sql nvl 关键字

如果第一个参数的值为空值，则返回第二个参数的值，否则返回第一个参数的值。如果两个参数的值都为空值，则返回空值。

这里我要判断这个值，是pi.sendflag 或者 N

<select id="***" resultType="com.***">
		<![CDATA[
		select distinct o.orderno, o.contact, o.campaignscriptid, o.domesticindicate,
						o.bookuser, o.bookagent, o.attribute3,o.attribute5,o.seatnum,o.createdate,o.lastupdate as lastUpdateTime,o.guarders,o.globalno,
						ch.channelid, ch.attribute1
		from orders o
				 left join b2corder_channel ch on o.orderno = ch.orderno
				 left join passengerinfo pi on pi.orderno = o.orderno
		 where o.status = 'E'
		  and nvl(pi.sendflag, 'N') = 'N'
		  and o.orderflag = 'B'
		  and o.lastupdate >= sysdate + INTERVAL '-2' DAY
		  and o.lastupdate < sysdate
		  and rownum < 1000
		]]>
	</select>