無法獲得MySQL中選擇查詢

Question

兩個領域我已經在我的route表四個字段

busid, routid, position and distance

我想顯示busid and distance from a select query。我選擇的查詢是如下：

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

但是當我使用這個查詢然後提示錯誤：unknown field distance in field list。普萊舍幫助我錯過了什麼

子查詢

Answer 1

失蹤距離

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 

即使是一個更好的版本：

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 

Answer 2

你不應該在MySQL中from子句中使用子查詢，除非必要。它們阻止優化器生成最佳查詢計劃。

一種更好的方式來編寫查詢：

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; 

您的特定問題，當然，是a.distance沒有定義，因爲它沒有在子查詢中定義。