MySQL Foreign Key Constraints: How to Avoid Data Errors in Table Relationships?

MySQL foreign key constraints are used to ensure the integrity of multi - table associated data, avoiding invalid references (such as non - existent user IDs in orders) and data inconsistencies (such as residual orders after a user is deleted). A foreign key constraint is a table - level constraint, which requires that the foreign key field of the child table references the primary key or unique key of the parent table. When creating, the parent table must be created first, and then in the child table, the association is specified using `FOREIGN KEY (foreign key field) REFERENCES parent_table(primary key field)`. The behavior can be set through `ON DELETE/ON UPDATE`, such as `CASCADE` (cascade operation), `SET NULL` (set to NULL), or `RESTRICT` (operation is prohibited by default). The functions of foreign key constraints are: preventing incorrect references, maintaining data consistency, and clarifying table relationships. Precautions for use: The referenced field in the parent table must be a primary key/unique key, the data types of the foreign key and the parent table field must be consistent, and when deleting records in the parent table, the child table associations must be processed first. Although it may affect performance, it can be ignored for small and medium - sized projects. Foreign key constraints are a core tool for multi - table association. It is recommended to use them first when designing related tables. Mastering the syntax and behavior settings can ensure data reliability.