Question

我正在尝试创建一个查询，该查询返回另一个表中最大次数的一个东西的名称和国家/地区。

这是结构：

ShipModels(mname, type, country, numGuns, gunSize)
Ships(sname, hasModel, launchYear)
Battles(bname, year)
Outcomes(ship, battle, result)

我尝试过很多不同的东西，但无法让它发挥作用。这是我目前的查询：

SELECT ship, country
FROM ShipModels, Ships, Outcomes
WHERE hasModel = mname AND ship = sname
GROUP BY ship
HAVING COUNT(ship) >= 
( SELECT COUNT (ship) FROM Outcomes GROUP BY ship);

Oracle错误：

ERROR at line 1:
ORA-00979: not a GROUP BY expression

从我读过的内容中，当SELECT语句中存在聚合函数时会抛出此错误，但是......好吧，没有。有什么我想念的吗？我怎样才能让它发挥作用？

Answer 1

您将country包含在SELECT：

中

SELECT ship, country

但不在GROUP BY。将该行更改为：

GROUP BY ship, country

当你纠正它时，你会遇到另一个错误，因为子查询不是标量，例如不会返回单个值：

( SELECT COUNT (ship) FROM Outcomes GROUP BY ship)

你可能想写：

( SELECT MAX(cnt)
  FROM
    ( SELECT COUNT(ship) AS cnt FROM Outcomes GROUP BY ship ) t
)

因此，完整查询将或多或少：

SELECT ship, country
FROM ShipModels, Ships, Outcomes
WHERE hasModel = mname AND ship = sname
GROUP BY ship, country
HAVING COUNT(ship) = 
    ( SELECT MAX(cnt)
      FROM
        ( SELECT COUNT(ship) AS cnt FROM Outcomes GROUP BY ship ) t
    ) ;

您还可以使用关键字ALL：

将其编写为更类似于您的尝试

SELECT ship, country
FROM ShipModels, Ships, Outcomes
WHERE hasModel = mname AND ship = sname
GROUP BY ship, country
HAVING COUNT(ship) >= ALL
    ( SELECT COUNT(ship) AS cnt FROM Outcomes GROUP BY ship ) ;

如果使用显式连接的“现代”（SQL-92）语法也会很好。而不是：

FROM ShipModels, Ships, Outcomes
WHERE hasModel = mname AND ship = sname

最好有各种理由写：

FROM ShipModels m
  JOIN Ships s ON s.hasModel = m.mname
  JOIN Outcomes o ON o.ship = s.sname

Answer 2

问题在于HAVING子句中的count（ship）。我认为这是select语句中的聚合函数。

SQL ORA-00979错误抛出

2 个答案: