spark sql group by 报错

报错信息，

org.apache.spark.sql.AnalysisException:
expression 'a.`site`' is neither present in the group by, nor is it an aggregate function. 
Add to group by or wrap in first() (or first_value)

报错原因：spark sql 使用group by的时候必须将select的所有字段都写进去。
例如:

select a,b,c from table group by a;

就会报错
解决办法，在不需要group by的字段上加first()

select a,first(b),first(c) from table group by a;

这样就可以了。