mysql 查询id如果为空则赋值为0

select
        IFNULL(
        (select DISTINCT  smvt.member_id
        from scs_mutation_verify_task smvt
        LEFT JOIN rpt_tools_procedure tp on  smvt.procedure_id=tp.id
        where tp.task_code =#{taskCode}  and smvt.member_type ="x")

        ,0)