无法在mysql select查询中获得两个字段

Question

I have four fields in my route table

busid, routid, position and distance

I want to show busid and distance from a select query . My select query is as follow :

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

but when I use this query then it gives error : unknown field distance in field list . Plese help what I am missing

Answer 1

You should not use subqueries in the from clause, unless necessary in MySQL. They prevent the optimizer from generating the best query plan.

A better way to write the query:

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;

Your specific problem, of course, is that a.distance is not defined because it was not defined in the subquery.

Answer 2

Missing distance in sub-query a

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

Even a better version:

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