SQLの値の同等性をnullと比較する方法は？
C＃に精通している方のために、null許容値を比較した結果を次に示します。
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
SQLで見つけた最も簡単な解決策は、null許容フィールドをいくつかの番兵値（例：'scoobydoo'）に合体させてから、それらを比較することです。
coalesce(A, 'scoobydoo') = coalesce(B, 'scoobydoo')
しかし、誰かが番兵の値を使用している場合、それは明白な恨みです。AがたまたまNULLで、Bが'scoobydoo'である場合、上記の式はtrueになります。
これはまさに、上記のコードのロジック（T-SQL UPDATEトリガー）を要求する私の目的です。
-- detect if the value changes
if (select invoice_date from inserted) <>
(select invoice_date from deleted) begin
-- do something to summary tables here
end
SQLでC＃のような動作で同等性を比較するにはどうすればよいですか？
[編集：ここで答えを見つけました]
コードをテストしました（Postgresの素敵なブールサポート、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)
[編集：Jonのコードをテストしました。平等に関する彼の答えは、半ば作業のようなものです（とにかく、nullをfalseとして扱います）が、不平等に関する彼の答えは爆破されます]
コードをテストしました（Postgresの素敵なブールサポート、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)
[編集：これに関連する別の質問を投稿しました]
[編集：不等式比較のための非機能セマンティクスに関するJonの問い合わせに基づいて投稿された結果]
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)
不平等のセマンティクスが機能しないため、別の質問を投稿するように促されました:-)
[編集：ジョンの新しい答えをテストしました]
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)
結果：
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)