SQL Postgres使用avg列作为外部查询中的变量

Question

我写了以下查询：

SELECT DISTINCT gno, avg(weight) as x
    FROM Edge
    GROUP BY gno
    HAVING avg(weight)<max(weight)/2

并得到这个（正确的）表格：

现在我想使用右列并在那里找到最大值并仅显示该行。 我怎么能这样做？

Answer 1

如果我理解正确，请使用order by和limit：

SELECT gno, avg(weight) as x
FROM Edge
GROUP BY gno
HAVING avg(weight)<max(weight)/2
ORDER BY avg(weight) desc
LIMIT 1;

请注意，使用group by时，您不需要select distinct。

Answer 2

您始终可以使用CTEs撰写此类查询：

WITH averages AS (
  SELECT DISTINCT gno, avg(weight) as x
    FROM Edge
    GROUP BY gno
    HAVING avg(weight)<max(weight)/2
)
SELECT MAX(x) FROM averages;

（但请注意CTEs are optimization fences）