How to distinguish primary key constraints and unique constraints in mysql

The difference between primary key and unique constraint: primary key forces each row of data to be unique and non-null, and is used to identify the row; unique constraint only enforces the uniqueness of a specific column or column combination, allowing other columns to be repeated.

The difference between primary key constraints and unique constraints in MySQL

Getting Started

In MySQL, primary key constraints and unique constraints are mechanisms used to ensure the integrity and uniqueness of data in the table. Their main difference is:

• Primary key constraintsEnforce that each row of data has a unique and non-null identifier, and no duplicate values ​​are allowed.
• Unique constraintsOnly enforce uniqueness for a specific column or combination of columns, allowing duplicate values ​​for other columns.

Detailed description

Primary key constraint

• Uniqueness:Every The primary key value of row data must be unique.
• Non-nullability: Primary key columns are not allowed to be NULL.
• Identification: The primary key is used to identify each row in the table.
• Foreign key relationships: Primary keys are often used as foreign keys in other tables to establish relationships between tables.

Unique Constraints

• Uniqueness: The value of a specific column or combination of columns must be unique, but others are allowed Columns are duplicated.
• Non-nullability: The unique constraint column can be NULL.
• Identity: Unique constraints do not guarantee the uniqueness of rows.
• Foreign key relationship: Unique constraints can also be used as foreign keys, but they are not as commonly used as primary keys.

Other differences

In addition, there are the following differences between primary key constraints and unique constraints:

• Create:Primary key constraints are usually defined when the table is created, while unique constraints can be added after the table is created.
• Index: Primary key constraints will automatically create a unique index, while unique constraints require explicit creation of the index.
• Delete: Deleting a primary key constraint will cascade delete the foreign keys that reference the primary key, while deleting a unique constraint will not affect the foreign keys.

Summary

By understanding the difference between primary key constraints and unique constraints, you can more effectively design and manage the data in your MySQL tables to ensure that it Completeness, uniqueness and correctness.

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