Question

我正在处理与以下内容非常相似的查询：

CREATE TABLE #test (a char(1), b char(1))

INSERT INTO #test(a,b) VALUES 
('A',NULL),
('A','B'),
('B',NULL),
('B',NULL)

SELECT DISTINCT a,b FROM #test

DROP TABLE #test

结果是，毫不奇怪，

a   b
-------
A   NULL
A   B
B   NULL

我希望实际看到的输出是：

a   b
-------
A   B
B   NULL

也就是说，如果列在某些记录中有值但在其他记录中没有，我想抛出该列为NULL的行。但是，如果列的所有记录都有NULL值，我想保留该NULL。

在单个查询中执行此操作的最简单/最优雅的方法是什么？

我觉得如果我在星期五下午没有筋疲力尽就会很简单。

Answer 1

试试这个：

select distinct * from test
where b is not null or a in (
  select a from test
  group by a
  having max(b) is null)

你可以得到小提琴here。

请注意，如果b中只有一个非空值，则可以简化为：

select a, max(b) from test
group by a

Answer 2

试试这个：

create table test(
x char(1),
y char(1)
);

insert into test(x,y) values
('a',null),
('a','b'),
('b', null),
('b', null)

查询：

with has_all_y_null as
(
    select x
    from test
    group by x
    having sum(case when y is null then 1 end) = count(x)
)
select distinct x,y from test
where 

    (
        -- if a column has a value in some records but not in others,
        x not in (select x from has_all_y_null) 

        -- I want to throw out the row with NULL
        and y is not null 
    )
    or 
    -- However, if a column has a NULL value for all records, 
    -- I want to preserve that NULL
    (x in (select x from has_all_y_null))

order by x,y

输出：

 X    Y
 A    B
 B    NULL

实时测试：http://sqlfiddle.com/#!3/259d6/16

修改

看到Mosty's answer，我简化了我的代码：

with has_all_y_null as ( select x from test group by x -- having sum(case when y is null then 1 end) = count(x) -- should have thought of this instead of the code above. Mosty's logic is good: having max(y) is null ) select distinct x,y from test where y is not null or (x in (select x from has_all_y_null)) order by x,y

我更喜欢CTE方法，它有一个更自我记录的逻辑： - ）

如果你有意识的话，你也可以把文件放在非CTE方法上：

select distinct * from test where b is not null or a in ( -- has all b null select a from test group by a having max(b) is null)

Answer 3

;WITH CTE
    AS
    (
    SELECT DISTINCT * FROM #test
    )
    SELECT a,b
    FROM CTE        
    ORDER BY CASE WHEN b IS NULL THEN 9999 ELSE b END ;

Answer 4

SELECT DISTINCT t.a, t.b
FROM   #test t
WHERE  b IS NOT NULL
OR     NOT EXISTS (SELECT 1 FROM #test u WHERE t.a = u.a AND u.b IS NOT NULL)
ORDER BY t.a, t.b

Answer 5

这是一个非常奇怪的要求。我想知道你是怎么需要的。

SELECT DISTINCT a, b
FROM   test t
WHERE  NOT ( b IS  NULL
          AND EXISTS 
              ( SELECT * 
                FROM test ta 
                WHERE ta.a = t.a 
                  AND ta.b IS NOT NULL
               ) 
             )
  AND  NOT ( a IS  NULL
          AND EXISTS 
              ( SELECT * 
                FROM test tb 
                WHERE tb.b = t.b 
                  AND tb.a IS NOT NULL
               ) 
             )

Answer 6

嗯，我不是特别喜欢这个解决方案，但它似乎对我来说最合适。请注意，您对所需内容的描述与LEFT JOIN的内容完全相同，因此：

SELECT DISTINCT a.a, b.b
FROM #test a
    LEFT JOIN #test b ON a.a = b.a
        AND b.b IS NOT NULL

Answer 7

SELECT a,b FROM #test t where b is not null
union
SELECT a,b FROM #test t where b is null
and not exists(select 1 from #test where a=t.a and b is not null)

结果：

a    b
---- ----
A    B
B    NULL

Answer 8

由于我的视图更复杂，我将把两个答案混合在一起解决了我的问题

    --IdCompe int,
    --Nome varchar(30),
    --IdVanBanco int,
    --IdVan int
    --FlagAtivo bit,
    --FlagPrincipal bit

    select IdCompe
           , Nome
           , max(IdVanBanco)
           , max(IdVan)
           , CAST(MAX(CAST(FlagAtivo as INT)) AS BIT) FlagAtivo
           , CAST(MAX(CAST(FlagPrincipal as INT)) AS BIT) FlagPrincipal
    from VwVanBanco
           where IdVan = {IdVan} or IdVan is null
           group by IdCompe, Nome order by IdCompe asc

感谢mosty mostacho和 kenwarner

在SELECT DISTINCT中消除NULL的最简单方法是什么？

8 个答案: