Text equality (including null values) in Postgres plpgsql stored function - sql

Question

I have a stored function which is supposed to compare three text values for equality. Some of these text values may be null, and if they are, then the comparison should return a false value.
CREATE OR REPLACE FUNCTION "subject_check_if_subjectName_exists"(name1IN text, name2IN text, name3IN text, name4IN text)
returns boolean as
$$
declare
results boolean;
subjectList record;
begin
results = false;
for subjectList in select name1, name2, name3, name4 from subject loop
if (name1In = subjectList.name1) and (name2In = subjectList.name2) and (name3In = subjectList.name3) and (name4In = subjectList.name4)
then
results = true;
EXIT; -- exit out of loop
end if;
end loop;
return results;
end;
$$ language 'plpgsql';
Of both name4IN and subjectList.name4 are null, and all the other values are equal, the function doesn't return a true value - which it should. How can I compare these text values even if they are null (null = null should return true)?

Answer 1

I think you want to use is not distinct from:
For non-null inputs, IS DISTINCT FROM is the same as the <> operator. However, if both inputs are null it returns false, and if only one input is null it returns true. Similarly, IS NOT DISTINCT FROM is identical to = for non-null inputs, but it returns true when both inputs are null, and false when only one input is null.
Essentially, A is not distinct from B is like A = B but it treats NULLs as "equal" (i.e. it behaves like most SQL newcomers think = should). For example, consider a simple function like this:
create function f(text,text) returns text as $$
begin
if $1 is distinct from $2 then
return '!=';
end if;
return '==';
end $$
language plpgsql;
That will give you results like this:
=> select f(null, null) as "1"
f(null, '') as "2",
f('', '') as "3",
f('pancakes','pancakes') as "4",
f('pancakes', null) as "5",
f('pancakes', 'house') as "6";
1 | 2 | 3 | 4 | 5 | 6
----+----+----+----+----+----
== | != | == | == | != | !=
So something like this is what you're looking for:
if (name1In is not distinct from subjectList.name1) and ...

Answer 2

Essentially, this is what I want to do. I figured there must be a quicker way to compare values including null ones. This does what I want it to do:
-- Function to check subject names (name1,2,3 and 4) already exists
CREATE OR REPLACE FUNCTION "subject_check_if_subjectName_exists"(name1IN text, name2IN text, name3IN text, name4IN text)
returns boolean as
$$
declare
results boolean;
subjectList record;
begin
results = false;
for subjectList in select name1, name2, name3, name4 from subject loop
if ((subjectList.name4 is null) and (name4IN is null)) then
if ((subjectList.name3 is null) and (name3IN is null)) then
if ((subjectList.name2 is null) and (name2IN is null)) then
if (name1IN = subjectList.name1) then
results = true;
EXIT;
end if;
else
if ((name1IN = subjectList.name1) and (name2IN = subjectList.name2)) then
results = true;
EXIT;
end if;
end if;
else
if ((name1IN = subjectList.name1) and (name2IN = subjectList.name2) and (name3IN = subjectList.name3)) then
results = true;
EXIT;
end if;
end if;
else
if ((name1IN = subjectList.name1) and (name2IN = subjectList.name2) and (name3IN = subjectList.name3) and (name4IN = subjectList.name4)) then
results = true;
EXIT;
end if;
end if;
end loop;
return results;
end;
$$ language 'plpgsql';
Will try IS NOT DISTINCT FROM now.
Thank you for your help. Much appreciated!

Answer 3

Use coalesce:
if (coalesce(name1In, '') = coalesce(subjectList.name1, '') ....