如何获取具有空值和值的行

Question

面试官问我以下问题：

考虑下表：

tpid data
100 1
100 2
100 NULL
101 6
101 5
101 NULL
102 NULL
103 9
103 65
104 NULL
..
..

如果tpid 有任何数据，则显示数据而不是空值 但如果 tpid 只得到 null 则只针对 id 显示 null。

结果集应该是这样的：

tpid data
100 1
100 2
101 6
101 5
102 NULL
103 9
103 65
104 NULL

我写了以下查询，但它没有给出想要的结果：

;with cte as
(select tpid,count(data) as num from a group by TPID)

 select a.TPID,
 (case when cte.num=0 then NULL else a.DATA end)col
 from cte
join A on a.TPID=cte.TPID

Answer 1

DECLARE @temp TABLE(tpid int, data int)

INSERT INTO @temp 
SELECT 100, 1 UNION
SELECT 100, 2UNION
SELECT 100, NULL UNION
SELECT 101, 6 UNION
SELECT 101, 5 UNION
SELECT 101, NULL UNION
SELECT 102, NULL UNION
SELECT 103, 9 UNION
SELECT 103, 65 UNION
SELECT 104, NULL




SELECT * FROM @temp a WHERE 
data IS not NULL OR (select COUNT(1) FROM @temp WHERE tpid=a.tpid)=1

答案：

tpid    data
100     1
100     2
101     5
101     6
102    NULL
103     9
103    65
104   NULL

Answer 2

你可以使用窗口函数来得到你想要的：

SELECT tpid, data
FROM (
  SELECT tpid, data,
         COUNT(*) OVER (PARTITION BY tpid) AS cnt,
         COUNT(CASE WHEN [data] IS NULL THEN 1 END) 
         OVER (PARTITION BY tpid) AS cntNulls
  FROM mytable) AS t
  WHERE (data IS NOT NULL) OR (cnt = cntNulls)

NULL 值被过滤掉，除非tpid 切片中只有 NULL 值。

Demo here

您也可以使用以下查询：

SELECT t1.tpid, t1.data
FROM mytable AS t1
LEFT JOIN (
  SELECT tpid
  FROM mytable
  GROUP BY tpid
  HAVING COUNT(*) = COUNT(CASE WHEN data IS NULL THEN 1 END)
) AS t2 ON t1.tpid = t2.tpid
WHERE (t1.data IS NOT NULL) OR (t2.tpid IS NOT NULL)

Demo here

Answer 3

select tpid,data from (SELECT tpid, data,
         COUNT(*) OVER (PARTITION BY tpid) AS cnt,
         COUNT(CASE WHEN [data] IS NULL THEN 1 END) 
         OVER (PARTITION BY tpid) AS cntNulls
  FROM mytable) t
  where t.cntNulls = t.cnt or data is not null