any number of columns.

2. Null value processing:

• Primary key constraints do not allow null or duplicate values.
• Unique constraints allow null values, but do not allow duplicate values.

3. Identify unique records:

• Primary key constraints are used to identify each row in the table. A table can only have one primary key constraint. It guarantees the uniqueness and non-repeatability of each record in the table.
• Unique constraints ensure that the combination of specified columns is unique in the table, but does not necessarily identify each record. It can be used to prevent the insertion of duplicate data, but allows null values.

4. Foreign key relationships:

• Primary key constraints can reference other tables as foreign keys to establish relationships between tables in a relational database. connect.
• Unique constraints cannot be used as foreign keys.

5. Performance:

• Primary key constraints usually have better performance than unique constraints because it can quickly find and retrieve keys with a specific primary key. Key-value records.
• The performance of unique constraints is usually low, especially when there are a large number of duplicate values ​​in the table.

Selection Guide:

Choosing to use a primary key constraint or a unique constraint depends on the needs of the specific table and data:

• If If you need to ensure the uniqueness and non-repeatability of each record in the table, use primary key constraints.
• If you need to prevent a combination of specific columns from being repeated but allow null values, use a unique constraint.

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