以下哪种写法，可以有效地找出目标列C1为空值的记录？其中C1列可能的值为正整数。BD A select * from t1 where c1 = null; B select * from t1 where decode(c1,null,1,2)=1; C select * from t1 where coalesce(c1,null)=null; D select * from t1 where c1 is null; E select * from t1 where nvl(c1,1)=1;