What is the relationship between primary key constraints and unique constraints in mysql

The relationship between primary key constraints and unique constraints in MySQL: Primary key constraints require column values ​​to be unique and non-null; unique constraints allow null values ​​but prevent duplicate rows.

The relationship between primary key constraints and unique constraints in MySQL

In the MySQL database, primary key constraints and unique constraints are two important constraints used to maintain data integrity and uniqueness. They have different purposes and implementation methods.

Primary key constraints

• Definition: Primary key constraints specify one or more columns in the table. The values ​​of these columns uniquely identify the table. of each line.
• Purpose: Enforce data uniqueness in the table and prevent rows with the same primary key value from being inserted.
• Implementation: Defined when creating the table, the PRIMARY KEY keyword is used to specify the primary key column.

Unique constraint

• Definition: Unique constraint specifies one or more columns in the table whose values ​​are in Must be unique in the table, but does not have to be the only combination.
• Purpose: Prevent the insertion of rows with the same constraint column value, but allow null values.
• Implementation: Use the UNIQUE keyword definition when creating a table or modifying an existing table.

Relationship

• Primary key constraintis more strict than Unique constraint**. Primary key constraints require column values ​​to be unique and non-null, while unique constraints allow null values.
• A column can have both a primary key constraint and a unique constraint, but this is uncommon.
• When a table has a composite primary key (containing multiple columns), a unique constraint can be applied to one or more of the primary key columns.
• Unique constraints can help prevent data duplication, but it does not guarantee data integrity because null values ​​are allowed.
• Primary key constraints not only ensure data integrity, but also prevent data duplication.

Summary

Primary key constraints and unique constraints are related constraint types in MySQL database, and they are used to maintain data integrity and uniqueness. The primary key constraint is more strict than the unique constraint. It requires that the value of the column is unique and non-null. A unique constraint allows null values, but it prevents duplicate rows from being inserted with the same constraint column value.

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