將一個表除以在同一查詢中創建的另一個表

Question

我有以下 SQL 查詢：

SELECT `NeighbourhoodName`,
count(NAME) as `Number of Parks`,
sum(CASE 
    WHEN `parks`.`Advisories` = 'Y' THEN 1
    ELSE 0 
END) as Advisories,
FROM parks
GROUP BY `NeighbourhoodName`;

在代碼的第二行，我創建了一個名為“Number of Parks”的列。 我希望下一列（建議）中的所有值除以“公園數量”中的值。 但是，當我嘗試像這樣在列之后插入除法語句時：

SELECT `NeighbourhoodName`,
count(NAME) as `Number of Parks`,
sum(CASE 
    WHEN `parks`.`Advisories` = 'Y' THEN 1
    ELSE 0 
END)/`Number of Parks` as Advisories
FROM parks
GROUP BY `NeighbourhoodName`;

我收到以下錯誤：

Unknown column, `Number of Parks` in field list.

如何在執行此除法的同時仍將其保留在一個查詢中？

Answer 1

您需要使用外部查詢來進行除法。

SELECT `NeighbourhoodName`,
       `Number of Parks`,
        Advisories/`Number of Parks` as Advisories
FROM    ( SELECT `NeighbourhoodName`,
                 count(NAME) as `Number of Parks`,
                 sum( CASE  WHEN `parks`.`Advisories` = 'Y' THEN 1 ELSE 0 END ) as Advisories
          FROM parks
          GROUP BY `NeighbourhoodName`
         ) as tbl;

列別名的問題

可以在查詢 select 列表中使用別名來為列賦予不同的名稱。 您可以在 GROUP BY、ORDER BY 或 HAVING 子句中使用別名來引用該列。

Answer 2

或者使用count(Name)的表達式代替Number of Parks ：

select NeighbourhoodName,
       count(Name)      as `Number of Parks`,
       sum(case when Advisories='Y' then 1 else 0 end)
       /count(Name)     as Advisories
  from parks
 group by NeighbourhoodName;