连贯操作total用于在查询数据的同时，返回合计行。比如要统计某一列的和、平均值等。
### 自动列名称
代码：
$m->headTotal(array('GrossWeight'=>'sum','TareWeight'=>'count','NetWeight'=>'avg'))->select();
结果格式：
array (size=38)
0 =>
array (size=3)
'GrossWeight' => string '894248.000' (length=10)
'TareWeight' => string '37' (length=2)
'NetWeight' => string '22375.6486486' (length=13)
1 =>
array (size=31)
'字段1' => string '89826' (length=5)
'字段2' => string '89826' (length=5)
……
### 指定列名称
代码：
$m->footTotal(array('GrossWeight'=>array('sum','a'),'TareWeight'=>array('count','b'),'NetWeight'=>array('avg','c')))->select();
结果格式：
array (size=38)
0 =>
array (size=31)
'字段1' => string '89826' (length=5)
'字段2' => string '89826' (length=5)
……
1 =>
array (size=3)
'a' => string '894248.000' (length=10)
'b' => string '37' (length=2)
'c' => string '22375.6486486' (length=13)
