Question

我在第7行创建了一个名为total_city_pop的列别名。

然后，在第8行中，我尝试在新计算中使用该别名。

但是，我收到一条错误消息，上面写着＆＃34;列＆＃34; total_city_pop＆＃34;不存在LINE 8：（ci.city_proper_pop + ci.metroarea_pop）/ total_city_pop AS ... ^＆＃34;

为什么我在创建别名后不能使用别名？

这是代码 -

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;

谢谢。

Answer 1

You need to repeat the expression in the subquery. However, I would suggest you simplify the expression using COALESCE():

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;

The most recent versions of MySQL support computed columns. These would allow you to put the computation in the definition of the table.

如何在SELECT子句中命名后立即使用列别名？

1 个答案: