MySQL函数里有一个很有用的函数IFNULL，它的形式是IFNULL(fieldA,fieldB),意义是当字段fieldA是NULL时取fieldB，不是NULL时取fieldA的值。
这个函数与外连接配合使用时能起到Oracle的IIF或是NVL的作用：
以下代码供参考：
StringBuilder sb=new StringBuilder();
sb.append(" select ");
sb.append(" t1.fullName,IFNULL(t2.actualHour,0) as actualHour,t1.planhour,t1.annotatorId,t1.annotatorId,IFNULL(t2.actualHour,0)/t1.planhour as ratio");
sb.append(" from ");
sb.append(" XXshipTB t1 ");
sb.append(" left outer join (select userid,sum(hours) as actualHour from dayworkTB where (Date(date) between '"+fromDate+"' and '"+endDate+"') group by userid) t2");
sb.append(" on ");
sb.append(" t1.annotatorId=t2.userid");
sb.append(" and abs(t2.actualHour-t1.planhour)>0.01");
sb.append(" ORDER by ");
sb.append(" ratio desc,t1.fullName asc");
String sql=sb.toString();