[求助]关于SQL语句的错误

程序如下
ADOQuery1.Close;
ADOQuery1.SQL.Clear;
ADOQuery1.SQL.Add('select avg(wendu) from 表1 where haoma=15978049552 and year(shijian)=a and Month(shijian)=b');
ADOQuery1.Parameters.ParamByName('a').Value:=strtoint(ComboBox2.Text);
ADOQuery1.Parameters.ParamByName('b').Value:=i;
ADOQuery1.open;
运行有错
但我把haoma=15978049552 and 删去运行就对了
说ADOQuery1: 什么 'a 'not found