Question

我的route表格中有四个字段

busid, routid, position and distance

我想展示busid and distance from a select query。我的选择查询如下：

$endb = mysql_query("select case when a.position < b.position then a.busid when a.position > b.position then a.busid else null end as busid, a.distance as distance from (select busid,position from route where routid=$result2) a join (select busid,position from route where routid=$end) b on a.busid = b.busid") or die(mysql_error());

但是当我使用此查询时，它会出错：unknown field distance in field list。请帮助我缺少的东西

Answer 1

除非在MySQL中有必要，否则不应在from子句中使用子查询。它们阻止优化器生成最佳查询计划。

编写查询的更好方法：

select (case when a.position < b.position then a.busid
             when a.position > b.position then a.busid 
        end) as busid,
       a.distance
from route a join
     route b
     on a.busid = b.busid and
        a.routid = $result2 and b.routid = $end;

当然，您的具体问题是a.distance未定义，因为它未在子查询中定义。

Answer 2

在子查询中缺少距离

select 
    case 
        when a.position < b.position then a.busid 
        when a.position > b.position then a.busid 
        else null 
    end as busid, 
    a.distance as distance 
from (
    select busid, position, distance
    from route 
    where routid=$result2
) as a join (
    select busid, position 
    from route 
    where routid=$end
) as b 
on a.busid = b.busid

即使是更好的版本：

SELECT if (a.position <> b.position, a.busid, null) busid, a.distance
FROM  route a, route b
WHERE a.busid = b.busid
AND   a.routid= $result2
AND   b.routid= $end

无法在mysql选择查询中获取两个字段

2 个答案: