DB2 SQL获得不同价值的分组行

Question

BUSINESSTABLE看起来像这样的时候：

HOTEL_CHAIN HOTEL_LOCATION HOTEL_OWNER 
_____________________________________________________ 
Marriott  Las Vegas  Nelson 
Best Western New York   Richards 
Best Western San Francisco Smith 
Marriott  New York   Nelson 
Hilton   Boston   James 

我想在DB2数据库组由HOTEL_CHAIN这些条目执行SQL语句。如果组合在一起的行包含相同的HOTEL_LOCATION或HOTEL_OWNER，则应保留该信息。否则，应显示“NULL”值。例如，两家万豪酒店都拥有相同的所有者纳尔逊，所以我想在新表格中显示这些信息。但是，每家万豪酒店都位于不同的位置，因此我想在该列中显示“NULL”。

结果表（HOTELTABLE）应该是这样的：

HOTEL_CHAIN HOTEL_LOCATION HOTEL_OWNER 
_____________________________________________________ 
Marriott  NULL    Nelson 
Best Western NULL    NULL 
Hilton   Boston   James 

我尝试使用下面的SQL语句来实现：

INSERT INTO HOTELTABLE(HOTEL_CHAIN,HOTEL_LOCATION,HOTEL_OWNER) 
SELECT 
HOTEL_CHAIN, 
CASE COUNT(DISTINCT(HOTEL_LOCATION)) WHEN 1 THEN HOTEL_LOCATION ELSE 'NULL' END, 
CASE COUNT(DISTINCT(HOTEL_OWNER)) WHEN 1 THEN HOTEL_OWNER ELSE 'NULL' END, 
FROM BUSINESSTABLE GROUP BY HOTEL_CHAIN 

我得到一个SQL错误SQLCODE- 119具有条款的列或表达式无效。这似乎是在抱怨我的病例报告中的第二个HOTEL_LOCATION和第二个HOTEL_OWNER。我也尝试使用DISTINCT（HOTEL_LOCATION），并抛出了另一个错误。有人可以请解释正确的方式来编码吗？谢谢！

Answer 1

请勿使用COUNT(DISTINCT)。使用MIN()和MAX()：

INSERT INTO HOTELTABLE(HOTEL_CHAIN,HOTEL_LOCATION,HOTEL_OWNER) 
    SELECT HOTEL_CHAIN, 
      (CASE WHEN MIN(HOTEL_LOCATION) = MAX(HOTEL_LOCATION) 
       THEN MIN(HOTEL_LOCATION) ELSE 'NULL' 
      END), 
      (CASE WHEN MIN(HOTEL_OWNER) = MAX(HOTEL_OWNER) 
       THEN MIN(HOTEL_OWNER) ELSE 'NULL' 
      END) 
    FROM BUSINESSTABLE 
    GROUP BY HOTEL_CHAIN; 

注：

• 为什么不COUNT(DISTINCT)？它通常比MIN()和MAX()贵得多，因为它需要维护所有值的内部列表。
• 我不认可名为'NULL'的字符串值。似乎它是为了引起混淆。也许只是NULL的价值本身？

Answer 2

我同意Gordon为null（gj Gordon）。

其他方法

INSERT INTO HOTELTABLE(HOTEL_CHAIN,HOTEL_LOCATION,HOTEL_OWNER) 

select distinct f1.HOTEL_CHAIN, 
     case when f2.HasDiffLocation is not null then 'NULL' else f1.HOTEL_LOCATION end as HOTEL_LOCATION, 
     case when f3.HasDiffOwner is not null then 'NULL' else f1.HOTEL_OWNER end as HOTEL_OWNER 
from BUSINESSTABLE f1 
left outer join lateral 
(
    select 1 HasDiffLocation from BUSINESSTABLE f2b 
    where f1.HOTEL_CHAIN=f2b.HOTEL_CHAIN and f1.HOTEL_LOCATION<>f2b.HOTEL_LOCATION 
    fetch first rows only 
) f2 on 1=1 

left outer join lateral 
(
    select 1 HasDiffOwner from BUSINESSTABLE f3b 
    where f1.HOTEL_CHAIN=f3b.HOTEL_CHAIN and f1.HOTEL_OWNER<>f3b.HOTEL_OWNER 
    fetch first rows only 
) f3 on 1=1 

或这样的：

INSERT INTO HOTELTABLE(HOTEL_CHAIN,HOTEL_LOCATION,HOTEL_OWNER) 
select distinct f1.HOTEL_CHAIN, 
ifnull(f2.result, f1.HOTEL_LOCATION) as HOTEL_LOCATION, 
ifnull(f3.result, f1.HOTEL_OWNER) as HOTEL_LOCATION, 
from BUSINESSTABLE f1 
left outer join lateral 
(
    select 'NULL' result from BUSINESSTABLE f2b 
    where f1.HOTEL_CHAIN=f2b.HOTEL_CHAIN and f1.HOTEL_LOCATION<>f2b.HOTEL_LOCATION 
    fetch first rows only 
) f2 on 1=1 

left outer join lateral 
(
    select 'NULL' result from BUSINESSTABLE f3b 
    where f1.HOTEL_CHAIN=f3b.HOTEL_CHAIN and f1.HOTEL_OWNER<>f3b.HOTEL_OWNER 
    fetch first rows only 
) f3 on 1=1