这样的语句加IF试什么意思？

"SELECT  talkTable.talkID,talkTable.talkTitle,talkTable.talkAuthor,talkTable.email,         talkTable.numHit,COUNT(talk_backTable.talkID) AS numTalkBack,         Max(If(talk_backTable.tbTime,talk_backTable.tbTime,talkTable.talkTime)) AS NewUpdate FROM talkTable LEFT JOIN talk_backTable ON   talkTable.talkID=talk_backTable.talkID GROUP BY talkTable.talkID,talkTable.talkTitle,talkTable.talkAuthor ORDER BY NewUpdate DESC";

双表查询语句，上面的Max(If(talk_backTable.tbTime,talk_backTable.tbTime,talkTable.talkTime)) 为什么要多加个if啊？ 即使是加判断也不能这样乱来吧。。。。。？？

If(talk_backTable.tbTime,talk_backTable.tbTime,talkTable.talkTime)
条件语句。条件成立取第一个值，否则去第二个值
mysql> SELECT IF(1<2,'yes ','no');
        -> 'yes'

鱼大哥 结合这个Max(If(talk_backTable.tbTime,talk_backTable.tbTime,talkTable.talkTime)) 语句说详细点好吗 谢谢了


PS:已经明白了  谢谢

[ 本帖最后由 xinxinge 于 2009-8-5 22:29 编辑 ]

顶顶顶！鱼大哥真是强悍！