联合投掷错误

【问题标题】：Union throwing error联合投掷错误
【发布时间】：2018-11-28 14:48:26
【问题描述】：

包含的查询抛出错误，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;

错误：

union 
* 
ERROR at line 3: 
ORA-00933: SQL command not properly ended

【问题讨论】：

标签： oracle join union

【解决方案1】：

您的整个查询只能有一个ORDER BY 子句，它位于最后。把中间的那个拿出来。

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;

此外，由于您的集合似乎没有重叠，union all 的性能会比union 稍好。

【讨论】：