我可以在Where子句中使用"虚拟列"吗？

Question

我有这个问题:

set @last_uid = 0;
set @last_tm = '00:00';
set @last_vid = 0;

SELECT v.*,
     CASE WHEN @last_uid = visitorid AND cast( @last_tm as datetime) >= subtime( timestamp, '01:00' )
     THEN 
          if( (@last_tm := timestamp ), @last_vid, @last_vid)
     ELSE 
          if( (@last_uid := visitorid) + (@last_vid := @last_vid+1) +(@last_tm := timestamp ),@last_vid, @last_vid )
     END  AS newVisitID

FROM visits v
ORDER BY timestamp DESC, visitorid

我试图在WHERE子句中使用newVisitID列,如下所示:

WHERE newVisitID <=5

这给了我错误:

#1054 - Unknown column 'newVisitID' in 'where clause'

谁能帮我解决这个问题？

Answer 1

您不能在WHERE子句中引用别名,但可以在HAVING以下位置引用它们:

SELECT v.*,
     CASE WHEN @last_uid = visitorid AND cast( @last_tm as datetime) >= subtime( timestamp, '01:00' )
     THEN 
          if( (@last_tm := timestamp ), @last_vid, @last_vid)
     ELSE 
          if( (@last_uid := visitorid) + (@last_vid := @last_vid+1) +(@last_tm := timestamp ),@last_vid, @last_vid )
     END  AS newVisitID

FROM visits v
HAVING newVisitID <- 5
ORDER BY timestamp DESC, visitorid