sql - 为什么不能在下一个 SELECT 表达式中使用列别名？

Question

我可以修改 next 以在表达式 ROUND(avg_time * cnt, 2) 中使用列别名 avg_time 和 cnt 吗？

SELECT 
    COALESCE(ROUND(stddev_samp(time), 2), 0) as stddev_time, 
    MAX(time) as max_time, 
    ROUND(AVG(time), 2) as avg_time, 
    MIN(time) as min_time, 
    COUNT(path) as cnt, 
    ROUND(avg_time * cnt, 2) as slowdown, path
FROM 
    loadtime
GROUP BY
    path
ORDER BY
    avg_time DESC
LIMIT 10;

它会引发下一个错误:

ERROR:  column "avg_time" does not exist
LINE 7:  ROUND(avg_time * cnt, 2) as slowdown, path

然而，下一个工作正常(使用主表达式而不是列别名:

SELECT 
    COALESCE(ROUND(stddev_samp(time), 2), 0) as stddev_time, 
    MAX(time) as max_time, 
    ROUND(AVG(time), 2) as avg_time, 
    MIN(time) as min_time, 
    COUNT(path) as cnt, 
    ROUND(AVG(time) * COUNT(path), 2) as slowdown, path
FROM 
    loadtime
GROUP BY
    path
ORDER BY
    avg_time DESC
LIMIT 10;

Answer 1

您可以在 GROUP BY 中使用之前创建的别名或 HAVING声明，但不在 SELECT 中或 WHERE陈述。这是因为程序处理了所有 SELECT同时声明并且还不知道别名的值。

解决方案是将查询封装在子查询中，然后别名在外部可用。

SELECT stddev_time, max_time, avg_time, min_time, cnt, 
       ROUND(avg_time * cnt, 2) as slowdown
FROM (
        SELECT 
            COALESCE(ROUND(stddev_samp(time), 2), 0) as stddev_time, 
            MAX(time) as max_time, 
            ROUND(AVG(time), 2) as avg_time, 
            MIN(time) as min_time, 
            COUNT(path) as cnt, 
            path
        FROM 
            loadtime
        GROUP BY
            path
        ORDER BY
            avg_time DESC
        LIMIT 10
   ) X;