方法1:缺点,不去重,不去空:见表1 with t as( select 'A' parent, 'A1' child union all select 'A', 'A1' union all select 'A', '' union all select 'A', null union all select 'A', 'A2' union all select 'B', 'B1' union all select 'B', 'B2' union all select 'C', 'C1' uni