DO
$do$
BEGIN    IF NOT EXISTS (
        SELECT *
        FROM information_schema.constraint_column_usage
        WHERE table_name = 'sys_user_register' AND constraint_name = 'sys_user_register_pk'
    ) THEN        ALTER TABLE sys_user_register
        ADD CONSTRAINT sys_user_register_pk PRIMARY KEY (f_id);
    END IF;
END;
$do$;