Question

如何比较SQL中的值与null的相等性？

对于熟悉C＃的人，以下是比较可空值的结果：

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空字段合并为一些标记值（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且B为'scoobydoo'，那么这就是普通的kludge，那么上面的表达式会产生真的

这正是我要求上面代码逻辑的目的（T-SQL UPDATE触发器）：

-- detect if the value changes

if (select invoice_date from inserted) <> 
   (select invoice_date from deleted) begin

    -- do something to summary tables here

end

如何在SQL中使用类似C＃的行为进行等式比较？

[编辑：找到答案here]

测试代码（Postgres nice boolean support，FTW！）：

select

    A, B,

    A = B,
    A IS NOT DISTINCT FROM B, -- "logically" same as above

    A <> B,
    A IS DISTINCT FROM B -- "logically" same as above

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

[编辑：测试了Jon的代码，他对平等各种半工作的回答（无论如何只是将null视为假），但他对不平等的回答却被炸出来了]

测试代码（Postgres nice boolean support，FTW！）：

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)

[编辑：发布了与此相关的另一个question]

[编辑：根据Jon对不平等比较的非工作语义的调查发布结果]

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)

不平等的非工作语义促使我发布另一个question： - ）

[编辑：测试Jon的新答案]

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)

结果：

  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)

Answer 1

再次编辑 ...合并结果应该有效并使事情变得更简单：

平等：

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

我同意这不是非常愉快。

编辑：Vilx指出A <> B存在问题。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))

虽然这样做可能更简单：

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

Answer 2

如果是Microsoft SQL Server，那么您正在寻找ANSI_NULLS选项。如果它是另一个DBMS，您将必须阅读它的文档。他们中的一些根本不支持这一点。

已添加：哦，我注意到你提到了T-SQL。然后 MSSQL！：）

如何在SQL中使用类似C＃的行为进行等式比较？

2 个答案: