5

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为一些标记值(例如“scoobydoo”),然后比较它们

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

但是,如果有人使用哨兵值,这很简单,如果 A 恰好为 NULL 而 B 是“scoobydoo”,那么上面的表达式将产生 true

这正是我询问上述代码逻辑的目的(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# 的行为进行相等比较?

[编辑:在这里找到答案]

测试了代码(Postgres 不错的布尔支持,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:测试了乔恩的代码,他对等式的回答是半工作的(只是将 null 视为假),但他对不等式的回答会爆炸]

测试了代码(Postgres 不错的布尔支持,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:发布了另一个与此相关的问题]

[编辑:基于 Jon 对不等式比较的非工作语义的调查发布的结果]

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问题:-)

[编辑:测试乔恩的新答案]

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)
4

2 回答 2

5

再次编辑...合并结果应该可以工作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 指出了A <> B. 我认为这会起作用:

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

不过,这样做可能更简单:

where !(COALESCE((A = B) or (A is null and B is null)), false)
于 2009-08-07T08:08:36.323 回答
3

如果它是 Microsoft SQL Server,那么您正在寻找ANSI_NULLS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。

补充:哦,我注意到你提到了 T-SQL。那MSSQL!:)

于 2009-08-07T08:10:33.903 回答