mysql - 我可以在 Where 子句中使用 "virtual column"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

我尝试像这样在 WHERE 子句中使用 newVisitID 列:

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，但您可以在 HAVING 中引用它们:

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