Oracle 联合投掷错误

包含的查询抛出了一个错误，请帮助我理解它

select name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks >= 70
  order by grade desc,name
union 
select to_char('NULL') as name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks <=69
  order by grade desc,marks asc;

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对于整个查询，只能有一个

orderby

子句，它位于末尾。取中间的一个。

select name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks >= 70
union
select to_char('NULL') as name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks <=69
  order by grade desc,marks asc;

选择名称、等级、标记
来自学生、年级
其中min_mark对于整个查询只能有一个
orderby
子句，它位于末尾。取中间的一个。

select name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks >= 70
union
select to_char('NULL') as name, grade, marks
  from students, grades
  where min_mark<=marks and
        marks <= max_mark and
        marks <=69
  order by grade desc,marks asc;

选择名称、等级、标记
来自学生、年级
min_mark在哪里