SQL嵌套联接（case语句和联接）

Question

Hive DBMS；两个表-A和B

表A

prnt_id      sub_id      ac_nm      cost    units
unknown      abc01       abc corp   34500   24
unknown      unknown     xyz corp   9800    10
856          abc03       jfk corp   9820    12

表B

prnt_id      sub_id      ac_nm 
123          abc01       abc corp
456          abc02       xyz corp
856          abc03       jfk corp
859          abc04       ops corp

问题->尝试执行查询，其中： 将表A与表B连接起来，首先在prnt_id上连接，如果它是“ unknown”，然后在sub_id上连接，如果它是“ unknown”，则在ac_nm上连接。

所需的输出：

prnt_id      sub_id      ac_nm      cost    units
123          abc01       abc corp   34500   24
456          abc02       xyz corp   9800    10
856          abc03       jfk corp   9820    12

Answer 1

您必须将LEFT的{​​{1}}联接到TableB的3个副本中，并过滤掉不匹配的行：

TableA

请参见demo。
结果：

select b.*,
       coalesce(a1.cost, a2.cost, a3.cost) cost,
       coalesce(a1.units, a2.units, a3.units) units
from TableB b
left join TableA a1 on a1.prnt_id = b.prnt_id
left join TableA a2 on a2.sub_id = b.sub_id and a1.prnt_id is null
left join TableA a3 on a3.ac_nm = b.ac_nm and a2.sub_id is null
where coalesce(a1.prnt_id, a2.sub_id, a3.ac_nm) is not null
order by b.prnt_id

Answer 2

您可以使用left join和coalesce()来选择第一个匹配项：

select a.*, coalesce(b.?, bs.?, ba.?) as new_col
from a left join
     b
     on b.prnt_id = a.prnt_id left join
     b bs
     on bs.sub_id = a.sub_id and b.prnt_id is null left join
     b ba
     on bs.ac_nm = a.ac_num and bs.sub_id is null;