mysql - 我可以在 Where 子句中使用“虚拟列”吗？

Question

我有这个查询：

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，但可以在以下位置引用它们HAVING：

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

Answer 2

您可以再次写出 case 语句，或者更简单的方法是将其包装然后过滤子查询：

select * from
(
    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
) x
WHERE x.newVisitID <= 5
ORDER BY x.timestamp DESC, x.visitorid

Answer 3

认为您可以使用变量

SELECT v.*,
     @newVisitID := 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
where @newVisitID <= 5
ORDER BY timestamp DESC, visitorid

或者如果您不想重复代码，请使用子查询。