How to compare equality of value in SQL with null?
For those familiar with C#, here are the results of comparing nullable values:
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
The easiest solution I found in SQL is to coalesced the nullable fields into some sentinel value(e.g. 'scoobydoo') then compare them
coalesce(A, 'scoobydoo') = coalesce(B, 'scoobydoo')
But that is plain kludge if somebody uses the sentinel value, if A happens to be NULL and B is 'scoobydoo', then the expression above would yield true
This is exactly my purpose on asking for the logic of the code above (T-SQL UPDATE trigger):
-- detect if the value changes
if (select invoice_date from inserted) <>
(select invoice_date from deleted) begin
-- do something to summary tables here
end
How to do equality comparison in SQL with C#-like behavior?
[EDIT: Found the answer here]
Tested the code (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)
[EDIT: Tested Jon's code, his answer on equality sorts of semi-work(just treat the null as false anyway), but his answer on inequality bombs out]
Tested the code (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)
[EDIT: posted another question related to this one]
[EDIT: posted results based on Jon's inquiry on non-working semantics for inequality comparison]
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)
the non-working semantics for inequality prompted me to post another question :-)
[EDIT: Tested Jon's new answer]
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)
Results:
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)