如何在SELECT子句中命名後立即使用列別名？

Question

我在第7行創建了一個名爲total_city_pop的列別名。

然後，在第8行中，我嘗試在新計算中使用該別名。

不過，我收到了寫着 「列 」total_city_pop錯誤消息「 不存在8號線：（ci.city_proper_pop + ci.metroarea_pop）/ total_city_pop AS ... ^」

爲什麼我不能我創建後使用別名？

下面的代碼 -

SELECT co.code, ci.name AS capital_city, ci.city_proper_pop, ci.metroarea_pop, ci.urbanarea_pop, 
    CASE 
     WHEN (ci.city_proper_pop + ci.metroarea_pop + ci.urbanarea_pop) IS NULL 
     THEN (ci.city_proper_pop + ci.urbanarea_pop) 
     ELSE (ci.city_proper_pop + ci.metroarea_pop + ci.urbanarea_pop) END AS total_city_pop, 
    (ci.city_proper_pop + ci.metroarea_pop)/total_city_pop AS percent_outside_urbanarea 
FROM countries AS co 
    INNER JOIN cities AS ci ON co.capital = ci.name 
    WHERE continent LIKE '%America%' OR continent LIKE 'Europe'  
    ORDER BY total_city_pop DESC 
    LIMIT 10; 

謝謝。

Answer 1

您需要在子查詢中重複表達式。不過，我建議你可以簡化使用COALESCE()表達：

SELECT co.code, ci.name AS capital_city, ci.city_proper_pop, ci.metroarea_pop, ci.urbanarea_pop, 
     (ci.city_proper_pop + ci.metroarea_pop + COALECE(ci.urbanarea_pop, 0)) as total_city_pop, 
     (ci.city_proper_pop + ci.metroarea_pop)/(ci.city_proper_pop + ci.metroarea_pop + COALECE(ci.urbanarea_pop, 0)) AS percent_outside_urbanarea 
FROM countries co INNER JOIN 
    cities ci 
    ON co.capital = ci.name 
WHERE continent LIKE '%America%' OR continent LIKE 'Europe'  
ORDER BY total_city_pop DESC 
LIMIT 10; 

MySQL支持計算列的最新版本。這些將允許您將計算放在表格的定義中。