C# 如何使用类似C的行为在SQL中进行等式比较？

如何将SQL中的值相等与null进行比较

对于熟悉C#的人，以下是比较可为空值的结果：

null == null : true
null == john : false
null == paul : false
john == null : false
john == john : true
john == paul : false
paul == null : false
paul == john : false
paul == paul : true

我在SQL中找到的最简单的解决方案是将可为空的字段合并成一些sentinel值（例如“scoobydoo”），然后比较它们

coalesce(A, 'scoobydoo') = coalesce(B, 'scoobydoo')

但如果有人使用sentinel值，如果A恰好为NULL，B为‘scoobydoo’，那么上面的表达式将产生true，这显然是一个难题

这正是我询问上述代码逻辑（T-SQL更新触发器）的目的：

再次编辑。。。合并结果应该会起作用，并使事情变得更简单：

平等：

where COALESCE((A = B) or (A is null and B is null), false)

我同意这不太令人愉快

编辑：维尔克斯指出了

ab

的一个问题。不过，我认为这会奏效：

where (A <> B) or ((A is null or B is null) and
                   (A is not null or B is not null))

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如果是Microsoft SQL Server，则您正在寻找该选项。如果它是另一个DBMS，您必须阅读它的文档。他们中的一些人根本不支持这一点

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补充道：哦，我注意到你提到了T-SQL。它就是MSSQL！：）

s/delete/deleted/在上一个示例中。明确给出paul&john的初始值？如果这是T-SQL，它肯定是Postgres问题的重复吗？同样的答案会起作用吗？@Jon Skeet:你只是厌倦了，迫不及待地想回答另一个问题：兴趣的撅嘴，你说的“炸弹爆炸”到底是什么意思？你有什么错误？我很好奇。。。（SQL不是我的强项。）+1用于提供的引擎盖下语义不同于Umm。。。如果我错了，请原谅，如果A为空，B不为空，那么第二个不是会失败吗？@Vilx:你关心哪一点

AB

将为真，

（A不为空或B不为空）

将为真，因为B不为空-因此根据需要，总体结果为真。不，AB将为假，因为其中一个为空，将任何内容与空值进行比较将导致空值。好吧，除非你玩弄ANSI_NULLS，但你无论如何都不需要这些。@Jon：这是有效的（如果我们将null视为false）：其中（ab）或（（A为null或B为null）和（A为非null或B为非null））；这不是：如果不（（A=B）或（A为null，B为null））看到我的查询结果，我喜欢它！我知道这不是标准，但该死！谢谢。理论上，它也可能是Sybase。嗯，好吧，我不知道。：）

select

    A, B,

    A = B,
    A IS NOT DISTINCT FROM B, -- "logically" same as above
    coalesce( (A = B) or (A is null and B is null), false ), 
    -- tested Jon's code for ==, semi-work, coalesced to make it true/false only


    A <> B,
    A IS DISTINCT FROM B, -- "logically" same as above
    (A <> B) and (A is not null or B is not null)  
    -- tested Jon's code for !=, bombs out

from(    
    values
    (null, null),
    (null, 'john'),
    (null, 'paul'),
    ('john', null),
    ('john', 'john'),
    ('john', 'paul'),
    ('paul', null),
    ('paul', 'john'),
    ('paul', 'paul')) as x(A,B)

select

    A, B,

    A = B,
    A IS NOT DISTINCT FROM B, -- "logically" same as above
    (A = B) or (A is null and B is null), 
    -- tested Jon's code for ==


    A <> B,
    A IS DISTINCT FROM B -- "logically" same as above,
    (A <> B) and (A is not null or B is not null)  
    -- tested Jon's code for !=, bombs out

from(    
    values
    (null, null),
    (null, 'john'),
    (null, 'paul'),
    ('john', null),
    ('john', 'john'),
    ('john', 'paul'),
    ('paul', null),
    ('paul', 'john'),
    ('paul', 'paul')) as x(A,B)


  a   |  b   | ?column? | ?column? | ?column? | ?column? | ?column? | ?column?
------+------+----------+----------+----------+----------+----------+----------
 null | null | null     | t        | t        | null     | f        | f
 null | john | null     | f        | null     | null     | t        | null
 null | paul | null     | f        | null     | null     | t        | null
 john | null | null     | f        | null     | null     | t        | null
 john | john | t        | t        | t        | f        | f        | f
 john | paul | f        | f        | f        | t        | t        | t
 paul | null | null     | f        | null     | null     | t        | null
 paul | john | f        | f        | f        | t        | t        | t
 paul | paul | t        | t        | t        | f        | f        | f
(9 rows)

select

    A, B,

    A = B as e,
    A IS NOT DISTINCT FROM B AS e_works, -- "logically" same as above
    (A = B) or (A is null and B is null) AS e_semi_work, -- tested Jon's code for ==, works if we treat null as false


    A <> B as ie,
    A IS DISTINCT FROM B as ie_works, -- "logically" same as above,
    (A <> B) and (A is not null or B is not null) as ie_not_work, -- tested Jon's code for !=, bombs out

    (A <> B) or ((A is null or B is null) and (A is not null or B is not null)) as ie_semi_works, -- this works(well it is, if you treat null as false),

     not ((A = B) or (A is null and B is null)) as ie_not_work2 -- this doesn't work


from(    
    values
    (null, null),
    (null, 'john'),
    (null, 'paul'),
    ('john', null),
    ('john', 'john'),
    ('john', 'paul'),
    ('paul', null),
    ('paul', 'john'),
    ('paul', 'paul')) as x(A,B)

  a   |  b   | e    | e_works | e_semi_work | ie   | ie_works | ie_not_work | ie_semi_works | ie_not_work2
------+------+------+---------+-------------+------+----------+-------------+---------------+--------------
 null | null | null | t       | t           | null | f        | f           | null          | f
 null | john | null | f       | null        | null | t        | null        | t             | null
 null | paul | null | f       | null        | null | t        | null        | t             | null
 john | null | null | f       | null        | null | t        | null        | t             | null
 john | john | t    | t       | t           | f    | f        | f           | f             | f
 john | paul | f    | f       | f           | t    | t        | t           | t             | t
 paul | null | null | f       | null        | null | t        | null        | t             | null
 paul | john | f    | f       | f           | t    | t        | t           | t             | t
 paul | paul | t    | t       | t           | f    | f        | f           | f             | f
(9 rows)

where COALESCE((A = B) or (A is null and B is null), false)

where (A <> B) or ((A is null or B is null) and
                   (A is not null or B is not null))

where !(COALESCE((A = B) or (A is null and B is null)), false)