Question

我有这个SQL语句：

select max(avrg) 
from (select sum(points) as avrg 
      from marks 
      group by num_eleve) s;

我在使其工作时遇到了问题，结果发现我在最后需要s，没有声明就失败了。

我想了解s的作用，换句话说，为什么上述工作会在以下失败时发挥作用？

select max(avrg) 
from (select sum(points) as avrg 
      from marks 
      group by num_eleve);

Answer 1

select max(avrg) 
from (select sum(points) as avrg 
      from marks 
      group by num_eleve) s;

在上面的查询中，s是sub-select

的别名

(select sum(points) as avrg from marks group by num_eleve)

它将用于引用Sub-select中的列。您的查询可以写成

select max(s.avrg)  --here I have added s.avrg 
from (select sum(points) as avrg 
      from marks 
      group by num_eleve) s;

另外，如果您想将子选择与另一个名为avrg的列的表连接，那么要区分您想要的avrg列值s

select max(s.avrg)  --here I have added s.avrg to pull valued from sub-select not from another_table 
from (select sum(points) as avrg 
      from marks  
      group by num_eleve) s Left join another_table b on s.avrg = b.avrg

Answer 2

这条查询：

(select sum(points) as avrg from marks group by num_eleve)

充当表格，只需要一个名称。＆＃34; s＆＃34;会做，但它可以是任何东西。如果你愿意，你可以称之为MyTable或Averages或Mxyzptlk。

SQL想问一下Select子句

2 个答案: