order by 对null的处理

【Oracle 结论】 
order by colum asc 时，null默认被放在最后
order by colum desc 时，null默认被放在最前
nulls first 时，强制null放在最前，不为null的按声明顺序[asc|desc]进行排序
nulls last 时，强制null放在最后，不为null的按声明顺序[asc|desc]进行排序 
【MySql 结论】
order by colum asc 时，null默认被放在最前
order by colum desc 时，null默认被放在最后
ORDER BY IF(ISNULL(update_date),0,1) null被强制放在最前，不为null的按声明顺序[asc|desc]进行排序
ORDER BY IF(ISNULL(update_date),1,0) null被强制放在最后，不为null的按声明顺序[asc|desc]进行排序

<select id="getPchsOrderReport" parameterClass="java.util.Map" resultClass="com.project.pojo.purchase.SupPurReport">
		SELECT s.unit_uid as supplierId,s.UNIT_NAME as supplierName,IFNULL(m.orderNum, 0) as orderNum,IFNULL(m.orderPrice, 0) as orderPrice,
		IFNULL(m.receiveNum, 0) as receiveNum ,IFNULL(m.receivePrice, 0) as receivePrice
		FROM bas_supplier s LEFT JOIN (
		SELECT
			b.supplier_uid AS supplierId,
			SUM(IFNULL(d.size, 0)) AS orderNum,
			SUM(IFNULL(d.sum, 0)) AS orderPrice,
			SUM(IFNULL(d.receive, 0)) AS receiveNum,
			SUM(
				IFNULL(d.receive, 0) * IFNULL(d.price, 0)
			) AS receivePrice
		FROM
		pchs_bill b
		LEFT JOIN pchs_detail d ON b.bill_uid = d.bill_uid
		WHERE b.com_uid = #comUid#
		<isNotEmpty property="status">
			and b.status  in <iterate property="status" conjunction="," close=")" open="(">#status[]:Integer#</iterate>
		</isNotEmpty>
		<isNotEmpty prepend="AND" property="startDate">
			<![CDATA[b.bill_date >= #startDate#]]>
		</isNotEmpty>
		<isNotEmpty prepend="AND"  property="endDate">
			<![CDATA[b.bill_date <= #endDate#]]>
		</isNotEmpty>
		<isNotEmpty prepend="and" property="storageId"> b.storage_uid = #storageId#  </isNotEmpty>
		<isNotEmpty prepend=" and b.storage_uid in " property="multi_storage_uid">
			<iterate property="multi_storage_uid" conjunction="," close=")" open="("> #multi_storage_uid[]# </iterate>
		</isNotEmpty>
		GROUP BY
			b.supplier_uid
		) m ON m.supplierId=s.unit_uid
		<isNotEmpty property="supplierId">
		WHERE s.unit_uid =#supplierId#
		</isNotEmpty>
		order by IF(ISNULL(orderNum),1,0),orderNum DESC,IF(ISNULL(orderPrice),1,0),orderPrice DESC
		<isNotEmpty property="offset">
			LIMIT #offset#,#limit#
		</isNotEmpty>
</select>