你需要使用 coalesce 获取要填充的非空值 b column 以及 case 决定填充的语句 c column .
例子：

hive> select t1.a,
      coalesce(t2.y,t1.b)b,
      case when t2.y is null then t1.c 
      else t2.z 
      end as c  
     from table1 t1 left join table2 t2 on t1.a=t2.x;

+----+-----+----+--+
| a  |  b  | c  |
+----+-----+----+--+
| a  | xx  | 5  |
| b  | bb  | 2  |
| c  | zz  | 7  |
| d  | dd  | 4  |
+----+-----+----+--+