Question

我有疑问：

SELECT  busCode, address, current.position-1 as position, movingbus.bus as bus,
        current.linea as line, destination, current.paletta as 'end',
        previous.paletta as 'start', previous.lapseToNext, current.ident,
        movingbus.latitude, movingbus.longitude,
        SQRT(POW(movingbus.latitude-41.717149, 2) + 
             POW(movingbus.longitude-12.312499,2)) as distance, 
        ROUND(distance*80) as `time`,
        current.previous as previous
    FROM  striscie as current, striscie as previous, movingbus,
        linee, palette
    WHERE  palette.number=current.paletta
      AND  current.linea=line
      AND  current.previous=previous.ident
      AND  current.ident=movingbus.striscia
    HAVING  distance<0.01

其中我计算一个遥远的，然后计算一个距离的时间：相对部分是：

SQRT(POW(movingbus.latitude-41.717149,2)+
     POW(movingbus.longitude-12.312499,2)) as distance,
ROUND(distance*80) as `time`

然而，当我执行它时，MariaDB报告错误：

“字段列表”中的未知列“距离”

如果我反过来在“时间”中用'完整操作'代替'距离'，那么产生它：

SQRT(POW(movingbus.latitude-41.717149,2)+
     POW(movingbus.longitude-12.312499,2)) as distance,
ROUND(SQRT(POW(movingbus.latitude-41.717149,2)+
           POW(movingbus.longitude-12.312499,2))*80) as `time`

我没有错误。但是我当然得到一个非常大且可能效率低下的查询。如何在同一个select中的操作中使用select返回的字段作为参数？

Answer 1

计划A：

@dist := SQRT(...) AS distance,
ROUND(@dist*80) as `time`,

计划B：计算子查询中的距离（以及许多其他值），然后在外部查询中使用distance两次。

对于此案例，计划A简单而有效。如果有GROUP BY，则可能根本不起作用。

计划B在这个的情况下效率不高，但是在到达distance<0.1之前不必携带大量的东西可以带来巨大的性能优势。我在一小时前讨论过这个问题：https://stackoverflow.com/a/35905072/1766831使用这种技术可以提高性能，而不是从计划A中获得的效果。

Answer 2

正确：查询转到：

SELECT  busCode, address, current.position-1 as position, movingbus.bus as bus,
    current.linea as line, destination, current.paletta as 'end',
    previous.paletta as 'start', previous.lapseToNext, current.ident,
    movingbus.latitude, movingbus.longitude,
    @dist := SQRT(POW(movingbus.latitude-41.717149, 2) + 
         POW(movingbus.longitude-12.312499,2)) as distance, 
    ROUND(@dist*80) as `time`,
    current.previous as previous
FROM  striscie as current, striscie as previous, movingbus,
    linee, palette
WHERE  palette.number=current.paletta
  AND  current.linea=line
  AND  current.previous=previous.ident
  AND  current.ident=movingbus.striscia
HAVING  distance<1

它确实有效，非常感谢。

在MARIADB中的另一个字段中使用选定的值

2 个答案: