td中聚合函数count（a）只计算a不为null的个数，avg（a）计算平均时候也是不会考虑为null的个数，所以有时需要转换一下，eg：
,AVG (NULLIF ( a, 0)) (TITLE 'NIZ//AVG') --如果a为0则设为null
,AVG (COALESCE ( a, 0)) (TITLE 'ZIN//AVG')--如果a为null则设为0
(FORMAT 'zz,zzz.z')
,COUNT (NULLIF ( la, 0)) (TITLE 'NIZ//COUNT')
(FORMAT 'zz9')
,COUNT (COALESCE ( a, 0)) (TITLE 'ZIN//COUNT')
(FORMAT'zz9')