如果关系已经存在，则跳过关系创建，而不是关于合并

-- 检查外键是否存在
SELECT COUNT(*) INTO @fk_exists FROM information_schema.table_constraints WHERE table_name = 'your_table' AND constraint_type = 'FOREIGN KEY' AND constraint_name = 'your_fk_name';

-- 如果不存在，则创建外键
SET @sql = IF(@fk_exists = 0, 'ALTER TABLE your_table ADD CONSTRAINT your_fk_name FOREIGN KEY (your_column) REFERENCES other_table(other_column)', '');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

CREATE TABLE IF NOT EXISTS your_table (
    id INT PRIMARY KEY,
    your_column INT,
    FOREIGN KEY (your_column) REFERENCES other_table(other_column) ON DELETE CASCADE
);