--你的表是什么,换了就可以了
select categories_id from mc_categories 
    where categories_status = 1
        and exists (select categories_no from 你的表 where parent_id='{$parent_id}'
        and mc_categories.categories_no like 你的表.categories_no+'%')

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这个我没试过这样
但我可以给你个建议
用一个文本框 或是什么控件
  parent_id = parent.text  
然后再 select categories_id from mc_categories where categories_status = 1 and categories_no like 'parent_id%'

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SQL code

select categories_id from mc_categories 
where categories_status = 1 and categories_no like (select categories_no where parent_id=@parent_id)+'%'

------解决方案--------------------

SQL code

--mssql写法
declare @parent_id varchar(12)
set @parent_id='000123456789'
select categories_id from mc_categories 
    where categories_status = 1
        and exists (select categories_no from tb where parent_id=@parent_id
        and mc_categories.categories_no like tb.categories_no+'%')