您希望按WHERE子句中的列进行筛选，而该列在执行WHERE子句时尚未计算。您可以使用子查询或具有。
应该是这样的：

SELECT team, CONCAT(firstname,' ',lastname) AS name, SUM(offreb + defreb) AS Rebounds
FROM boxscore
WHERE round = 'Finals' AND game = 7
GROUP BY team, CONCAT(firstname,' ',lastname)
HAVING SUM(offreb + defreb) > 0
ORDER BY team, Rebounds;

这里使用HAVING子句解决了您的问题。
如果一个表已经使用GROUP BY进行了分组，但只对某些组感兴趣，则可以使用HAVING子句（很像WHERE子句）从结果中删除组。官方postgres docs

SELECT
  team,
  CONCAT(firstname,' ',lastname) AS name,
  SUM(offreb + defreb) AS "Rebounds"
FROM 
  boxscore
WHERE
  round = 'Finals' AND game = 7
GROUP BY
  team,
  CONCAT(firstname,' ',lastname)
HAVING
  SUM(offreb + defreb) > 0
ORDER BY
  team, "Rebounds";

请注意，不能在WHERE和GROUP BY子句中使用列别名，但可以在ORDER BY子句中使用列别名，并用双引号括起来以保持区分大小写。