$field = 'user_id, sum(quantity) as total, sum(if(goods_id=2,`quantity`, 0)) as total2, sum(if(goods_id=3,`quantity`, 0)) as total3 ';

生成SQL语句后：变成：
`user_id`,sum(quantity) as total,sum(if(goods_id=2,`quantity`,0)) as total2,sum(if(goods_id=3,0)) as total3

少了,`quantity`，结果SQL报错。