问题是avg是一个聚合函数。如果要Map一对多关系，则需要使用交叉联接函数：

select 
  count(*), avg(v1.wage),
  sum(case when v.wage < v2.avgwage then 1 else 0 end) below_average
from 
emp_wages v cross join (select avg(wage) as avgwage from emp_wages) as v2

SELECT COUNT(*) 
FROM (SELECT wage, AVG(wage) AS avgWage FROM emp_wages group by wage)avg --group by needed
WHERE wage < avgWage;

语法错误。派生表应具有别名。

SELECT COUNT(*) 
FROM (SELECT wage, AVG(wage) AS avgWage FROM emp_wages group by wage)  t --alias needed here
WHERE wage < avgWage;

就查询而言，它需要改变。

select count(*)
from (SELECT wage, AVG(wage) over() AS avgWage 
      FROM emp_wages
     ) t
where wage < avgWage

正确的查询是：

select count(*) where wage <(select avg(wage) from emp_wages);

由于子查询中有wage和avgwage，您将得到一个解析错误。